Research Area A
Project A1
Controlling Fragmentation of the Acetylene Cation in the Vacuum-Ultraviolet via Transient Molecular Alignment
L. Varvarezos, J. Delgado-Guerrero, M. Di Fraia, T. J. Kelly, A. Palacios, C. Callegari, A. L. Cavalieri, R. Coffee, M. Danailov, P. Decleva, A. Demidovich, L. DiMauro, S. Düsterer, L. Giannessi, W. Helml, M. Ilchen, R. Kienberger, T. Mazza, M. Meyer, R. Moshammer, C. Pedersini, O. Plekan, K. C. Prince, A. Simoncig, A. Schletter, K. Ueda, M. Wurzer, M. Zangrando, F. Martín, and J. T. Costello
An open-loop control scheme of molecular fragmentation based on transient molecular alignment combined with single-photon ionization induced by a short-wavelength free electron laser (FEL) is demonstrated for the acetylene cation. Photoelectron spectra are recorded, complementing the ion yield measurements, to demonstrate that such control is the consequence of changes in the electronic response with molecular orientation relative to the ionizing field. We show that stable C2H2+ cations are mainly produced when the molecules are parallel or nearly parallel to the FEL polarization, while the hydrogen fragmentation channel (C2H2+ → C2H+ + H) predominates when the molecule is perpendicular to that direction, thus allowing one to distinguish between the two photochemical processes. The experimental findings are supported by state-of-the art theoretical calculations.
Influence of an atomic resonance on the coherent control of the photoionization process
E. V. Gryzlova, P. Carpeggiani, M. M. Popova, M. D. Kiselev, N. Douguet, M. Reduzzi, M. Negro, A. Comby, H. Ahmadi, V. Wanie, M. C. Castrovilli, A. Fischer, P. Eng-Johnsson, M. Meyer, K. Bartschat, S. M. Burkov, T. Csizmadia, M. Dumergue, S. Kühn, N. G. Harshitha, M. Fule, F. Aeenehvand, F. Stienkemeier, D. Iablonskyi, K. Ueda, P. Finetti, M. Zangrando, N. Mahne, K. L. Ishikawa, O. Plekan, K. C. Prince, E. Allaria, L. Giannessi, C. Callegari, A. N. Grum-Grzhimailo, and G. Sansone
In coherent control schemes, pathways connecting an initial and a final state can be independently controlled by manipulating the complex amplitudes of their transition matrix elements. For paths characterized by the absorption of multiple photons, these quantities depend on the magnitude and phase between the intermediate steps, and are expected to be strongly affected by the presence of resonances. We investigate the coherent control of the photoemission process in neon using a phase-controlled two-color extreme ultraviolet pulse with frequency in proximity of an excited energy state. Using helium as a reference, we show that the presence of such a resonance in neon modifies the amplitude and phase of the asymmetric emission of photoelectrons. Theoretical simulations based on perturbation theory are in fair agreement with the experimental observations.
High-temporal-resolution X-ray spectroscopy with free-electron and optical lasers
D. E. Rivas, S. Serkez, T. M. Baumann, R. Boll, M. K. Czwalinna, S. Dold, A. de Fanis, N. Gerasimova, P. Grychtol, B. Lautenschlager, M. Lederer, T. Jezynksi, D. Kane, T. Mazza, J. Meier, J. Muller, F. Pallas, D. Rompotis, P. Schmidt, S. Schulz, S. Usenko, S. Venkatesan, J. Wang, and M. Meyer
Ultrafast X-ray spectroscopies require flexible X-ray properties together with high temporal and spectral resolution. Here, we demonstrate simultaneous sub-20 fs and sub-eV resolutions for pump/probe experiments, without the need for additional photon arrival-time monitors.
Complex attosecond waveform synthesis at FEL FERMI
P. K. Maroju, C. Grazioli, M. Di Fraia, M. Moioli, D. Ertel, H. Ahmadi, O. Plekan, P. Finetti, E. Allaria, L. Giannessi, G. De Ninno, A. A. Lutman, R. J. Squibb, R. Feifel, P. Carpeggiani, M. Reduzzi, T. Mazza, M. Meyer, S. Bengtsson, N. Ibrakovic, E. R. Simpson, J. Mauritsson, T. Csizmadia, M. Dumergue, S. Kühn, H. N. Gopalakrishnan, D. You, K. Ueda, M. Labeye, J. E. Bækhøj, K. J. Schafer, E. V. Gryzlova, A. N. Grum-Grzhimailo, K. C. Prince, C. Callegari, and G. Sansone
Free-electron lasers (FELs) can produce radiation in the short wavelength range extending from the extreme ultraviolet (XUV) to the X-rays with a few to a few tens of femtoseconds pulse duration. These facilities have enabled significant breakthroughs in the field of atomic, molecular, and optical physics, implementing different schemes based on two-color photoionization mechanisms. In this article, we present the generation of attosecond pulse trains (APTs) at the seeded FEL FERMI using the beating of multiple phase-locked harmonics. We demonstrate the complex attosecond waveform shaping of the generated APTs, exploiting the ability to manipulate independently the amplitudes and the phases of the harmonics. The described generalized attosecond waveform synthesis technique with an arbitrary number of phase-locked harmonics will allow the generation of sub-100 as pulses with programmable electric fields.
Analysis of two-color photoelectron spectroscopy for attosecond metrology at seeded free-electron lasers
P. K. Maroju, C. Grazioli, M. Di Fraia, M. Moioli, D. Ertel, H. Ahmadi, O. Plekan, P. Finetti, E. Allaria, L. Giannessi, G. De Ninno, R. J. Squibb, R. Feifel, P. Carpeggiani, M. Reduzzi, T. Mazza, M. Meyer, S. Bengtsson, N. Ibrakovic, E. R. Simpson, J. Mauritsson, T. Csizmadia, M. Dumergue, S. Kühn, N. G. Harshitha, D. You, K. Ueda, M. Labeye, J. E. Baekhoj, K. J. Schafer, E. V. Gryzlova, A. N. Grum-Grzhimailo, K. C. Prince, C. Callegari, and G. Sansone
The generation of attosecond pulse trains at free-electron lasers opens new opportunities in ultrafast science, as it gives access, for the first time, to reproducible, programmable, extreme ultraviolet (XUV) waveforms with high intensity. In this work, we present a detailed analysis of the theoretical model underlying the temporal characterization of the attosecond pulse trains recently generated at the free-electron laser FERMI. In particular, the validity of the approximations used for the correlated analysis of the photoelectron spectra generated in the two-color photoionization experiments are thoroughly discussed. The ranges of validity of the assumptions, in connection with the main experimental parameters, are derived.
Timing and X-ray pulse characterization at the Small Quantum Systems instrument of the European XFEL
P. Grychtol, D. E. Rivas, T. M. Baumann, R. Boll, A. De Fanid, B. Erk, M. Ilchen, J. Liu, T. Mazza, J. Montano, J. Müller, V. Music, Y. Ovcharenko, N. Rennhack, A. Rouzee, P. Schmidt, S. Schulz, S. Usenko, R. Wagner, P. Ziolkowski, H. Schlarb, J. Grünert, N. Kabachnik and M. Meyer
This contribution presents the initial characterization of the pump-probe performance at the Small Quantum Systems (SQS) instrument of the European X-ray Free Electron Laser. It is demonstrated that time-resolved experiments can be performed by measuring the X-ray/optical cross-correlation exploiting the laser-assisted Auger decay in neon. Applying time-of-arrival corrections based on simultaneous spectral encoding measurements allow us to significantly improve the temporal resolution of this experiment. These results pave the way for ultrafast pump-probe investigations of gaseous media at the SQS instrument combining intense and tunable soft X-rays with versatile optical laser capabilities.
Clocking Auger electrons
D. C. Haynes, M. Wurzer, A. Schletter, A. Al-Haddad, C. Blaga, C. Bostedt, J. Bozek, H. Bromberger, M. Bucher, A. Camper, S. Carron, R. Coffee, J. T. Costello, L. F. DiMauro, Y. Ding, K. Ferguson, I. Grguraš, W. Helml, M. C. Hoffmann, M. Ilchen, S. Jalas, N. M. Kabachnik, A. K. Kazansky, R. Kienberger, A. R. Maier, T. Maxwell, T. Mazza, M. Meyer, H. Park, J. Robinson, C. Roedig, H. Schlarb, R. Singla, F. Tellkamp, P. A. Walker, K. Zhang, G. Doumy, C. Behrens, A. L. Cavalieri
Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving it in inherently unstable states that decay on femtosecond timescales. The relaxation occurs primarily via Auger emission, so excited-state observations are constrained by Auger decay. In situ measurement of this process is therefore crucial, yet it has thus far remained elusive in XFELs owing to inherent timing and phase jitter, which can be orders of magnitude larger than the timescale of Auger decay. Here we develop an approach termed ‘self-referenced attosecond streaking’ that provides subfemtosecond resolution in spite of jitter, enabling time-domain measurement of the delay between photoemission and Auger emission in atomic neon excited by intense, femtosecond pulses from an XFEL. Using a fully quantum-mechanical description that treats the ionization, core-hole formation and Auger emission as a single process, the observed delay yields an Auger decay lifetime of 2.2+0.2−0.3 fs for the KLL decay channel.
Near-threshold two-photon double ionization of Kr in the vacuum ultraviolet
Lazaros Varvarezos, Stefan Düsterer, Maksim D. Kiselev, Rebecca Boll, Cedric Bomme, Alberto De Fanis, Benjamin Erk, Christopher Passow, Sergei M. Burkov, Gregor Hartmann, Markus Ilchen, Per Johnsson, Thomas J. Kelly, Bastian Manschwetus, Tommaso Mazza, Michael Meyer, Dimitrios Rompotis, Oleg Zatsarinny, Elena V. Gryzlova, Alexei N. Grum-Grzhimailo, and John T. Costello
We report angle-resolved measurements on photoelectrons emitted upon near-threshold two-photon double ionization (TPDI) of Kr irradiated by free-electron laser (FEL) pulses. These photoelectron angular distributions (PADs) are compared with the results of semirelativistic R-matrix calculations. As reported by Augustin et al. [Phys. Rev. A 98, 033408 (2018)], it is found that the presence of autoionizing resonances within the bandwidth of the exciting FEL pulse strongly influences the PADs. In contrast to measurements on lower-Z targets such as Ne and Ar, the larger spin-orbit interaction, inherent in 4p-subshell hole states of Kr, permits us to resolve and study PADs associated with some of the fine-structure components of the Kr+ and Kr2+ ions.
Deriving x-ray pulse duration from center-of-energy shifts in THz-streaked ionized electron spectra
Marek Wieland, Nikolay M. Kabachnik, Markus Drescher, Yunpei Deng, Yunieski Arbelo, Nikola Stojanovic, Bernd Steffen, Juliane Roensch-Schulenburg, Rasmus Ischebeck, Alexander Malyzhenkov, Eduard Prat, and Pavle Juranić
A fast and robust, yet simple, method has been developed for the immediate characterization of x-ray pulse durations via IR/THz streaking that uses the center of energy (COE) of the photoelectron spectrum for the evaluation. The manuscript presents theory and numerical models demonstrating that the maximum COEs shift as a function of the pulse duration and compares them to existing data for validation. It further establishes that the maximum COE can be derived from two COE measurements set at a phase of π/2 apart. The theory, model, and data agree with each other very well, and they present a way to measure pulse durations ranging from sub-fs to tens of fs on-the-fly with a fairly simple experimental setup.
Photoelectron spectra and angular distribution in sequential two-photon double ionization in the region of autoionizing resonances of ArII and KrII
M. D. Kiselev, P. A. Carpeggiani, E. V. Gryzlova, S. M. Burkov, M. Reduzzi, A. Dubrouil, D. Faccial, M. Negro, K. Ueda, F. Frassetto, F. Stienkemeier, Y. Ovcharenko, M. Meyer, O. Plekan, P. Finetti, K. C. Prince, C. Callegari, G. Sansone, and A. N. Grum-Grzhimailo
Autoionizing hole states with electron configuration nsnp5mp are studied in Ar+and Kr+. Total and partial photoionization cross sections, photoelectron spectra and photoelectron angular distributions in the region of the resonances are obtained theoretically in extensive R-matrix calculations. The states of Ar+are observed by means of excitation by a free-electron laser operating in the vacuum- and extreme-ultraviolet wavelength regime combined with angle-resolved photoelectron spectroscopy. Fine tuning of the photon energy allows scanning of the resonances and the observation of the shape of the partial cross section ratio, as well as the asymmetry parameter of the angular distribution of the photoelectrons. The calculations are in good agreement with the experimental data.
Attosecond pulse shaping using a seeded free-electron laser
P. K. Maroju, C. Grazioli, M. Di Fraia, M. Moioli, D. Ertel, H. Ahmadi, O. Plekan, P. Finetti, E. Allaria, L. Giannessi, G. De Ninno, C. Spezzani, G. Penco, A. Demidovich, M. Danailov, R. Borghes, G. Kourousias, C. E. Sanches Dos Reis, F. Bill´e, A. A. Lutman, R. J. Squibb, R. Feifel, P. Carpeggiani, M. Reduzzi, T. Mazza, M. Meyer, S. Bengtsson, N. Ibrakovic, E. R. Simpson, J. Mauritsson, T. Csizmadia, M. Dumergue, S. Kühn, H. Ng, D. You, K. Ueda, M. Labeye, J. Egebjerg Bækhøj, K. J. Schafer, E. V. Gryzlova, A. N. Grum-Grzhimailo, K. C. Prince,C. Callegari, and G. Sansone
Attosecond pulses are central to the investigation of valence- and core-electron dynamics on their natural timescales1,2,3. The reproducible generation and characterization of attosecond waveforms has been demonstrated so far only through the process of high-order harmonic generation4,5,6,7. Several methods for shaping attosecond waveforms have been proposed, including the use of metallic filters8,9, multilayer mirrors10 and manipulation of the driving field11. However, none of these approaches allows the flexible manipulation of the temporal characteristics of the attosecond waveforms, and they suffer from the low conversion efficiency of the high-order harmonic generation process. Free-electron lasers, by contrast, deliver femtosecond, extreme-ultraviolet and X-ray pulses with energies ranging from tens of microjoules to a few millijoules12,13. Recent experiments have shown that they can generate subfemtosecond spikes, but with temporal characteristics that change shot-to-shot14,15,16. Here we report reproducible generation of high-energy (microjoule level) attosecond waveforms using a seeded free-electron laser17. We demonstrate amplitude and phase manipulation of the harmonic components of an attosecond pulse train in combination with an approach for its temporal reconstruction. The results presented here open the way to performing attosecond time-resolved experiments with free-electron lasers.
New Method for Measuring Angle-Resolved Phases in Photoemission
D. You, K. Ueda, E. V. Gryzlova, A. N. Grum-Grzhimailo, M. M. Popova, E. I. Staroselskaya, O. Tugs, Y. Orimo, T. Sato, K. L. Ishikawa, P. A. Carpeggiani, T. Csizmadia, M. Füle, G. Sansone, P. K. Maroju, A. D'Elia, T. Mazza, M. Meyer, C. Callegari, M. Di Fraia, O. Plekan, R. Richter, L. Giannessi, E. Allaria, G. De Ninno, M. Trovo, L. Badano, B. Diviacco, G. Gaio, D. Gauthier, N. Mirian, G. Penco, P. R. Ribic, S. Spampinati, C. Spezzani, and K. C. Prince
Quantum mechanically, photoionization can be fully described by the complex photoionization amplitudes that describe the transition between the ground state and the continuum state. Knowledge of the value of the phase of these amplitudes has been a central interest in photoionization studies and newly developing attosecond science, since the phase can reveal important information about phenomena such as electron correlation. We present a new attosecond-precision interferometric method of angle-resolved measurement for the phase of the photoionization amplitudes, using two phase-locked extreme ultraviolet pulses of frequency ω and 2ω, from a free-electron laser. Phase differences Δ˜η between one- and two-photon ionization channels, averaged over multiple wave packets, are extracted for neon 2p electrons as a function of the emission angle at photoelectron energies 7.9, 10.2, and 16.6 eV. Δ˜η is nearly constant for emission parallel to the electric vector but increases at 10.2 eV for emission perpendicular to the electric vector. We model our observations with both perturbation and ab initio theory and find excellent agreement. In the existing method for attosecond measurement, reconstruction of attosecond beating by interference of two-photon transitions (RABBITT), a phase difference between two-photon pathways involving absorption and emission of an infrared photon is extracted. Our method can be used for extraction of a phase difference between single-photon and two-photon pathways and provides a new tool for attosecond science, which is complementary to RABBITT.
A THz streak camera based on a highly efficient velocity map imaging spectrometer in collinear geometry
Mamuna Anwar , Marek Wieland and Markus Drescher
In this paper we report on a velocity map imaging spectrometer with the implementation of an on-axis geometry where the electrons travel parallel to the ionizing beam towards the detector. Owing to a usually low photon flux from table-top soft x-ray sources, the instrument is optimized for high detector efficiency and high target density. The collinear design offers the possibility to freely choose the state and orientation of the polarization of light fields in streaking experiments. First results of an experiment using THz pulses for streaking using a Xe gas target are presented and compared to the simulated performance of the spectrometer.
Single-shot temporal characterization of XUV pulses with duration from ∼10 fs to ∼350 fs at FLASH
Rosen Ivanov , Ivette J Bermúdez Macias, Jia Liu , Günter Brenner, Juliane Roensch-Schulenburg, Gabor Kurdi, Ulrike Frühling, Katharina Wenig, Sophie Walther, Anastasios Dimitriou, Markus Drescher, Irina P Sazhina, Andrey K Kazansky, Nikolay M Kabachnik, and Stefan Düsterer
Ultra-short extreme ultraviolet pulses from the free-electron laser FLASH are characterized using terahertz-field driven streaking. Measurements at different ultra-short extreme ultraviolet wavelengths and pulse durations as well as numerical simulations were performed to explore the application range and accuracy of the method. For the simulation of streaking, a standard classical approach is used which is compared to quantum mechanical theory, based on strong field approximation. Various factors limiting the temporal resolution of the presented terahertz streaking setup are investigated and discussed. Special attention is paid to the cases of very short (∼10 fs) and long (up to ∼350 fs) pulses.
Field-enabled quantum interference in atomic Auger decay
Murali Krishna Ganesa Subramanian, Roman Brannath, Ralph Welsch, Robin Santra, and Markus Drescher
We demonstrate that an external terahertz (THz) field enables the formation of interference between two distinct Auger pathways leading to the same final ionic state. The kinetic energy of Auger electrons ejected from either of two spin-orbit split one-hole states of magnesium cations is recorded. In the presence of the THz field, a clear oscillatory structure in the Auger spectrum emerges, which we find to be in very good agreement with an analytical model based on perturbation theory. For this interference to occur, the THz field has to chirp the energy of both Auger electrons and photoelectrons simultaneously, in order to create states with indistinguishable quantum properties.
Complete Characterization of Phase and Amplitude of Bichromatic Extreme Ultraviolet Light
M. Di Fraia, O. Plekan, C. Callegari, K. C. Prince, L. Giannessi, E. Allaria, L. Badano, G. De Ninno, M. Trovò, B. Diviacco, D. Gauthier, N. Mirian, G. Penco, P. R. Ribič, S. Spampinati, C. Spezzani, G. Gaio, Y. Orimo, O. Tugs, T. Sato, K. L. Ishikawa, P. A. Carpeggiani, T. Csizmadia, M. Füle, G. Sansone, P. K. Maroju, A. D’Elia, T. Mazza, M. Meyer, E. V. Gryzlova, A. N. Grum-Grzhimailo, D. You, and K. Ueda
Intense, mutually coherent beams of multiharmonic extreme ultraviolet light can now be created using seeded free-electron lasers, and the phase difference between harmonics can be tuned with attosecond accuracy. However, the absolute value of the phase is generally not determined. We present a method for determining precisely the absolute phase relationship of a fundamental wavelength and its second harmonic, as well as the amplitude ratio. Only a few easily calculated theoretical parameters are required in addition to the experimental data.
Electronic decay of core-excited HCl molecules probed by THz streaking
K. Wenig, M. Wieland, A. Baumann , S. Walther, A. Dimitriou , M. J. Prandolini , O. Schepp , I. Bermúdez Macias, M. Sumfleth , N. Stojanovic, S. Düsterer, J. Rönsch-Schulenburg, E. Zapolnova, R. Pan, M. Drescher, and U. Frühling
The ultrafast electronic decay of HCl molecules in the time domain after resonant core excitation was measured. Here, a Cl-2p core electron was promoted to the antibonding σ* orbital initiating molecular dissociation, and simultaneously, the electronic excitation relaxes via an Auger decay. For HCl, both processes compete on similar ultrashort femtosecond time scales. In order to measure the lifetime of the core hole excitation, we collinearly superimposed 40 fs soft x-ray pulses with intense terahertz (THz) radiation from the free-electron laser in Hamburg (FLASH). Electrons emitted from the molecules are accelerated (streaked) by the THz electric field where the resulting momentum change depends on the field's phase at the instant of ionization. Evaluation of a time-shift between the delay-dependent streaking spectra of photo- and Auger electrons yields a decay constant of (11 ± 2) fs for LMM Auger electrons. For further validation, the method was also applied to the MNN Auger decay of krypton. Reproduction of the value already published in the literature confirms that a temporal resolution much below the duration of the exciting x-ray pulses can be reached.
Struct. Dyn. 6, 034301 (2019)
Two-color XUV+NIR multiphoton near-threshold ionization of the helium ion by circularly polarized light in the vicinity of the 3p resonance
A. N. Grum-Grzhimailo, N. Douguet, M. Meyer, and K. Bartschat
Two-color XUV plus near-IR multiphoton ionization of the helium ion by circularly polarized light is studied in the vicinity of the 3p resonance. Combining the analysis of results obtained by solving the time-dependent Schrödinger equation and that of the quasienergy spectrum of He+ reveals the physical mechanisms that determine the photoelectron spectra and the variation of the circular dichroism as a function of the near-IR intensity.
Roadmap of ultrafast x-ray atomic and molecular physics
L. Young, K. Ueda, M. Gühr, P. H. Bucksbaum, M. Simon, S. Mukamel, N. Rohringer, K. C. Prince, C. Masciovecchio, M. Meyer, A. Rudenko, D. Rolles, C. Bostedt, M. Fuchs, D. A. Reis, R. Santra, H. Kapteyn, M. Murnane, H. Ibrahim, F. Légaré, et. al.
X-ray free-electron lasers (XFELs) and table-top sources of x-rays based upon high harmonic generation (HHG) have revolutionized the field of ultrafast x-ray atomic and molecular physics, largely due to an explosive growth in capabilities in the past decade. XFELs now provide unprecedented intensity (1020 W cm−2) of x-rays at wavelengths down to ~1 Ångstrom, and HHG provides unprecedented time resolution (~50 attoseconds) and a correspondingly large coherent bandwidth at longer wavelengths. For context, timescales can be referenced to the Bohr orbital period in hydrogen atom of 150 attoseconds and the hydrogen-molecule vibrational period of 8 femtoseconds; wavelength scales can be referenced to the chemically significant carbon K-edge at a photon energy of ~280 eV (44 Ångstroms) and the bond length in methane of ~1 Ångstrom. With these modern x-ray sources one now has the ability to focus on individual atoms, even when embedded in a complex molecule, and view electronic and nuclear motion on their intrinsic scales (attoseconds and Ångstroms). These sources have enabled coherent diffractive imaging, where one can image non-crystalline objects in three dimensions on ultrafast timescales, potentially with atomic resolution. The unprecedented intensity available with XFELs has opened new fields of multiphoton and nonlinear x-ray physics where behavior of matter under extreme conditions can be explored. The unprecedented time resolution and pulse synchronization provided by HHG sources has kindled fundamental investigations of time delays in photoionization, charge migration in molecules, and dynamics near conical intersections that are foundational to AMO physics and chemistry. This roadmap coincides with the year when three new XFEL facilities, operating at Ångstrom wavelengths, opened for users (European XFEL, Swiss-FEL and PAL-FEL in Korea) almost doubling the present worldwide number of XFELs, and documents the remarkable progress in HHG capabilities since its discovery roughly 30 years ago, showcasing experiments in AMO physics and other applications. Here we capture the perspectives of 17 leading groups and organize the contributions into four categories: ultrafast molecular dynamics, multidimensional x-ray spectroscopies; high-intensity x-ray phenomena; attosecond x-ray science.
Observation and Control of Laser-Enabled Auger Decay
D. Iablonskyi, K. Ueda, Kenichi L. Ishikawa, A. S. Kheifets, P. Carpeggiani, M. Reduzzi, H. Ahmadi, A. Comby, G. Sansone, T. Csizmadia, S. Kuehn, E. Ovcharenko, T. Mazza, M. Meyer, A. Fischer, C. Callegari, O. Plekan, P. Finetti, E. Allaria, E. Ferrari, E. Roussel, D. Gauthier, L. Giannessi, K. C. Prince
Single-photon laser-enabled Auger decay (spLEAD) is predicted theoretically [B. Cooper and V. Averbukh, Phys. Rev. Lett. 111, 083004 (2013)] and here we report its first experimental observation in neon. Using coherent, bichromatic free-electron laser pulses, we detect the process and coherently control the angular distribution of the emitted electrons by varying the phase difference between the two laser fields. Since spLEAD is highly sensitive to electron correlation, this is a promising method for probing both correlation and ultrafast hole migration in more complex systems.
Ultrashort free-electron laser X-ray pulses
W. Helml, I. Grguras, P. N. Juranic, S. Düsterer, T. Mazza, A. R. Maier, N. Hartmann, M. Ilchen, G. Hartmann, L. Patthey, C. Callegari, J. T. Costello, M. Meyer, R. N. Coffee, A. L. Cavalieri, R. Kienberger
For the investigation of processes happening on the time scale of the motion of bound electrons, well-controlled X-ray pulses with durations in the few-femtosecond and even sub-femtosecond range are a necessary prerequisite. Novel free-electron lasers sources provide these ultrashort, high-brightness X-ray pulses, but their unique aspects open up concomitant challenges for their characterization on a suitable time scale. In this review paper we describe progress and results of recent work on ultrafast pulse characterization at soft and hard X-ray free-electron lasers. We report on different approaches to laser-assisted time-domain measurements, with specific focus on single-shot characterization of ultrashort X-ray pulses from self-amplified spontaneous emission-based and seeded free-electron lasers. The method relying on the sideband measurement of X-ray electron ionization in the presence of a dressing optical laser field is described first. When the X-ray pulse duration is shorter than half the oscillation period of the streaking field, few-femtosecond characterization becomes feasible via linear streaking spectroscopy. Finally, using terahertz fields alleviates the issue of arrival time jitter between streaking laser and X-ray pulse, but compromises the achievable temporal resolution. Possible solutions to these remaining challenges for single-shot, full time–energy characterization of X-ray free-electron laser pulses are proposed in the outlook at the end of the review.
Circular Dichroism in Multiphoton Ionization of Resonantly Excited He+ Ions
M. Ilchen, N. Douguet, T. Mazza, A. J. Rafipoor et. al.
Intense, circularly polarized extreme-ultraviolet and near-infrared (NIR) laser pulses are combined to double ionize atomic helium via the oriented intermediate He+(3p) resonance state. Applying angle-resolved electron spectroscopy, we find a large photon helicity dependence of the spectrum and the angular distribution of the electrons ejected from the resonance by NIR multiphoton absorption. The measured circular dichroism is unexpectedly found to vary strongly as a function of the NIR intensity. The experimental data are well described by theoretical modeling and possible mechanisms are discussed.
Angle resolved photoelectron spectroscopy of two-color XUV-NIR ionization with polarization control
S. Düsterer, G. Hartmann, F. Babies, A. Beckmann et. al.
Electron emission caused by extreme ultraviolet (XUV) radiation in the presence of a strong near infrared (NIR) field leads to multiphoton interactions that depend on several parameters. Here, a comprehensive study of the influence of the angle between the polarization directions of the NIR and XUV fields on the two-color angle-resolved photoelectron spectra of He and Ne is presented. The resulting photoelectron angular distribution strongly depends on the orientation of the NIR polarization plane with respect to that of the XUV field. The prevailing influence of the intense NIR field over the angular emission characteristics for He(1s) and Ne(2p) ionization lines is shown. The underlying processes are modeled in the frame of the strong field approximation (SFA) which shows very consistent agreement with the experiment reaffirming the power of the SFA for multicolor-multiphoton ionization in this regime.
Coherent control with a short-wavelength Free Electron Laser
K.C. Prince, E. Allaria, C. Callegari, R. Cucini et. al
Extreme ultraviolet and X-ray free-electron lasers (FELs) produce short-wavelength pulses with high intensity, ultrashort duration, well-defined polarization and transverse coherence, and have been utilized for many experiments previously possible only at long wavelengths: multiphoton ionization1, pumping an atomic laser2 and four-wave mixing spectroscopy3. However one important optical technique, coherent control, has not yet been demonstrated, because self-amplified spontaneous emission FELs have limited longitudinal coherence4, 5, 6, 7. Single-colour pulses from the FERMI seeded FEL are longitudinally coherent8, 9, and two-colour emission is predicted to be coherent. Here, we demonstrate the phase correlation of two colours, and manipulate it to control an experiment. Light of wavelengths 63.0 and 31.5 nm ionized neon, and we controlled the asymmetry of the photoelectron angular distribution10, 11 by adjusting the phase, with a temporal resolution of 3 as. This opens the door to new short-wavelength coherent control experiments with ultrahigh time resolution and chemical sensitivity.
Angular distribution and circular dichroism in the two-colour XUV+NIR above-threshold ionization of helium
T. Mazza, M. Ilchen, A. J. Rafipoor, C. Callegari et. al.
The photoelectron angular distribution and the circular dichroism in two-colour XUV+NIR above-threshold ionization of helium atoms have been investigated both experimentally and theoretically. Circularly polarized XUV pulses from the free electron laser FERMI have been spatially and temporally overlapped with circularly polarized optical pulses in the interaction region with an atomic helium jet. The emitted electrons were energy and angle analyzed by means of a velocity map imaging spectrometer. Asymmetry parameters of the angular distribution were determined and compared to theoretical predictions based on the strong field approximation and perturbation theory, respectively. For low NIR intensities, the ratio of the partial waves in the two-photon ionization process and their relative phase could be deduced. For high NIR intensities, the influence of multi-photon processes is discussed. Circular dichroism was revealed in both cases and is in good agreement with the results of the strong field approximation.
Femtosecond dynamics of correlated many-body states in C60 fullerenes
S. Usenko, M. Schüler, A. Azima, M. Jakob, L.L. Lazzarino, Y. Pavlyukh, A. Przystawik, M. Drescher, T. Laarmann, J. Berakdar
Fullerene complexes may play a key role in the design of future molecular electronics and nanostructured devices with potential applications in light harvesting using organic solar cells. Charge and energy flow in these systems is mediated by many-body effects. We studied the structure and dynamics of laser-induced multi-electron excitations in isolated C60 by two-photon photoionization as a function of excitation wavelength using a tunable fs UV laser and developed a corresponding theoretical framework on the basis of ab initio calculations. The measured resonance line width gives direct information on the excited state lifetime. From the spectral deconvolution we derive a lower limit for purely electronic relaxation on the order of Tel=10+5-3 fs. Energy dissipation towards nuclear degrees of freedom is studied with time-resolved techniques. The evaluation of the nonlinear autocorrelation trace gives a characteristic time constant of Tvib=400+-100 fs for the exponential decay. In line with the experiment, the observed transient dynamics is explained theoretically by nonadiabatic (vibronic) couplings involving the correlated electronic, the nuclear degrees of freedom (accounting for the Herzberg–Teller coupling), and their interplay.
Sensitivity of nonlinear photoionization to resonance substructure in collective excitation
T. Mazza, A. Karamatskou, M. Ilchen, S. Bakhtiarzadeh, A. J. Rafipoor, P. O’Keeffe, T. J. Kelly, N. Walsh, J. T. Costello, M. Meyer, R. Santra
Collective behaviour is a characteristic feature in many-body systems, important for developments in fields such as magnetism, superconductivity, photonics and electronics. Recently, there has been increasing interest in the optically nonlinear response of collective excitations. Here we demonstrate how the nonlinear interaction of a many-body system with intense XUV radiation can be used as an effective probe for characterizing otherwise unresolved features of its collective response. Resonant photoionization of atomic xenon was chosen as a case study. The excellent agreement between experiment and theory strongly supports the prediction that two distinct poles underlie the giant dipole resonance. Our results pave the way towards a deeper understanding of collective behaviour in atoms, molecules and solid-state systems using nonlinear spectroscopic techniques enabled by modern short-wavelength light sources.
Dichroism in the photoionisation of atoms at XUV Free Electron Lasers
T. Mazza, E.V. Gryzlova, A.N. Grum-Grzhimailo, A.K. Kazansky, N.M. Kabachnik, M. Meyer
Two-color photoionization of atomic He has been investigated by angle-integrated and angle-resolved electron spectroscopy. The combined action of intense radiation pulses from the XUV free-electron laser (FEL), FERMI or FLASH, and a synchronized optical laser on the target atom gives rise to a rich sideband structure in the photoemission spectrum. Measurements of the angular distribution parameters and the determination of the circular and linear dichroism for the two-color photoionization enable a detailed analysis of the symmetry of the outgoing electron waves and of the dynamics underlying the multi-photon processes. The experimental results are in excellent agreement with theoretical results obtained using perturbation theory (low intensity regime) and the strong field approximation. For the particular case of two-photon ionization the measurements represent an ideal tool for characterizing certain FEL parameters, here for example the degree and the sign of circular polarization. Finally, new features of the dichroism are theoretically predicted originating from the non-dipole contribution into the photoionization amplitudes.
Probing photoelectron angular distributions in molecules with polarization-controlled two-color above-threshold ionization
T. Leitner, R. Taieb, M. Meyer, Ph. Wernet
We present polarization-controlled multiphoton two-color above-threshold ionization (TCATI) of molecules. The intensity modulations of valence photoelectron intensities of molecules arising from varying the relative orientation of the linear polarization vectors of femtosecond infrared (IR) and vacuum-ultraviolet (VUV) radiation in TCATI of the highest occupied molecular orbitals of H2O, O2, and N2 are reported. The results on the molecular systems are compared to the 3p photoionization of atomic Ar, which serves as a reference system. Modeling the large differences of the modulation amplitudes within the soft-photon approximation enables us to extract the one-photon-ionization anisotropy parameter β2. Accounting only for the first sideband due to two-photon TCATI by one VUV and one IR photon we find satisfactory agreement between experiment and simulation for H2O and O2. However, the model fails for N2 and possible reasons are discussed. We discuss that the described approach may represent an alternative way of determining photoelectron angular distributions from valence shells of molecules and indicate future directions for modeling TCATI of molecules.
Isotope effects in resonant two-color photoionization of Xe in the region of the 5p5(2P1/2)4f [5/2]2 autoionizing state
E. V. Gryzlova, P. O. Keeffe, D. Cubaynes, G. A. Garcia, L. Nahon, A. N. Grum-Grzhimailo, M. Meyer
Isotope effects in two-photon two-color photoionization are investigated by a combined theoretical and experimental study of the ionization of xenon atoms. A combination of variable polarization synchrotron and laser radiations are used to excite the 5p5(2P1/2)4f[5/2]2 autoionizing resonance via the intermediate 5p5(2P3/2)5d[3/2]1 state. Electrons and ions are detected in coincidence in order to extract the photoelectron angular distributions and the values of the linear and circular dichroism and to determine how these depend on the isotope. A complete theoretical model of the two-photon process in atoms is given in order to describe these parameters as a function of the polarization of the exciting light sources (both linear and circular polarization). Furthermore, the hyperfine depolarization due to the coupling of the electronic and nuclear angular momenta in the intermediate state is taken into account. The results of the theoretical model are in agreement with the experimental results and allow estimation of the previously unknown hyperfine structure (HFS) constant for the case of overlapping HFS levels.
Isotope effects in resonant two-color photoionization of Xe in the region of the 5p5(2P1/2)4f[5/2]2 autoionizing state (PDF Download Available). Available from: https://www.researchgate.net/publication/275584741_Isotope_effects_in_resonant_two-color_photoionization_of_Xe_in_the_region_of_the_5p52P124f522_autoionizing_state [accessed Feb 17, 2016].
Femtosecond all-optical synchronization of an X-ray free-electron laser
S. Schulz, I. Grguraš, C. Behrens, H. Bromberger, J. T. Costello, M. K. Czwalinna, M. Felber, M. C. Hoffmann, M. Ilchen, H. Y. Liu, T. Mazza, M. Meyer, S. Pfeiffer, P. Prędki, S. Schefer, C. Schmidt, U. Wegner, H. Schlarb, A. L. Cavalieri
Many advanced applications of X-ray free-electron lasers require pulse durations and time resolutions of only a few femtoseconds. To generate these pulses and to apply them in time-resolved experiments, synchronization techniques that can simultaneously lock all independent components, including all accelerator modules and all external optical lasers, to better than the delivered free-electron laser pulse duration, are needed. Here we achieve all-optical synchronization at the soft X-ray free-electron laser FLASH and demonstrate facility-wide timing to better than 30 fs r.m.s. for 90 fs X-ray photon pulses. Crucially, our analysis indicates that the performance of this optical synchronization is limited primarily by the free-electron laser pulse duration, and should naturally scale to the sub-10 femtosecond level with shorter X-ray pulses.
Determining the polarization state of an extreme ultraviolet free-electron laser beam using atomic circular dichroism
T. Mazza, M. Ilchen, A. J. Rafipoor, C. Callegari, P. Finetti, O. Plekan, K. C. Prince, R. Richter, M. B. Danailov, A. Demidovich, G. De Ninno, C. Grazioli, R. Ivanov, N. Mahne, M. Meyer et al
Ultrafast extreme ultraviolet and X-ray free-electron lasers are set to revolutionize many domains such as bio-photonics and materials science, in a manner similar to optical lasers over the past two decades. Although their number will grow steadily over the coming decade, their complete characterization remains an elusive goal. This represents a significant barrier to their wider adoption and hence to the full realization of their potential in modern photon sciences. Although a great deal of progress has been made on temporal characterization and wavefront measurements at ultrahigh extreme ultraviolet and X-ray intensities, only few, if any progress on accurately measuring other key parameters such as the state of polarization has emerged. Here we show that by combining ultra-short extreme ultraviolet free electron laser pulses from FERMI with near-infrared laser pulses, we can accurately measure the polarization state of a free electron laser beam in an elegant, non-invasive and straightforward manner using circular dichroism.
Accelerator- and laser-based sources of high-field terahertz pulses
N. Stojanovic, M. Drescher
At present we are witnessing a rapid development of sources for terahertz (THz) pulses with very strong electromagnetic fields. These pulses are reaching a stage where they can be used to not only probe, but also uniquely control a variety of processes that range from fundamental dynamics in individual atoms and molecules, through phase transitions in solids to a wealth of interactions in biological materials. In this review, we are presenting an overview of two major directions in the generation of such radiation. Large-scale accelerator-based sources offer unprecedented pulse energies coupled with a wide tuning range and extreme repetition rates. Laser-based sources, on the other hand, are laboratory-scale instruments and thus are very attractive in their availability to the wide scientific community. The capabilities of different variants of these THz sources are evaluated and compared with each other. In addition, powerful techniques for the temporal characterization of THz pulses are discussed.
Evidence for Chirped Auger-Electron Emission
B. Schuette, S. Bauch, U. Fruehling, M. Wieland, M. Gensch, E. Ploenjes, T. Gaumnitz, A. Azima, M. Bonitz, M. Drescher
Auger decay carries valuable information about the electronic structure and dynamics of atoms, molecules, and solids. Here we furnish evidence that under certain conditions Auger electrons are subject to an energetic chirp. The effect is disclosed in time-resolved streaking experiments on the Xe NOO and Kr MNN Auger decay using extreme-ultraviolet pulses from the free-electron laser in Hamburg as well as from a high-order harmonic laser source. The origin of this effect is found to be an exchange of energy between the Auger electron and an earlier emitted correlated photoelectron. The observed time-dependent spectral modulations are understood within an analytical model and confirmed by extensive computer simulations.
Project A2
Charge-induced chemical dynamics in glycine probed with time-resolved Auger electron spectroscopy
D. Schwickert, M. Ruberti, P. Kolorenč, A. Przystawik, S. Skruszewicz, M. Sumfleth, M. Braune, L. Bocklage, L. Carretero, M.K. Czwalinna, D. Diaman, S. Düsterer, M. Kuhlmann, S. Palutke, R. Röhlsberger, J. Rönsch-Schulenburg, S. Toleikis, S. Usenko, J. Viefhaus, A. Vorobiov, M. Martins, D. Kip, V. Averbukh, J.P. Marangos, and T. Laarmann
In the present contribution, we use x-rays to monitor charge-induced chemical dynamics in the photoionized amino acid glycine with femtosecond time resolution. The outgoing photoelectron leaves behind the cation in a coherent superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay. Temporal modulation of the Auger electron signal correlated with specific ions is observed, which is governed by the initial electronic coherence and subsequent vibronic coupling to nuclear degrees of freedom. In the time-resolved x-ray absorption measurement, we monitor the time-frequency spectra of the resulting many-body quantum wave packets for a period of 175 fs along different reaction coordinates. Our experiment proves that by measuring specific fragments associated with the glycine dication as a function of the pump-probe delay, one can selectively probe electronic coherences at early times associated with a few distinguishable components of the broad electronic wave packet created initially by the pump pulse in the cation. The corresponding coherent superpositions formed by subsets of electronic eigenstates and evolving along parallel dynamical pathways show different phases and time periods in the range of... .
Electronic quantum coherence in glycine molecules probed with ultrashort x-ray pulses in real time
D. Schwickert, M. Ruberti, P. Kolorenč, S. Usenko, A. Przystawik, K. Baev, I. Baev, M. Braune, L. Bocklage, M.K. Czwalinna, S. Deinert, S. Düsterer, A. Hans, G. Hartmann, C. Haunhorst, M. Kuhlmann, S. Palutke, R. Röhlsberger, J. Rönsch-Schulenburg, P. Schmidt, S. Toleikis, J. Viefhaus, M. Martins, A. Knie, D. Kip, V. Averbukh, J.P. Marangos, and T. Laarmann
Here, we use x-rays to create and probe quantum coherence in the photoionized amino acid glycine. The outgoing photoelectron leaves behind the cation in a coherent superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay and by photoelectron emission from sequential double photoionization. Sinusoidal temporal modulation of the detected signal at early times (0 to 25 fs) is observed in both measurements. Advanced ab initio many-electron simulations allow us to explain the first 25 fs of the detected coherent quantum evolution in terms of the electronic coherence. In the kinematically complete x-ray absorption measurement, we monitor its dynamics for a period of 175 fs and observe an evolving modulation that may implicate the coupling of electronic to vibronic coherence at longer time scales. Our experiment provides a direct support for the existence of long-lived electronic coherence in photoionized biomolecules.
Full characterization of a phase-locked DUV double pulse generated in an all-reflective shaping setup working under grazing incidence in a broad spectral range
S. Hartwell, A. Azima, C. Haunhorst, M. Kazemi, M. Namboodiri, A. Przystawik, D. Schwickert, S. Skruszewicz, D. Kip, M. Drescher & T. Laarmann
Controlling the temporal and spectral properties of ultrashort laser pulses in the visible and near-infrared spectral range by means of a femtosecond pulse-shaping device is a powerful tool with many applications in ultrafast spectroscopy. A major and successful concept is known as the 4f design, which has a symmetric zero-dispersion-compressor geometry. Most 4f pulse shapers rely on using transmissive optics in their beam path limiting the operational wavelength ranges. In the present contribution, we use an all-reflective shaping setup to generate a phase-locked 266 nm double pulse to benchmark its performance in the limit of short wavelengths. The setup comprises the complete spectral amplitude and phase diagnostics for quantitative analysis of the pulse properties before and after the shaper using the technique of frequency-resolved optical gating. The measured time–frequency spectra are in good agreement with optical simulations. The geometry and hardware of the device including the optical components are designed, such that all harmonics of the deep UV pulses travel the same path, giving the instrument the ability to work with soft X-ray pulses, under vacuum conditions, down to the few-nanometer wavelength scale.
Table-top interferometry on extreme time and wavelength scales
S. Skruszewicz, A. Przystawik, D. Schwickert, M. Sumfleth, M. Namboodiri, V. Hilbert, R. Klas, P. Gierschke, V. Schuster, A. Vorobiov, C. Haunhorst, D. Kip, J. Limpert, J. Rothhardt, and T. Laarmann
Short-pulse metrology and dynamic studies in the extreme ultraviolet (XUV) spectral range greatly benefit from interferometric measurements. In this contribution a Michelson-type all-reflective split-and-delay autocorrelator operating in a quasi amplitude splitting mode is presented. The autocorrelator works under a grazing incidence angle in a broad spectral range (10 nm – 1 μm) providing collinear propagation of both pulse replicas and thus a constant phase difference across the beam profile. The compact instrument allows for XUV pulse autocorrelation measurements in the time domain with a single-digit attosecond precision and a useful scan length of about 1 ps enabling a decent resolution of E/ΔE = 2000 at 26.6 eV. Its performance for selected spectroscopic applications requiring moderate resolution at short wavelengths is demonstrated by characterizing a sharp electronic transition at 26.6 eV in Ar gas. The absorption of the 11th harmonic of a frequency-doubled Yb-fiber laser leads to the well-known 3s3p64p1P1 Fano resonance of Ar atoms. We benchmark our time-domain interferometry results with a high-resolution XUV grating spectrometer and find an excellent agreement. The common-path interferometer opens up new opportunities for short-wavelength femtosecond and attosecond pulse metrology and dynamic studies on extreme time scales in various research fields.
Auger electron wave packet interferometry on extreme timescales with coherent soft x-rays
S. Usenko et al.
Wave packet interferometry provides benchmark information on light-induced electronic quantum states by monitoring their relative amplitudes and phases during coherent excitation, propagation, and decay. The relative phase control of soft x-ray pulse replicas on the single-digit attosecond timescale achieved in our experiments makes this method a powerful tool to probe ultrafast quantum phenomena such as the excitation of Auger shake-up states with sub-cycle precision. In this contribution we present first results obtained for different Auger decay channels upon generating L-shell vacancies in argon atoms using Michelson-type all-reflective interferometric autocorrelation at a central free-electron laser photon energy of 274.7 eV.
Shaping femtosecond laser pulses at short wavelength with grazing-incidence optics
L. L. Lazzarino, M. M. Kazemi, C. Haunhorst, C. Becker, S. Hartwell, M. A. Jakob, A. Przystawik, S. Usenko, D. Kip, I. Hartl, and T. Laarmann
We present the design of an extreme ultraviolet (XUV) pulse shaper relying on reflective optics. The instrument will allow tailoring of the time-frequency spectrum of femtosecond pulses generated by seeded free-electron lasers (FEL) and high-harmonic generation (HHG) sources down to a central wavelength of ~15 nm. The device is based on the geometry of a 4f grating compressor that is a standard concept in ultrafast laser science and technology. We apply it to shorter wavelengths using grazing-incidence optics operated under ultra-high vacuum conditions. The design blaze angle and the line density of the gratings allow the manipulation of all different harmonics typical for seeded FEL and HHG photon sources without the need of realignment of the instrument and even simultaneously in multi-color experiments. A proof-of-principle pulse shaping experiment using 266 nm laser light has been performed, demonstrating relative phase-control of femtosecond UV pulses.
VUV-induced dynamics of the electronically excited C2D4 molecule in a single-color pump-probe experiment
Oliver Schepp, Arne Baumann, Marek Wieland, Armin Azima, Markus Drescher
The ultrafast non-adiabatic dynamics of the electronically excited ethylene molecule C2H4 and its deuterated isotopologue C2D4 are studied with time-resolved photoelectron spectroscopy after excitation via irradiation with light in the vacuum ultraviolet spectral range. Sub-20-fs pulses, generated as the fifth harmonic of a Ti:Sa laser system, are split and delayed in an all-reflective Michelson-type interferometer, enabling a single-color pump-probe experiment. In addition to the ultrafast non-adiabatic relaxation process of C2H4, we find more complex dynamics exhibited by the delay-dependent photoelectron yield of C2D4, identified as a signature of the delayed dissociative ionization of the parent ion.
Chemical Physics Letters: X, Volume 3, 1000024 (2019);
DOI: 10.1016/j.cpletx.2019.100024
Time-Resolved Dissociation Dynamics of Iodomethane Resulting from Rydberg and Valence Excitation
Arne Baumann, Dimitrios Rompotis, Oliver Schepp, Marek Wieland, and Markus Drescher
Rydberg excitations in the vacuum ultraviolet spectral range may open up molecular photoreaction pathways not accessible from lower-lying valence states. Here, single-shot UV/VUV pump–probe spectroscopy was used to study the photodissociation dynamics of iodomethane after 268 nm excitation in the A-band and excitation of the 6p (2E3/2) Rydberg state at 161 nm. By combining weak-field VUV single-photon ionization with sub-10 fs temporal resolution and the superior statistical accuracy of the single-shot technique, sub-30 fs wave packet dynamics upon excitation in the A-band by a UV pump pulse were disclosed. Population transfer from the Rydberg state to the 2 1A1 valence state leading to 100 fs dissociation dynamics was observed by utilizing the same methodology in a VUV-pump/UV-probe scheme.
J. Phys. Chem. A, 2018, 122 (21), pp 4779–4784
DOI: 10.1021/acs.jpca.8b01248
Weak-field few-femtosecond VUV photodissociation dynamics of water isotopologues
A. Baumann, S. Bazzi, D. Rompotis, O. Schepp, A. Azima, M. Wieland, D. Popova-Gorelova, O. Vendrell, R. Santra, M. Drescher
We present a joint experimental and theoretical study of the VUV-induced dynamics of H2O and its deuterated isotopologues in the first excited state (˜A1B1) utilizing a VUV-pump VUV-probe scheme combined with ab initio classical trajectory calculations. 16-fs VUV pulses centered at 161 nm created by fifth-order harmonic generation are employed for single-shot pump-probe measurements. Combined with a precise determination of the VUV pulses' temporal profile, they provide the necessary temporal resolution to elucidate sub-10-fs dissociation dynamics in the 1+1 photon ionization time window. Ionization with a single VUV photon complements established strong-field ionization schemes by disclosing the molecular dynamics under perturbative conditions. Kinetic isotope effects derived from the pump-probe experiment are found to be in agreement with our by ab initio classical trajectory calculations, taking into account photoionization cross sections for the ground and first excited state of the water cation.
Single-shot nonlinear spectroscopy in the vacuum-ultraviolet
D. Rompotis, A. Baumann, O. Schepp, T. Maltezopoulos, M. Wieland, M. Drescher
Time-resolved spectroscopy in the vacuum/extreme ultraviolet (VUV/XUV) spectral range promises to disclose ultrafast dynamics in all states of matter. Completing a measurement within a single shot eliminates the influence of experimental parameter fluctuations and enhances its statistical significance. We demonstrate a single-shot temporal metrology scheme operating in the vacuum/extreme-ultraviolet spectral range, combining few-femtosecond resolution in a wide temporal window with high detection efficiency. An anticollinear geometry encodes temporal delay information on the beam propagation coordinate. The spatial distribution of ions created in the common focus is captured with a mass/charge-state-selective ion imaging spectrometer, resulting in a single-shot pump–probe measurement. We demonstrate a proof-of-principle single-shot VUV-pump/VUV-probe experiment by investigating ultrafast dissociation dynamics of O2 excited at 162 nm. The experimental determination of the finite instrument response in the same apparatus enables robust deconvolution of the investigated dynamics well beyond the instrument’s intrinsic temporal resolution.
Attosecond interferometry with self-amplified spontaneous emission of a free-electron laser
Usenko, A. Przystawik, M.A. Jakob, L.L. Lazzarino, G. Brenner, S. Toleikis, Ch. Haunhorst, D. Kip, T. Laarmann
Light-phase-sensitive techniques, such as coherent multidimensional spectroscopy, are well-established in a broad spectral range, already spanning from radio-frequencies in nuclear magnetic resonance spectroscopy to visible and ultraviolet wavelengths in nonlinear optics with table-top lasers. In these cases, the ability to tailor the phases of electromagnetic waves with high precision is essential. Here we achieve phase control of extreme-ultraviolet pulses from a free-electron laser (FEL) on the attosecond timescale in a Michelson-type all-reflective interferometric autocorrelator. By varying the relative phase of the generated pulse replicas with sub-cycle precision we observe the field interference, that is, the light-wave oscillation with a period of 129 as. The successful transfer of a powerful optical method towards short-wavelength FEL science and technology paves the way towards utilization of advanced nonlinear methodologies even at partially coherent soft X-ray FEL sources that rely on self-amplified spontaneous emission.
Split-and-delay unit for FEL interferometry in the XUV spectral range
S. Usenko, A. Przystawik, L.L. Lazzarino, M.A. Jakob, F. Jacobs, C. Becker, C. Haunhorst, D. Kip, and T. Laarmann
In this work we present a reflective split-and-delay unit (SDU) developed for interferometric time-resolved experiments utilizing an (extreme ultraviolet) XUV pump–XUV probe scheme with focused free-electron laser beams. The developed SDU overcomes limitations for phase-resolved measurements inherent to conventional two-element split mirrors by a special design using two reflective lamellar gratings. The gratings produce a high-contrast interference signal controlled by the grating displacement in every diffraction order. The orders are separated in the focal plane of the focusing optics, which enables one to avoid phase averaging by spatially selective detection of a single interference state of the two light fields. Interferometry requires a precise relative phase control of the light fields, which presents a challenge at short wavelengths. In our setup the phase delay is determined by an in-vacuum white light interferometer (WLI) that monitors the surface profile of the SDU in real time and thus measures the delay for each laser shot. The precision of the WLI is 1 nm as determined by optical laser interferometry. In the presented experimental geometry it corresponds to a time delay accuracy of 3 as, which enables phase-resolved XUV pump–XUV probe experiments at free-electron laser (FEL) repetition rates up to 60 Hz.
Femtosecond dynamics of correlated many-body states in C60 fullerenes
S. Usenko, M. Schüler, A. Azima, M. Jakob, L.L. Lazzarino, Y. Pavlyukh, A. Przystawik, M. Drescher, T. Laarmann, J. Berakdar
Fullerene complexes may play a key role in the design of future molecular electronics and nanostructured devices with potential applications in light harvesting using organic solar cells. Charge and energy flow in these systems is mediated by many-body effects. We studied the structure and dynamics of laser-induced multi-electron excitations in isolated C60 by two-photon photoionization as a function of excitation wavelength using a tunable fs UV laser and developed a corresponding theoretical framework on the basis of ab initio calculations. The measured resonance line width gives direct information on the excited state lifetime. From the spectral deconvolution we derive a lower limit for purely electronic relaxation on the order of Tel=10+5-3 fs. Energy dissipation towards nuclear degrees of freedom is studied with time-resolved techniques. The evaluation of the nonlinear autocorrelation trace gives a characteristic time constant of Tvib=400+-100 fs for the exponential decay. In line with the experiment, the observed transient dynamics is explained theoretically by nonadiabatic (vibronic) couplings involving the correlated electronic, the nuclear degrees of freedom (accounting for the Herzberg–Teller coupling), and their interplay.
Tracing few-femtosecond photodissociation dynamics on molecular oxygen with a single-color pump-probe scheme in the VUV
O. Schepp, A. Baumann, D. Rompotis, T. Gebert, A. Azima, M. Wieland, M. Drescher
Efficient generation of below-threshold harmonics for high-fidelity multi-photon physics in the VUV spectral range
D. Rompotis, T. Gebert, M. Wieland, F. Karimi, M. Drescher
We demonstrate the generation of microjoule level, sub-20-fs, Ti:Sa fifth-harmonic pulses utilizing a loose-focusing geometry in a long Ar gas cell. The VUV pulses centered at 161.8 nm reach pulse energies of 1.1 μJ per pulse, while the corresponding pulse duration is measured with a second-order, fringe-resolved autocorrelation scheme to be 18±1 fs. Nonresonant, two-photon ionization of Kr and three-photon ionization of Ne verify the fifth-harmonic pulse high-intensity content and indicate the feasibility of multi-photon VUV pump-VUV probe studies of ultrafast atomic and molecular dynamics.
Ionisation dynamics of Xe nanoplasma formation studied with XUV fluorescence spectroscopy
A. Przystawik, L. Schroedter, M. Müller, M. Adolph, C. Bostedt, L. Flückiger, T. Gorkhover, A. Kickermann, M. Krikunova, Nösel, T. Oelze, Y. Ovcharenko, D. Rupp, L. M. Sauppe, S. Schorb, S. Usenko, T. Möller, T. Laarmann
Intense pulses from a short wavelength free-electron laser turn xenon nanoparticles into a high energy density nanoplasma within femtoseconds. Recently, the generation of multiply charged xenon ions during the initial phase of plasma evolution has been studied by energy-resolved XUV fluorescence detection as a function of cluster size and cluster composition [1]. In the present contribution we give a detailed analysis of the corresponding radiative transitions after resonant excitation of the 4d electron shell at intensities of 2 × 1012 − 2.45 × 1015 W cm−2. The evaluation of charge-state specific fluorescence yields as a function of FEL power density demonstrates that plasma effects such as ionization potential lowering, electron impact excitation, ionization, and energy redistribution govern the laser-induced non-equilibrium dynamics in xenon clusters.
Ionization dynamics of XUV excited clusters: the role of inelastic electron collisions
M. Müller, L. Schroedter, T. Oelze, L. Nösel, A. Przystawik, A. Kickermann, M. Adolph, T. Gorkhover, L. Flückiger, M. Krikunova, M. Sauppe, Y. Ovcharenko, S. Schorb, C. Bostedt, D. Rupp, T. Laarmann, T. Möller
We report on the ionization and nanoplasma dynamics of small xenon clusters irradiated by intense, short pulses of a short-wavelength free-electron laser. Fluorescence spectroscopy indicates that inelastic electron collisions play a prominent role in the formation of the highest charge states. From the spectral distribution an electron temperature of 27 eV is deduced which corresponds to the average excess energy of the Auger- and photoelectrons ionized from individual atoms but trapped in the cluster core. This suggests that fluorescence spectra reflect a very early stage within the nanoplasma dynamics and shows how a part of the kinetic energy of the plasma electrons trapped in the cluster potential is transferred to the ions.
Hidden Charge States in Soft-X-Ray Laser-Produced Nanoplasmas Revealed by Fluorescence Spectroscopy
L. Schroedter, M. Müller, A. Kickermann, A. Przystawik, S. Toleikis, M. Adolph, L. Flückiger, T. Gorkhover, L. Nösel, M. Krikunova, T. Oelze, Y. Ovcharenko, D. Rupp, M. Sauppe, D. Wolter, S. Schorb, C. Bostedt, T. Möller, T. Laarmann
Highly charged ions are formed in the center of composite clusters by strong free-electron laser pulses and they emit fluorescence on a femtosecond time scale before competing recombination leads to neutralization of the nanoplasma core. In contrast to mass spectrometry that detects remnants of the interaction, fluorescence in the extreme ultraviolet spectral range provides fingerprints of transient states of high energy density matter. Spectra from clusters consisting of a xenon core and a surrounding argon shell show that a small fraction of the fluorescence signal comes from multiply charged xenon ions in the cluster core. Initially, these ions are as highly charged as the ions in the outer shells of pure xenon clusters with charge states up to at least.
A high-harmonic generation source for seeding a free-electron laser at 38 nm
T. Maltezopoulos, M. Mittenzwey, A. Azima, J. Bödewadt, H. Dachraoui, M. Rehders, C. Lechner, M. Schulz, M. Wieland, T. Laarmann, J. Roßbach, M. Drescher
Direct seeding with a high-harmonic generation (HHG) source can improve the spectral, temporal, and coherence properties of a free-electron laser (FEL) and shall reduce intensity and arrival-time fluctuations. In the seeding experiment sFLASH at the extreme ultraviolet FEL in Hamburg FLASH, which operates in the self-amplified spontaneous emission mode (SASE), the 21st harmonic of an 800 nm laser is refocused into a dedicated seeding undulator. For seeding, the external light field has to overcome the noise level of SASE; therefore, an efficient coupling between seed pulse and electron bunch is mandatory. Thus, an HHG beam with a proper divergence, width, beam quality, Rayleigh length, pointing stability, single-shot pulse energy, and stability in the 21st harmonic is needed. Here, we present the setup of the HHG source that seeds sFLASH at 38.1 nm, the optimization procedures, and the necessary diagnostics.
Generation of the simplest rotational wave packet in a diatomic molecule: Tracing a two-level superposition in the time domain
A. Przystawik, A. Kickermann, A. Al-Shemmary, S. Düsterer, A. M. Ellis, K. von Haeften, M. Harmand, S. Ramakrishna, H. Redlin, L. Schroedter, M. Schulz, T. Seideman, N. Stojanovic, J. Szekely, F. Tavella, S. Toleikis, T. Laarmann
We introduce a time-domain approach to explore rotational dynamics caused by intramolecular coupling or the interaction with dissipative media. It pushes the time resolution toward the ultimate limit determined by the rotational period. Femtosecond pulses create a coherent superposition of two rotational states of carbon monoxide. The wave-packet motion is observed by subsequent Coulomb explosion, which results in a time-dependent asymmetry of spatial fragmentation patterns. The asymmetry oscillation prevails for at least 1 ns, covering more than 300 periods with no decoherence. Long time scans will allow weak perturbations of the order of ΔE/E=10−4 to be discerned. Our conclusions are confirmed by a fully quantum-mechanical model.
Michelson-type all-reflective interferometric autocorrelation in the VUV regime
T. Gebert, D. Rompotis, M. Wieland, F. Karimi, A. Azima, M. Drescher
We demonstrate second-order interferometric autocorrelation of a pulse in the vacuum-ultraviolet (VUV) spectral range using an optical arrangement equivalent to a Michelson interferometer. In an all-reflective design, wavefront splitting is realized with two moveable interdigitated reflective gratings forming a diffraction pattern with well separated orders and an intensity distribution depending on the precisely adjustable path-length difference. An imaging time-of-flight spectrometer is able to spatially select ions created by nonlinear two-photon absorption in the focus of the zeroth diffraction order. This arrangement is used to demonstrate interferometric autocorrelation in krypton with femtosecond VUV pulses at 160 nm wavelength. In addition to the pulse duration, which is already accessible with non-collinear intensity autocorrelation, the full interferometric contrast of the presented approach enables us to extract also information on temporal phases.
Project A3
Investigating charge-up and fragmentation dynamics of oxygen molecules after interaction with strong X-ray free-electron laser pulses
G. Kastirke, F. Ota, D. V. Rezvan, M. S. Schöffler, M. Weller, J. Rist, R. Boll, N.Anders, T. M. Baumann, S. Eckart, B. Erk, A. De Fanis, K. Fehre, A. Gatton, S. Grundmann, P. Grychtol, A. Hartung, M. Hofmann, M. Ilchen, C. Janke, M. Kircher, M. Kunitski, X. Li, T. Mazza, N. Melzer, J. Montano, V. Music, G. Nalin, Y. Ovcharenko, A. Pier, N. Rennhack, D. E. Rivas, R. Dörner, D. Rolles, A. Rudenko, P. Schmidt, J. Siebert, N. Strenger, D. Trabert, I. Vela-Perez, R. Wagner, T. Weber, J. B. Williams, P. Ziolkowski, L. Ph. H. Schmidt, A. Czasch, Y. Tamura, N. Hara, K. Yamazaki, K. Hatada, F. Trinter, M. Meyer, K. Ueda, Ph. V. Demekhin, and T. Jahnke
During the last decade, X-ray free-electron lasers (XFELs) have enabled the study of light–matter interaction under extreme conditions. Atoms which are subject to XFEL radiation are charged by a complex interplay of (several subsequent) photoionization events and electronic decay processes within a few femtoseconds. The interaction with molecules is even more intriguing, since intricate nuclear dynamics occur as the molecules start to dissociate during the charge-up process. Here, we demonstrate that by analyzing photoelectron angular emission distributions and kinetic energy release of charge states of ionic molecular fragments, we can obtain a detailed understanding of the charge-up and fragmentation dynamics. Our novel approach allows for gathering such information without the need of complex ab initio modeling. As an example, we provide a detailed view on the processes happening on a femtosecond time scale in oxygen molecules exposed to intense XFEL pulses.
X-ray diffractive imaging of highly ionized helium nanodroplets
Alexandra J. Feinberg et al.
Finding the lowest energy configuration of N unit charges on a sphere, known as Thomson's problem, is a long-standing query which has only been studied via numerical simulations. We present its physical realization using multiply charged He nanodroplets. The charge positions are determined by x-ray coherent diffractive imaging with Xe as a contrast agent. In neutral droplets, filaments resulting from Xe atoms condensing on quantum vortices are observed. Unique to charged droplets, however, Xe clusters that condense on charges are distributed on the surface in lattice-like structures, introducing He droplets as experimental model systems for the study of Thomson's problem.
Resonance-enhanced x-ray multiple ionization of a polyatomic molecule
X. Li, A. Rudenko, T. Mazza, A. Rörig, N. Anders, Th. M. Baumann, S. Eckart, B. Erk, A. De Fanis, K. Fehre, R. Dörner, L. Foucar, S. Grundmann, P. Grychtol, A. Hartung, M. Hofmann, M. Ilchen, Ch. Janke, G. Kastirke, M. Kircher, K. Kubicek, M. Kunitski, S. Meister, N. Melzer, J. Montano, V. Music, G. Nalin, Y. Ovcharenko, Ch. Passow, A. Pier, N. Rennhack, J. Rist, D. E. Rivas, I. Schlichting, L. Ph. H. Schmidt, Ph. Schmidt, M. S. Schöffler, J. Siebert, N. Strenger, D. Trabert, F. Trinter, I. Vela-Perez, R. Wagner, P. Walter, M. Weller, P. Ziolkowski, A. Czasch, M. Meyer, T. Jahnke, D. Rolles, and R. Boll
Extremely high charge states of atoms and molecules can be created when they are irradiated by intense x-ray pulses. At certain x-ray photon energies, electron ejection from atoms can be drastically enhanced by transient resonances created during the sequential ionization process. Here we report on the observation of such resonance effects in a molecule, CH3I, and show the photon-energy-dependent shift of resonance-induced structures in ion charge state distributions. By comparing the ion charge state distribution of CH3I with that from ionization of atomic xenon, molecule-specific features are observed, which can be attributed to ultrafast intramolecular charge rearrangement. In addition, we experimentally demonstrate that the charge-rearrangement-enhanced x-ray ionization of molecules, previously found with hard x rays, also plays a role in the soft x-ray regime.
X-ray multiphoton-induced Coulomb explosion images complex single molecules
R. Boll, J. M. Schäfer, B. Richard, K. Fehre, G. Kastirke, Z. Jurek, M. S. Schöffler, M. M. Abdullah, N. Anders, T. M. Baumann, S. Eckart, B. Erk, A. De Fanis, R. Dörner, S. Grundmann, P. Grychto, A. Hartung, M. Hofmann, M. Ilchen, L. Inhester, C. Janke, R. Jin, M. Kircher, K. Kubicek, M. Kunitski, X. Li, T. Mazza, S. Meister, N. Melzer, J. Montano, V. Music, G. Nalin, Y. Ovcharenko, C. Passow, A. Pier, N. Rennhack, J. Rist, D. E. Rivas, D. Rolles, I. Schlichting, L. Ph. H. Schmidt, P. Schmidt, J. Siebert, N. Strenger, D. Trabert, F. Trinter, I. Vela-Perez, R. Wagner, P. Walter, M. Weller, P. Ziolkowski, S.-K. Son, A. Rudenko, M. Meyer, R. Santra, and T. Jahnke
Following structural dynamics in real time is a fundamental goal towards a better understanding of chemical reactions. Recording snapshots of individual molecules with ultrashort exposure times is a key ingredient towards this goal, as atoms move on femtosecond (10−15 s) timescales. For condensed-phase samples, ultrafast, atomically resolved structure determination has been demonstrated using X-ray and electron diffraction. Pioneering experiments have also started addressing gaseous samples. However, they face the problem of low target densities, low scattering cross sections and random spatial orientation of the molecules. Therefore, obtaining images of entire, isolated molecules capturing all constituents, including hydrogen atoms, remains challenging. Here we demonstrate that intense femtosecond pulses from an X-ray free-electron laser trigger rapid and complete Coulomb explosions of 2-iodopyridine and 2-iodopyrazine molecules. We obtain intriguingly clear momentum images depicting ten or eleven atoms, including all the hydrogens, and thus overcome a so-far impregnable barrier for complete Coulomb explosion imaging—its limitation on molecules consisting of three to five atoms. In combination with state-of-the-art multi-coincidence techniques and elaborate theoretical modelling, this allows tracing ultrafast hydrogen emission and obtaining information on the result of intramolecular electron rearrangement. Our work represents an important step towards imaging femtosecond chemistry via Coulomb explosion.
Resonance-enhanced multiphoton ionization in the x-ray regime
A. C. LaForge, S.-K. Son, D. Mishra, M. Ilchen, S. Duncanson, E. Eronen, E. Kukk, S. Wirok-Stoletow,, D. Kolbasova, P. Walter, R. Boll, A. De Fanis, M. Meyer, Y. Ovcharenko, D. E. Rivas, P. Schmidt, S. Usenko, R. Santra, and N. Berrah
Here, we report on the nonlinear ionization of argon atoms in the short wavelength regime using ultraintense x rays from the European XFEL. After sequential multiphoton ionization, high charge states are obtained. For photon energies that are insufficient to directly ionize a 1s electron, a different mechanism is required to obtain ionization to Ar17+. We propose this occurs through a two-color process where the second harmonic of the FEL pulse resonantly excites the system via a 1s→2p transition followed by ionization by the fundamental FEL pulse, which is a type of x-ray resonance-enhanced multiphoton ionization (REMPI). This resonant phenomenon occurs not only for Ar16+, but also through lower charge states, where multiple ionization competes with decay lifetimes, making x-ray REMPI distinctive from conventional REMPI. With the aid of state-of-the-art theoretical calculations, we explain the effects of x-ray REMPI on the relevant ion yields and spectral profile.
Ionization – dissociation of methane in ultrashort 400 nm and 800 nm laser fields
L. Varvarezos, J.C. Costello, C. Long, A.J. Achner, R. Wagner, M. Meyer and P. Grychtol
The effect of laser wavelength on the dissociation mechanisms in methane is examined over a broad range of intensities for both 800 nm and 400 nm laser fields. It is found that, at lower laser intensities, the dissociation pathways identified with the aid of theoretical calculations for the methane cation can account for most of the experimental findings, including the differences observed for irradiation by 800 nm and the 400 nm fields. As the laser intensity increases, the significance of the Coulomb explosion mechanism, along with the contribution of the rescattering process and the concomitant dissociation pathways, is highlighted.
Mapping Resonance Structures in Transient Core-Ionized Atoms
T. Mazza, M. Ilchen, M. D. Kiselev, E. V. Gryzlova, T. M. Baumann, R. Boll, A. De Fanis, P. Grychtol, J. Montaño, V. Music, Y. Ovcharenko, N. Rennhack, D. E. Rivas, Ph. Schmidt, R. Wagner, P. Ziolkowski, N. Berrah, B. Erk, P. Johnsson, C. Küstner-Wetekam, L. Marder, M. Martins, C. Ott, S. Pathak, T. Pfeifer, D. Rolles, O. Zatsarinny, A. N. Grum-Grzhimailo, and M. Meyer
The nature of transient electronic states created by photoabsorption critically determines the dynamics of the subsequently evolving system. Here, we investigate K-shell photoionized atomic neon by absorbing a second photon within the Auger-decay lifetime of 2.4 fs using the European XFEL, a unique high-repetition-rate, wavelength-tunable x-ray free-electron laser. By high-resolution electron spectroscopy, we map out the transient Rydberg resonances unraveling the details of the subsequent decay of the hollow atom. So far, ultra-short-lived electronic transients, which are often inaccessible by experiments, were mainly inferred from theory but are now addressed by nonlinear x-ray absorption. The successful characterization of these resonances with femtosecond lifetimes provides the basis for a novel class of site-specific, nonlinear, and time-resolved studies with strong impact for a wide range of topics in physics and chemistry.
Auger electron wave packet interferometry on extreme timescales with coherent soft x-rays
S. Usenko et al.
Wave packet interferometry provides benchmark information on light-induced electronic quantum states by monitoring their relative amplitudes and phases during coherent excitation, propagation, and decay. The relative phase control of soft x-ray pulse replicas on the single-digit attosecond timescale achieved in our experiments makes this method a powerful tool to probe ultrafast quantum phenomena such as the excitation of Auger shake-up states with sub-cycle precision. In this contribution we present first results obtained for different Auger decay channels upon generating L-shell vacancies in argon atoms using Michelson-type all-reflective interferometric autocorrelation at a central free-electron laser photon energy of 274.7 eV.
Inner-shell X-ray absorption spectra of the cationic series NHy+ (y=0-3)
Sadia Bari, Ludger Inhester, Kaja Schubert, Karolin Mertens, Jan O. Schunck, Simon Dörner, Sascha Deinert, Schwob Lucas, Stefan Schippers, Alfred Müller, Stephan Klumpp and Michael Martins
On yields following X-ray absorption of the cationic series NHy+(y= 0–3) were measured to identify the characteristic absorption resonances in the energy range of the atomic nitrogen K-edge. Significant changes in the position of the absorption resonances were observed depending on the number of hydrogen atoms bound to the central nitrogen atom. Configuration interaction (CI) calculations were performed to obtain line assignments in the frame of molecular group theory. To validate the calculations, our assignment for the atomic cation N+, measured as a reference, was compared with published theoretical and experimental data.
Photoionization and photo- fragmentation of singly charged positive and negative Sc3N@C80 endohedral fullerene ions
A. Müller, M. Martins, A. L. D. Kilcoyne, R. A. Phaneuf, J. Hellhund, A. Borovik, Jr., K. Holste, S. Bari, T. Buhr, S. Klumpp, A. Perry-Sassmannshausen, S. Reinwardt, S. Ricz, K. Schubert, and S. Schippers
Photoprocesses of the endohedral fullerene ions Sc3N@C+80 and Sc3N@C80− in the gas phase have been investigated in the photon energy ranges 30–50 eV and 280–420 eV. Single and double ionization as well as single ionization accompanied by the release of a C2 dimer were observed as a function of the photon energy for the positive parent ion and double detachment was measured for the negative parent ion. The emphasis of the experiments was on the specific effects of the encapsulated trimetallic nitride cluster Sc3N on the observed reactions. Clear evidence of photoexcitation near the Sc L edge is obtained with the dominating contributions visible in the one- and two-electron-removal channels. K-vacancy production in the encapsulated central nitrogen atom is seen in the single ionization of Sc3N@C+80 but is much less pronounced in the photoionization-with-fragmentation channel. Comparison of the cross sections near the carbon K edge with the corresponding channels measured previously in the photoionization of Lu3N@C+80 reveal strong similarities. Previously predicted sharp resonance features in the ionization of Sc3N@C+80 ions below the Sc M edge are not confirmed. The experiments are accompanied by quantum-chemistry calculations in the Hartree-Fock approximation and by model calculations employing density functional theory (DFT).
The photonion merged-beams experiment
Stefan Schippers, Ticia Buhr, Alexander Borovik Jr., Kristof Holste, Alexander Perry-Sassmannshausen, Karolin Mertens, Simon Reinwardt, Michael Martins, Stephan Klumpp, Kaja Schubert, Sadia Bari, Randolf Beerwerth, Stephan Fritzsche, Sandor Ricz, Jonas Hellhund, and Alfred Müller
The Photon‐Ion Spectrometer at PETRA III—in short, PIPE—is a permanently installed user facility at the "Variable Polarization XUV Beamline" P04 of the synchrotron light source PETRA III operated by DESY in Hamburg, Germany. The careful design of the PIPE ion‐optics in combination with the record‐high photon flux at P04 has lead to a breakthrough in experimental studies of photon interactions with ionized small quantum systems. This short review provides an overview over the published scientific results from photon‐ion merged‐beams experiments at PIPE that were obtained since the start of P04 operations in 2013. The topics covered comprise photoionization of ions of astrophysical relevance, quantitative studies of multi‐electron processes upon inner‐shell photoexcitation and photoionization of negative and positive atomic ions, precision spectroscopy of photoionization resonances, photoionization and photofragmentation of molecular ions, and of endohedral fullerene ions.
Ultrafast charge redistribution in small iodine containing molecules
M. Hollstein, K. Mertens, S. Klumpp, N. Gerken, S. Palutke, I. Baev, G. Brenner, S. Dziarzhytski, M. Meyer, W. Wurth, D. Pfannkuche1, M. Martins
We present studies on intra-molecular charge redistribution in iodine containing molecules upon iodine-4d photoionization. For this, we employed an XUV-pump-XUV-probe scheme based on time-delayed femtosecond pulses delivered by the free-electron laser at DESY in Hamburg (FLASH). The experimental results show delay dependent and molecule-specific iodine charge state distributions that arise upon multiple iodine-4d photoionization. Using the example of CH3I and CH2I2, we compare the delay-dependent yields of I3+. We model the involved processes using advanced ab initio electronic structure calculations which include electron correlations combined with a classical model of the nuclear motion. The qualitative agreement of our model with the experimental results allows us to relate the observed, strongly molecule-specific efficiencies of the intra-molecular charge rearrangement not only to molecule-specific fragmentation timescales but also to molecule-specific electronic structure and molecular environment.
Photoabsorption of the molecular IH cation at the iodine 3d absorption edge
Stephan Klumpp, Alexander A. Guda, Kaja Schubert, Karolin Mertens, Jonas Hellhund, Alfred Müller, Stefan Schippers, Sadia Bari, and Michael Martins
Yields of atomic iodine Iq+ (q≥2) fragments resulting from photoexcitation and photoionization of the target ions IH+ and I+ have been measured in the photon-energy range 610–680 eV, which comprises the thresholds for iodine 3d ionization. The measured ion-yield spectra show two strong and broad resonance features due to the excitation of the 3d3/2,5/2 electrons into ɛf states rather similar for both parent ions. In the 3d pre-edge range, excitations into (npπ)-like orbitals and into an additional σ∗ orbital are found for IH+, which have been identified by comparison of the atomic I+ and molecular IH+ data and with the help of (time-dependent) density functional theory (DFT) and atomic Hartree-Fock calculations. The (5p π) orbital is almost atomlike, whereas all other resonances of the IH+ primary ion show a more pronounced molecular character, which is deduced from the chemical shifts of the resonances and the theoretical analysis.
Photoionization of metastable heliumlike C4+(1s2s 3S1 ) ions: Precision study of intermediate doubly excited states
A. Müller, E. Lindroth, S. Bari, A. Borovik, Jr., P.-M. Hillenbrand, K. Holste, P. Indelicato, A. L. D. Kilcoyne, S. Klumpp, M. Martins, J. Viefhaus, P. Wilhelm, and S. Schippers
In a joint experimental and theoretical endeavor, photoionization of metastable C4+(1s2s3S1) ions via intermediate levels with hollow, double-K-vacancy configurations 2s2p, 2s3p, 2p3s, 2p3d, 2s4p, 2p4s, and 2p4d has been investigated. High-resolution photon-ion merged-beams measurements were carried out with the resolving power reaching up to 25 800 which is sufficient to separate the leading fine-structure components of the 2s2p3P term. Many-body perturbation theory was employed to determine level-to-level cross sections for K-shell excitation with subsequent autoionization. The resonance energies were calculated with inclusion of electron correlation and radiative contributions. Their uncertainties are estimated to be below ±1 meV. Detailed balance confirms the present photoionization cross-section results by comparison with previous dielectronic-recombination measurements. The high accuracy of the theoretical transition energies together with the present experimental results qualify photoabsorption resonances in heliumlike ions as new, greatly improved energy-reference standards at synchrotron radiation facilities.
Near-K-Edge Double and Triple Detachment of the F - Negative Ion: Observation of Direct Two-Electron Ejection by a Single Photon
A. Müller, A. Borovik, Jr., S. Bari, T. Buhr, K. Holste, M. Martins, A. Perry-Saßmannshausen, R. A. Phaneuf, S. Reinwardt, S. Ricz, K. Schubert, and S. Schippers
Double and triple detachment of the F−(1s22s22p6) negative ion by a single photon have been investigated in the photon energy range 660 to 1000 eV. The experimental data provide unambiguous evidence for the dominant role of direct photodouble detachment with a subsequent single-Auger process in the reaction channel leading to F2+ product ions. Absolute cross sections were determined for the direct removal of a (1s+2p) pair of electrons from F− by the absorption of a single photon.
Soft X-ray Transmission Polarizer Based on Ferromagnetic Thin Films
L. Müller, G. Hartmann, S. Schleitzer, M. H. Berntsen, M. Walther, R. Rysov, W. Roseker, F. Scholz, J. Seltmann, L. Glaser, J. Viefhaus, K. Mertens, K. Bagschik, R. Frömter, A. De Fanis, I. Shevchuk, K. Medjanik, G. Öhrwall, H. P. Oepen, M. Martins, M. Meyer, and G. Grübel
A transmission polarizer for producing elliptically polarized soft X-ray radiation from linearly polarized light is presented. The setup is intended for use at synchrotron and free-electron laser beamlines that do not directly offer circularly polarized light for, e.g., X-ray magnetic circular dichroism (XMCD) measurements or holographic imaging. Here, we investigate the degree of ellipticity upon transmission of linearly polarized radiation through a cobalt thin film. The experiment was performed at a photon energy resonant to the Co L3-edge, i.e., 778 eV, and the polarization of the transmitted radiation was determined using a polarization analyzer that measures the directional dependence of photo electrons emitted from a gas target. Elliptically polarized radiation can be created at any absorption edge showing the XMCD effect by using the respective magnetic element.
Ultrashort free-electron laser X-ray pulses
W. Helml, I. Grguras, P. N. Juranic, S. Düsterer, T. Mazza, A. R. Maier, N. Hartmann, M. Ilchen, G. Hartmann, L. Patthey, C. Callegari, J. T. Costello, M. Meyer, R. N. Coffee, A. L. Cavalieri, R. Kienberger
For the investigation of processes happening on the time scale of the motion of bound electrons, well-controlled X-ray pulses with durations in the few-femtosecond and even sub-femtosecond range are a necessary prerequisite. Novel free-electron lasers sources provide these ultrashort, high-brightness X-ray pulses, but their unique aspects open up concomitant challenges for their characterization on a suitable time scale. In this review paper we describe progress and results of recent work on ultrafast pulse characterization at soft and hard X-ray free-electron lasers. We report on different approaches to laser-assisted time-domain measurements, with specific focus on single-shot characterization of ultrashort X-ray pulses from self-amplified spontaneous emission-based and seeded free-electron lasers. The method relying on the sideband measurement of X-ray electron ionization in the presence of a dressing optical laser field is described first. When the X-ray pulse duration is shorter than half the oscillation period of the streaking field, few-femtosecond characterization becomes feasible via linear streaking spectroscopy. Finally, using terahertz fields alleviates the issue of arrival time jitter between streaking laser and X-ray pulse, but compromises the achievable temporal resolution. Possible solutions to these remaining challenges for single-shot, full time–energy characterization of X-ray free-electron laser pulses are proposed in the outlook at the end of the review.
Imaging the square of the correlated two-electron wave function of a hydrogen molecule
M. Waitz, R.Y. Bello, D. Metz, J. Lower, F. Trinter, C. Schober, M. Keiling, U. Lenz, M. Pitzer, K. Mertens, M. Martins, J. Viefhaus, S. Klumpp, T. Weber, L.Ph.H. Schmidt, J.B. Williams, M.S. Schöffler, V.V. Serov, A.S. Kheifets, L. Argenti, A. Palacios, F. Martı́n, T. Jahnke, and R. Dörner
The toolbox for imaging molecules is well-equipped today. Some techniques visualize the geometrical structure, others the electron density or electron orbitals. Molecules are many-body systems for which the correlation between the constituents is decisive and the spatial and the momentum distribution of one electron depends on those of the other electrons and the nuclei. Such correlations have escaped direct observation by imaging techniques so far. Here, we implement an imaging scheme which visualizes correlations between electrons by coincident detection of the reaction fragments after high energy photofragmentation. With this technique, we examine the H2 two-electron wave function in which electron–electron correlation beyond the mean-field level is prominent. We visualize the dependence of the wave function on the internuclear distance. High energy photoelectrons are shown to be a powerful tool for molecular imaging. Our study paves the way for future time resolved correlation imaging at FELs and laser based X-ray sources.
Multiple Auger cycle photoionisation of manganese atoms by short soft x-ray pulses
S. Klumpp, N. Gerken, K. Mertens, M. Richter, B. Sonntag, A. A. Sorokin, M. Braune, K. Tiedtke, P. Zimmermann and M. Martins
The multiple ionisation of atomic Mn, excited at (photon energy: 52.1 eV) and above (photon energy: 61.1 eV) the discrete giant 3p-3d resonance, was studied using high irradiation free-electron-laser soft x-ray pulses from the BL2 beamline of FLASH, DESY, Hamburg. In particular, the impact of the giant resonance on the ionisation mechanism was investigated. Ion mass-over-charge spectra were obtained by means of ion time-of-flight spectrometry. For the two photon energies, the yield of the different ionic charge states Mnq+ (q = 0–7) was determined as a function of the irradiance of the soft x-ray pulses. The maximum charge state observed was Mn6+ for resonant excitation at 52.1 eV and Mn7+ for non-resonant excitation at 61.1 eV at a maximum irradiation of 3 x 10 13 Wcm -2 .
Two-electron Processes in Multiple Ionization under Strong Soft X-ray Radiation
M. Ilchen, T. Mazza, E. T. Karamatskos, D. Markellos, S. Bakhtiarzadeh, A. J. Rafipoor, T. J. Kelly, N. Walsh, J. T. Costello, P. O’Keeffe, N. Gerken, M. Martins, P. Lambropoulos, M. Meyer
In a combined experimental and theoretical study we have investigated the ionization of atomic argon upon irradiation with intense soft-x-ray pulses of 105 eV photon energy from the free-electron laser FLASH. The measured ion yields show charge states up to Ar7+. The comparison with the theoretical study of the underlying photoionization dynamics highlights the importance of excited states in general and of processes governed by electron correlation in particular, namely, ionization with excitation and shake-off, processes usually inaccessible by measurements of ionic yields only. The Ar7+ yield shows a clear deviation from the predictions of the commonly used model of sequential ionization via single-electron processes and the observed signal can only be explained by taking into account the full multiplet structure of the involved configurations and by inclusion of two-electron processes. The competing process of two-photon ionization from the ground state of Ar6+ is calculated to be orders of magnitude smaller.
Soft X-ray multiphoton excitation of small iodine methane derivatives
K. Mertens, N. Gerken, S. Klumpp, M. Braune and M. Martins
The fragmentation pattern of the iodine-containing molecules CH2I2 and CH3I following a strong multiphoton excitation in the vicinity of the iodine 4d giant resonance regime is studied using soft X-ray free electron laser radiation. A strong difference of the charge distribution and the kinetic energy release (KER) for the two molecules is found. The effects can be attributed to charge rearrangement processes induced by the photoexcitation. The difference in the observed distribution for higher charge states of iodine and carbon fragments is consistent with an over-the-barrier model for the charge rearrangement in the dissociating molecules. The KER for singly ionised carbon fragments indicates an ultrafast charge rearrangement before the dissociation starts.
Sensitivity of nonlinear photoionization to resonance substructure in collective excitation
T. Mazza, A. Karamatskou, M. Ilchen, S. Bakhtiarzadeh, A. J. Rafipoor, P. O’Keeffe, T. J. Kelly, N. Walsh, J. T. Costello, M. Meyer, R. Santra
Collective behaviour is a characteristic feature in many-body systems, important for developments in fields such as magnetism, superconductivity, photonics and electronics. Recently, there has been increasing interest in the optically nonlinear response of collective excitations. Here we demonstrate how the nonlinear interaction of a many-body system with intense XUV radiation can be used as an effective probe for characterizing otherwise unresolved features of its collective response. Resonant photoionization of atomic xenon was chosen as a case study. The excellent agreement between experiment and theory strongly supports the prediction that two distinct poles underlie the giant dipole resonance. Our results pave the way towards a deeper understanding of collective behaviour in atoms, molecules and solid-state systems using nonlinear spectroscopic techniques enabled by modern short-wavelength light sources.
Observation of a Four-Electron Auger Process in Near-K-Edge Photoionization of Singly Charged Carbon Ions
A. Müller, A. Borovik, Jr., T. Buhr, J. Hellhund, K. Holste, A. L. D. Kilcoyne, S. Klumpp, M. Martins, S. Ricz, J. Viefhaus, S. Schippers
High resolution multi-photon spectroscopy by a tunable free-electron-laser light
M. Zitnik, A. Mihelic, K. Bucar, M. Kavcic, J.-E. Rubensson, M. Svanquist, J. Söderström, R. Feifel, C. Sathe, Y. Ovcharenko, V. Lyamayev, T. Mazza, M. Meyer, M. Simon, L. Journel, J. Lüning, O. Plekan, M. Coreno, M. Devetta, M. Di Fraia, et. al
Seeded free electron lasers theoretically have the intensity, tunability, and resolution required for multiphoton spectroscopy of atomic and molecular species. Using the seeded free electron laser FERMI and a novel detection scheme, we have revealed the two-photon excitation spectra of dipole-forbidden doubly excited states in helium. The spectral profiles of the lowest (−1,0)+1 Se1 and (0,1)0 De1 resonances display energy shifts in the meV range that depend on the pulse intensity. The results are explained by an effective two-level model based on calculated Rabi frequencies and decay rates.
Time-Dependent Multiphoton Ionization of Xenon in the Soft-X-Ray Regime
N. Gerken, S. Klumpp, A. A. Sorokin, K. Tiedtke, M. Richter, V. Bürk, K. Mertens, P. Jurani, M. Martins
The time-dependent multiphoton ionization of xenon atoms is studied with femtosecond pulses in the excitation range of the 4d giant resonance at the photon energy of 93 eV. Benefiting from a new operation mode of the free electron laser FLASH, the measurements are performed with varying pulse durations. A strong dependence of the ion charge distribution on the pulse duration allows the different multiphoton mechanisms behind the multiple photoionization of xenon to be disentangled up to a charge state of Xe10+. The results up to Xe8+ are well explained by sequences of single photon, multiphoton, and Auger processes, but higher charge state generation suggests the need for collective electron multiphoton excitations.
Project A4
A sensitive high repetition rate arrival time monitor for X-ray Free Electron Lasers
Michael Diez, Henning Kirchberg, Andreas Galler, Sebastian Schulz, Mykola Biednov, Christina Bömer, Tae-Kyu Choi, Angel Rodriguez-Fernandez, Wojciech Gawelda, Dmitry Khakhulin, Katharina Kubicek, Frederico Lima, Florian Otte, Peter Zalden, Ryan Coffee, Michael Thorwart, and Christian Bressler
X-ray free-electron laser sources enable time-resolved X-ray studies with unmatched temporal resolution. To fully exploit ultrashort X-ray pulses, timing tools are essential. However, new high repetition rate X-ray facilities present challenges for currently used timing tool schemes. Here we address this issue by demonstrating a sensitive timing tool scheme to enhance experimental time resolution in pump-probe experiments at very high pulse repetition rates. Our method employs a self-referenced detection scheme using a time-sheared chirped optical pulse traversing an X-ray stimulated diamond plate. By formulating an effective medium theory, we confirm subtle refractive index changes, induced by sub-milli-Joule intense X-ray pulses, that are measured in our experiment. The system utilizes a Common-Path-Interferometer to detect X-ray-induced phase shifts of the optical probe pulse transmitted through the diamond sample. Owing to the thermal stability of diamond, our approach is well-suited for MHz pulse repetition rates in superconducting linear accelerator-based free-electron lasers.
Tracking structural solvent reorganization and recombination dynamics following e − photoabstraction from aqueous I− with femtosecond X-ray spectroscopy and scattering
P. Vester, K. Kubicek, T. Assefa, E. Biasin, M. Christensen, A. O. Dohn, T. B. van Driehl, A. Galler, W. Gawelda, T. C. B. Harlang, N. E. Henriksen, K. S. Kjaer, T. S. Kuhlmann, Z. Németh, Z. Nurekeyev, M. Papai, G. Vankó, H. Yavas, D. B. Zederkof, U. Bergmann, M. M. Nielsen, K. B. Moller, K. Haldrup, and C. Bressler
We present a sub-picosecond resolved investigation of the structural solvent reorganization and geminate recombination dynamics following 400 nm two-photon excitation and photodetachment of a valence p electron from the aqueous atomic solute, I−(aq). The measurements utilized time-resolved X-ray Absorption Near Edge Structure (TR-XANES) spectroscopy and X-ray Solution Scattering (TR-XSS) at the Linac Coherent Light Source x-ray free electron laser in a laser pump/x-ray probe experiment. The XANES measurements around the L1-edge of the generated nascent iodine atoms (I0) yield an average electron ejection distance from the iodine parent of 7.4 ± 1.5 Å with an excitation yield of about 1/3 of the 0.1M NaI aqueous solution. The kinetic traces of the XANES measurement are in agreement with a purely diffusion-driven geminate iodine–electron recombination model without the need for a long-lived (I0:e−) contact pair. Nonequilibrium classical molecular dynamics simulations indicate a delayed response of the caging H2O solvent shell and this is supported by the structural analysis of the XSS data: We identify a two-step process exhibiting a 0.1 ps delayed solvent shell reorganization time within the tight H-bond network and a 0.3 ps time constant for the mean iodine–oxygen distance changes. The results indicate that most of the reorganization can be explained classically by a transition from a hydrophilic cavity with a well-ordered first solvation shell (hydrogens pointing toward I−) to an expanded cavity around I0 with a more random orientation of the H2O molecules in a broadened first solvation shell.
R-Group Stabilization in Methylated Formamides observed by Resonant Inelastic X-ray Scattering
Miguel Ochmann, Vinícius Vaz da Cruz, Sebastian Eckert, Nils Huse and Alexander Föhlisch
The inherent stability of methylated formamides is traced to a stabilization of the deep-lying σ-framework by resonant inelastic X ray scattering at the nitrogen K-edge. Charge transfer from the amide nitrogen to the methyl groups underlie this stabilization mechanism that leaves the aldehyde group essentially unaltered and explains the stability of secondary and tertiary amides.
Intraband dynamics of mid-infrared HgTe quantum dots
Matthias Ruppert, Hanh Bui, Laxmi Kishore Sagar, Pieter Geiregat, Zeger Hens, Gabriel Bester and Nils Huse
Femtosecond pump–probe spectroscopy reveals ultrafast carrier dynamics in mid-infrared (MIR) colloidal HgTe nanoparticles with a bandgap of 2.5 μm. We observe intraband relaxation processes after photoexcitation ranging from resonant excitation up to the multi-exciton generation (MEG) regime by identifying initially excited states from atomic effective pseudopotential calculations. Our study elucidates the earliest dynamics below 10 ps in this technologically relevant material. With increasing photon energy, we find carrier relaxation times as long as 2.1 ps in the MEG regime close to the ionization threshold of the particles. For all photon energies, we extract a constant mean carrier energy dissipation rate of 0.36 eV ps−1 from which we infer negligible impact of the density of states on carrier cooling.
Site-Selective Real-Time Observation of Bimolecular Electron Transfer in a Photocatalytic System Using L-Edge X-Ray Absorption Spectroscopy
A. Britz, S. I. Bokarev, T. A. Assefa, E. G. Bajnóczi, Z. Németh, G. Vankó, N. Rockstroh, H. Junge, M. Beller, G. Doumy, A. M. March, S. H. Southworth, S. Lochbrunner, C. Bressler, W. Gawelda
Time-resolved X-ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X-ray transitions with insights from high-level ab initio calculations. The specific target has been a heteroleptic [IrIII(ppy)2(bpy)]+ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe3 (CO)12 as a water reduction catalyst. The relevant molecular transitions have been characterized via high-resolution Ir L-edge X-ray absorption spectroscopy on the picosecond time scale and restricted active space self-consistent field calculations. The presented methods and results will enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance.
Breaking the Symmetry of Pyrimidine: Solvent Effects and Core-Excited State Dynamics
Sebastian Eckert, Vinícius Vaz da Cruz, Miguel Ochmann, Inga von Ahnen, Alexander Föhlisch, and Nils Huse
Symmetry and its breaking crucially define the chemical properties of molecules and their functionality. Resonant inelastic X-ray scattering is a local electronic structure probe reporting on molecular symmetry and its dynamical breaking within the femtosecond scattering duration. Here, we study pyrimidine, a system from the C2v point group, in an aqueous solution environment, using scattering though its 2a2 resonance. Despite the absence of clean parity selection rules for decay transitions from in-plane orbitals, scattering channels including decay from the 7b2 and 11a1 orbitals with nitrogen lone pair character are a direct probe for molecular symmetry. Computed spectra of explicitly solvated molecules sampled from a molecular dynamics simulation are combined with the results of a quantum dynamical description of the X-ray scattering process. We observe dominant signatures of core-excited Jahn–Teller induced symmetry breaking for resonant excitation. Solvent contributions are separable by shortening of the effective scattering duration through excitation energy detuning.
Following Metal-to-Ligand Charge-Transfer Dynamics with Ligand and Spin Specificity Using Femtosecond Resonant Inelastic X-ray Scattering at the Nitrogen K-Edge
Raphael M. Jay, Sebastian Eckert, Benjamin E. Van Kuiken, Miguel Ochmann, Markus Hantschmann, Amy A. Cordones, Hana Cho, Kiryong Hong, Rory Ma, Jae Hyuk Lee, Georgi L. Dakovski, Joshua J. Turner, Michael P. Minitti, Wilson Quevedo, Annette Pietzsch, Martin Beye, Tae Kyu Kim, Robert W. Schoenlein, Philippe Wernet, Alexander Föhlisch, and Nils Huse
We demonstrate for the case of photoexcited [Ru(2,2′-bipyridine)3]2+ how femtosecond resonant inelastic X-ray scattering (RIXS) at the ligand K-edge allows one to uniquely probe changes in the valence electronic structure following a metal-to-ligand charge-transfer (MLCT) excitation. Metal–ligand hybridization is probed by nitrogen-1s resonances providing information on both the electron-accepting ligand in the MLCT state and the hole density of the metal center. By comparing to spectrum calculations based on density functional theory, we are able to distinguish the electronic structure of the electron-accepting ligand and the other ligands and determine a temporal upper limit of (250 ± 40) fs for electron localization following the charge-transfer excitation. The spin of the localized electron is deduced from the selection rules of the RIXS process establishing new experimental capabilities for probing transient charge and spin densities.
Femtosecond Charge Density Modulations in Photoexcited CuWO4
Yohei Uemura, Ahmed S. M. Ismail, Sang Han Park, Soonnam Kwon, Minseok Kim, Yasuhiro Niwa, Hiroki Wadati, Hebatalla Elnaggar, Federica Frati, Ties Haarman, Niko Höppel, Nils Huse, Yasuyuki Hirata, Yujun Zhang, Kohei Yamagami, Susumu Yamamoto, Iwao Matsuda, Tetsuo Katayama, Tadashi Togashi, Shigeki Owada, Makina Yabashi, Uufuk Halisdemir, Gertjan Koster, Toshihiko Yokoyama, Bert M. Weckhuysen, and Frank M. F. de Groot
Copper tungstate (CuWO4) is an important semiconductor with a sophisticated and debatable electronic structure that has a direct impact on its chemistry. Using the PAL-XFEL source, we study the electronic dynamics of photoexcited CuWO4. The Cu L3 X-ray absorption spectrum shifts to lower energy upon photoexcitation, which implies that the photoexcitation process from the oxygen valence band to the tungsten conduction band effectively increases the charge density on the Cu atoms. The decay time of this spectral change is 400 fs indicating that the increased charge density exists only for a very short time and relaxes electronically. The initial increased charge density gives rise to a structural change on a time scale longer than 200 ps.
A self-referenced in-situ arrival time monitor for X-ray free-electron lasers
R. N. Coffee, N. Hartmann, R. Heider, M. S. Wagner, W. Helml, T. Katayama, T. Sato, T. Sato, M. Yabashi, C. Bressler
We present a novel, highly versatile, and self-referenced arrival time monitor for measuring the femtosecond time delay between a hard X-ray pulse from a free-electron laser and an optical laser pulse, measured directly on the same sample used for pump-probe experiments. Two chirped and picosecond long optical supercontinuum pulses traverse the sample with a mutually fixed time delay of 970 fs, while a femtosecond X-ray pulse arrives at an instant in between both pulses. Behind the sample the supercontinuum pulses are temporally overlapped to yield near-perfect destructive interference in the absence of the X-ray pulse. Stimulation of the sample with an X-ray pulse delivers non-zero contributions at certain optical wavelengths, which serve as a measure of the relative arrival time of the X-ray pulse with an accuracy of better than 25 fs. We find an excellent agreement of our monitor with the existing timing diagnostics at the SACLA XFEL with a Pearson correlation value of 0.98. We demonstrate a high sensitivity to measure X-ray pulses with pulse energies as low as 30 μJ. Using a free-flowing liquid jet as interaction sample ensures the full replacement of the sample volume for each X-ray/optical event, thus enabling its utility even at MHz repetition rate XFEL sources.
Shot noise limited soft x-ray absorption spectroscopy in solution at a SASE-FEL using a transmission grating beam splitter
Robin Y. Engel, Maria Ekimova, Piter S. Miedema, Carlo Kleine, Jan Ludwig, Miguel Ochmann, BenjaminGrimm-Lebsanft, Rory Ma, Melissa Teubner, Siarhei Dziarzhytski, Günter Brenner, Marie Kristin Czwalinna,Benedikt Rösner, Tae Kyu Kim, Christian David, Sonja Herres-Pawlis, Michael Rübhausen, Erik T. J. Nibbering, Nils Huse, and Martin Beye
X-ray absorption near-edge structure (XANES) spectroscopy provides element specificity and is a powerful experimental method to probelocal unoccupied electronic structures. In the soft x-ray regime, it is especially well suited for the study of 3d-metals and light elements suchas nitrogen. Recent developments in vacuum-compatible liquid flat jets have facilitated soft x-ray transmission spectroscopy on molecules insolution, providing information on valence charge distributions of heteroatoms and metal centers. Here, we demonstrate XANES spectros-copy of molecules in solution at the nitrogen K-edge, performed at FLASH, the Free-Electron Laser (FEL) in Hamburg. A split-beamreferencing scheme optimally characterizes the strong shot-to-shot fluctuations intrinsic to the process of self-amplified spontaneousemission on which most FELs are based. Due to this normalization, a sensitivity of 1% relative transmission change is achieved, limited byfundamental photon shot noise. The effective FEL bandwidth is increased by streaking the electron energy over the FEL pulse train tomeasure a wider spectral window without changing FEL parameters. We propose modifications to the experimental setup with the potentialof improving the instrument sensitivity by two orders of magnitude, thereby exploiting the high peak fluence of FELs to enableunprecedented sensitivity for femtosecond XANES spectroscopy on liquids in the soft x-ray spectral region.
Exploring the light-induced dynamics in solvated metallogrid complexes with femtosecond pulses across the electromagnetic spectrum
M. Naumova, A.Kalinko, J. W. L. Wong, S. Alvarez Gutierez, J. Meng, M. Liang, M. Abdellah, H. Geng, W.Lin, K. Kubicek, M. Biednov, F. Lima, A. Galler, P. Zalden, S. Checchia, P. Mante, J. Zimara, D. Schwarzer, S. Demeshko, V. Y. Murzin, D. J Gosztola, M. Jarenmark, J. Zhang, M. Bauer, L. Max Lawson Daku, D. Khakhulin, W. Gawelda, C. Bressler, F. Meyer, K. Zheng, S. E. Canton
Oligonuclear complexes of d4–d7 transition metal ion centers that undergo spin-switching have long been developed for their practical role in molecular electronics. Recently, they also have appeared as promising photochemical reactants demonstrating improved stability. However, the lack of knowledge about their photophysical properties in the solution phase compared to mononuclear complexes is currently hampering their inclusion into advanced light-driven reactions. In the present study, the ultrafast photoinduced dynamics in a solvated [2 × 2] iron(II) metallogrid complex are characterized by combining measurements with transient optical-infrared absorption and x-ray emission spectroscopy on the femtosecond time scale. The analysis is supported by density functional theory calculations. The photocycle can be described in terms of intra-site transitions, where the FeII centers in the low-spin state are independently photoexcited. The Franck–Condon state decays via the formation of a vibrationally hot high-spin (HS) state that displays coherent behavior within a few picoseconds and thermalizes within tens of picoseconds to yield a metastable HS state living for several hundreds of nanoseconds. Systematic comparison with the closely related mononuclear complex [Fe(terpy)2]2+ reveals that nuclearity has a profound impact on the photoinduced dynamics. More generally, this work provides guidelines for expanding the integration of oligonuclear complexes into new photoconversion schemes that may be triggered by ultrafast spin-switching.
Time-Resolved Probing of the Nonequilibrium Structural Solvation Dynamics by the Time-Dependent Stokes Shift
Henning Kirchberg and Michael Thorwart
The time-dependent fluorescence Stokes shift monitors the relaxation of the polarization of a polar solvent in the surroundings of a photoexcited solute molecule but also the structural variation of the solute following photoexcitation and the subsequent molecular charge redistribution. Here, we formulate a simple nonequilibrium quantum theory of solvation for an explicitly time-dependent continuous solvent. The time-dependent solvent induces nonequilibrium fluctuations on the solvent dynamics which are directly reflected in different time components in the time-dependent Stokes shift. We illustrate the structural dynamics in the presence of an explicitly time-dependent solvent by the example of a dynamically shrinking solute which leads to a bimodal Stokes shift. Interestingly, both contributions are mutually coupled. Furthermore, we can explain a prominent long-tail decay of the Stokes shift associated with slow structural dynamical variations.
Revealing Hot and Long-Lived Metastable Spin-States in the Photoinduced Switching of Solvated Metallogrid Complexes with Femtosecond Optical and X-ray Spectroscopies
M. Naumova, A.Kalinko, J. W. L. Wong, M. Abdellah, H. Geng, E. Domenichini, J. Meng, S. Alvarez Gutierez, P.-A. Mante, W.Lin, P. Zalden, A. Galler, F. Alves Lima, K. Kubicek, M. Biednov, A. Britz, S. Checchia, V. Kabanova, M. Wulff, J. Zimara, D. Schwarzer, S. Demeshko, V. Y. Murzin, D. J Gosztola, M. Jarenmark, J. Zhang, M. Bauer, L. Max Lawson Daku, W. Gawelda, D. Khakhulin, C. Bressler, F. Meyer, K. Zheng, S. E. Canton
An atomistic understanding of the photoinduced spin-state switching (PSS) within polynuclear systems of d4–d7 transition metal ion complexes is required for their rational integration into light-driven reactions of chemical and biological interest. However, in contrast to mononuclear systems, the multidimensional dynamics of the PSS in solvated molecular arrays have not yet been elucidated due to the expected complications associated with the connectivity between the metal centers and the strong interactions with the surroundings. In this work, the PSS in a solvated triiron(II) metallogrid complex is characterized using transient optical absorption and X-ray emission spectroscopies on the femtosecond time scale. The complementary measurements reveal the photoinduced creation of energy-rich (hot) and long-lived quintet states, whose dynamics differ critically from their mononuclear congeners. This finding opens major prospects for developing novel schemes in solution-phase spin chemistry that are driven by the dynamic PSS process in compact oligometallic arrays.
Ultrafast X-ray Photochemistry at European XFEL: Capabilities of the Femtosecond X-ray Experiments (FXE) Instrument
D. Khakhulin, F. Otte, M. Biednov, C. Bömer, T.-K. Choi, M. Diez, A. Galler, Y. Jiang, K. Kubicek, F. A. Lima, A. Rodriguez-Fernandez, P. Zalden, W. Gawelda, C. Bressler
Time-resolved X-ray methods are widely used for monitoring transient intermediates over the course of photochemical reactions. Ultrafast X-ray absorption and emission spectroscopies as well as elastic X-ray scattering deliver detailed electronic and structural information on chemical dynamics in the solution phase. In this work, we describe the opportunities at the Femtosecond X-ray Experiments (FXE) instrument of European XFEL. Guided by the idea of combining spectroscopic and scattering techniques in one experiment, the FXE instrument has completed the initial commissioning phase for most of its components and performed first successful experiments within the baseline capabilities. This is demonstrated by its currently 115 fs (FWHM) temporal resolution to acquire ultrafast X-ray emission spectra by simultaneously recording iron Kα and Kβ lines, next to wide angle X-ray scattering patterns on a photoexcited aqueous solution of [Fe(bpy)3]2+, a transition metal model compound.
Spectroscopic Signatures of the Dynamical Hydrophobic Solvation Shell Formation
Henning Kirchberg, Peter Nalbach, Christian Bressler, and Michael Thorwart
When a hydrophilic solute in water is suddenly turned into a hydrophobic species, for instance, by photoionization, a layer of hydrated water molecules forms around the solute on a time scale of a few picoseconds. We study the dynamic buildup of the hydration shell around a hydrophobic solute on the basis of a time-dependent dielectric continuum model. Information about the solvent is spectroscopically extracted from the relaxation dynamics of a test dipole inside a static Onsager sphere in the nonequilibrium solvent. The growth process is described phenomenologically within two approaches. First, we consider a time-dependent thickness of the hydration layer that grows from zero to a finite value over a finite time. Second, we assume a time-dependent complex permittivity within a finite layer region around the Onsager sphere. The layer is modeled as a continuous dielectric with a much slower fluctuation dynamics. We find a time-dependent frequency shift down to the blue of the resonant absorption of the dipole, together with a dynamically decreasing line width, as compared to bulk water. The blue shift reflects the work performed against the hydrogen-bonded network of the bulk solvent and is a directly measurable quantity. Our results are in agreement with an experiment on the hydrophobic solvation of iodine in water.
J. Phys. Chem. B 123, 2106 (2019)
Using Ultrafast X-ray Spectroscopy To Address Questions in Ligand-Field Theory: The Excited State Spin and Structure of [Fe(dcpp)2]2+
Alexander Britz, Wojciech Gawelda, Tadesse A. Assefa, Lindsey L. Jamula, Jonathan T. Yarranton, Andreas Galler, Dmitry Khakhulin, Michael Diez, Manuel Harder, Gilles Doumy, Anne Marie March, Éva Bajnóczi, Zoltán Németh, Mátyás Pápai, Emese Rozsályi, Dorottya Sárosiné Szemes, Hana Cho, Sriparna Mukherjee, Chang Liu, Tae Kyu Kim, Robert W. Schoenlein, Stephen H. Southworth, Linda Young, Elena Jakubikova, Nils Huse, György Vankó, Christian Bressler, and James K. McCusker
We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)2]2+ (where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the 5T2/3T1 crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the 5T2 state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a 5T2 state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited 5T2 state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin 1A1 ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules—in particular, transition metal complexes—that are difficult if not impossible to obtain by other means.
Transient Metal-Centered States Mediate Isomerization of a Photochromic Ru-Sulfoxide Complex
A. Cordones, J. Hyuk Lee, K. Hong, H. Cho, K. Garg, M. Boggio-Pasqua, J. Rack, N. Huse, R. W. Schoenlein, T. Kyu Kim
Ultrafast isomerization reactions underpin many processes in (bio)chemical systems and molecular materials. Understanding the coupled evolution of atomic and molecular structure during isomerization is paramount for control and rational design in molecular science. Here we report transient X-ray absorption studies of the photo-induced linkage isomerization of a Ru-based photochromic molecule. X-ray spectra reveal the spin and valence charge of the Ru atom and provide experimental evidence that metal-centered excited states mediate isomerization. Complementary X-ray spectra of the functional ligand S atoms probe the nuclear structural rearrangements, highlighting the formation of two metal-centered states with different metal-ligand bonding. These results address an essential open question regarding the relative roles of transient charge-transfer and metal-centered states in mediating photoisomerization. Global temporal and spectral data analysis combined with time-dependent density functional theory reveals a complex mechanism for photoisomerization with atomic details of the transient molecular and electronic structure not accessible by other means.
Nonequilibrium quantum solvation with a time-dependent Onsager cavity
Henning Kirchberg, Peter Nalbach, and Michael Thorwart
We formulate a theory of nonequilibrium quantum solvation in which parameters of the solvent are explicitly depending on time. We assume in a simplest approach a spherical molecular Onsager cavity with a time-dependent radius. We analyze the relaxation properties of a test molecular point dipole in a dielectric solvent and consider two cases: (i) a shrinking Onsager sphere and (ii) a breathing Onsager sphere. Due to the time-dependent solvent, the frequency-dependent response function of the dipole becomes time-dependent. For a shrinking Onsager sphere, the dipole relaxation is in general enhanced. This is reflected in a temporally increasing linewidth of the absorptive part of the response. Furthermore, the effective frequency-dependent response function shows two peaks in the absorptive part which are symmetrically shifted around the eigenfrequency. By contrast, a breathing sphere reduces damping as compared to the static sphere. Interestingly, we find a non-monotonous dependence of the relaxation rate on the breathing rate and a resonant suppression of damping when both rates are comparable. Moreover, the linewidth of the absorptive part of the response function is strongly reduced for times when the breathing sphere reaches its maximal extension.
J. Chem. Phys. 148, 164301 (2018)
UV-Photochemistry of the Disulfide Bond: Evolution of Early Photoproducts from Picosecond X-ray Absorption Spectroscopy at the Sulfur K-Edge
M. Ochmann, A. Hussain, I. von Ahnen, A. A. Cordones, K. Hong, J. H. Lee, R. Ma, K. Adamczyk, O. Vendrell, T. K. Kim, R. W. Schoenlein, and N. Huse
We have investigated dimethyl disulfide as the basic moiety for understanding the photochemistry of disulfide bonds, which are central to a broad range of biochemical processes. Picosecond time-resolved X-ray absorption spectroscopy at the sulfur K-edge provides unique element-specific insight into the photochemistry of the disulfide bond initiated by 267 nm femtosecond pulses. We observe a broad but distinct transient induced absorption spectrum which recovers on at least two time scales in the nanosecond range. We employed RASSCF electronic structure calculations to simulate the sulfur-1s transitions of multiple possible chemical species, and identified the methylthiyl and methylperthiyl radicals as the primary reaction products. In addition, we identify disulfur and the CH2S thione as the secondary reaction products of the perthiyl radical that are most likely to explain the observed spectral and kinetic signatures of our experiment. Our study underscores the importance of elemental specificity and the potential of time-resolved X-ray spectroscopy to identify short-lived reaction products in complex reaction schemes that underlie the rich photochemistry of disulfide systems.
Transferring the entatic-state principle to copper photochemistry
B. Maerz, D. Göries, M. Naumova, M. Biednov, G. Neuber, A. Wetzel, S. M. Hofmann, P. Roedig, A. Meents, J. Bielecki, J. Andreasson, K. R. Beyerlein, H. N. Chapman, C. Bressler, W. Zinth, M. Rübhausen, and S. Herres-Pawlis
The entatic state denotes a distorted coordination geometry of a complex from its typical arrangement that generates an improvement to its function. The entatic-state principle has been observed to apply to copper electron-transfer proteins and it results in a lowering of the reorganization energy of the electron-transfer process. It is thus crucial for a multitude of biochemical processes, but its importance to photoactive complexes is unexplored. Here we study a copper complex—with a specifically designed constraining ligand geometry—that exhibits metal-to-ligand charge-transfer state lifetimes that are very short. The guanidine–quinoline ligand used here acts on the bis(chelated) copper(I) centre, allowing only small structural changes after photoexcitation that result in very fast structural dynamics. The data were collected using a multimethod approach that featured time-resolved ultraviolet–visible, infrared and X-ray absorption and optical emission spectroscopy. Through supporting density functional calculations, we deliver a detailed picture of the structural dynamics in the picosecond-to-nanosecond time range.
Probing Transient Valence Orbital Changes with Picosecond Valence-to-Core X-ray Emission Spectroscopy
A. March, T. Assefa, C. Bömer, C. Bressler, A. Britz, M. Diez, G. Doumy, A. Galler, M. Harder, D. Khakhulin, Z. Németh, M. Pápai, S. Schulz, S. H. Southworth, H. Yavas, L. Young, W. Gawelda, and G. Vankó
We probe the dynamics of valence electrons in photoexcited [Fe(terpy)2]2+ in solution to gain deeper insight into the Fe–ligand bond changes. We use hard X-ray emission spectroscopy (XES), which combines element specificity and high penetration with sensitivity to orbital structure, making it a powerful technique for molecular studies in a wide variety of environments. A picosecond-time-resolved measurement of the complete 1s X-ray emission spectrum captures the transient photoinduced changes and includes the weak valence-to-core (vtc) emission lines that correspond to transitions from occupied valence orbitals to the nascent core-hole. Vtc-XES offers particular insight into the molecular orbitals directly involved in the light-driven dynamics; a change in the metal–ligand orbital overlap results in an intensity reduction and a blue energy shift in agreement with our theoretical calculations and more subtle features at the highest energies reflect changes in the frontier orbital populations.
Parametric Down-Conversion of X Rays into the Optical Regime
A. Schori, C. Bömer, D. Borodin, S. P. Collins, B. Detlefs, M. Moretti Sala, S. Yudovich, and S. Shwartz
We report the observation of parametrically down-converted x-ray signal photons at photon energies that correspond to idler photons at optical wavelengths. The count-rate dependence on the angles of the input beam and of the detector and on the slit sizes agrees with theory within the experimental uncertainties. The nonlinear susceptibility, which we calculated from the measured efficiencies, is comparable to the nonlinear susceptibility evaluated from the measurements of x-ray and optical wave mixing. The results of the present Letter advance the development of a spectroscopy method for probing valence-electron charges and the microscopic optical response of crystals with atomic-scale resolution.
Light-Induced Radical Formation and Isomerization of an Aromatic Thiol in Solution Followed by Time-Resolved X-ray Absorption Spectroscopy at the Sulfur K-Edge
M. Ochmann, I. von Ahnen, A. A. Cordones, A. Hussain, J. H. Lee, K. Hong, K. Adamczyk, O. Vendrell , T. K. Kim , R. W. Schoenlein, N. Huse
We applied time-resolved sulfur-1s absorption spectroscopy to a model aromatic thiol system as a promising method for tracking chemical reactions in solution. Sulfur-1s absorption spectroscopy allows tracking multiple sulfur species with a time resolution of ∼70 ps at synchrotron radiation facilities. Experimental transient spectra combined with high-level electronic structure theory allow identification of a radical and two thione isomers, which are generated upon illumination with 267 nm radiation. Moreover, the regioselectivity of the thione isomerization is explained by the resulting radical frontier orbitals. This work demonstrates the usefulness and potential of time-resolved sulfur-1s absorption spectroscopy for tracking multiple chemical reaction pathways and transient products of sulfur-containing molecules in solution.
Time-resolved pump and probe x-ray absorption fine structure spectroscopy at beamline P11 at PETRA III
D. Göries, B. Dicke, P. Roedig, N. Stübe, J. Meyer, A. Galler, W. Gawelda, A. Britz, P. Geßler, H. Sotoudi Namin, A. Beckmann, M. Schlie, M. Warmer, M. Naumova, C. Bressler, M. Rübhausen, E. Weckert, and A. Meents
We report about the development and implementation of a new setup for time-resolved X-ray absorption fine structure spectroscopy at beamline P11 utilizing the outstanding source properties of the low-emittance PETRA III synchrotron storage ring in Hamburg. Using a high intensity micrometer-sized X-ray beam in combination with two positional feedback systems, measurements were performed on the transition metal complex fac-Tris[2-phenylpyridinato-C2,N]iridium(III) also referred to as fac-Ir(ppy)3. This compound is a representative of the phosphorescent iridium(III) complexes, which play an important role in organic light emitting diode (OLED) technology. The experiment could directly prove the anticipated photoinduced charge transfer reaction. Our results further reveal that the temporal resolution of the experiment is limited by the PETRA III X-ray bunch length of ∼103 ps full width at half maximum (FWHM).
Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated [Co(terpy)2]2+
E. Basin, T. B. van Driel, K. S. Kjaer, A. O. Dohn, M. Christensen, T. Harlang, P. Chabera, Y. Liu, J. Uhlig, M. Pápai, Z. Németh, R. Hartsock, W. Liang, J. Zhang, R. Alonso-Mori, M. Chollet, J. M. Glawnia, S. Nelsen, D. Sokara, T. A. Assefa, A. Britz, A. Galler, W. Gawelda, C. Bressler, K. J. Gaffney, H. T. Lemke, K. B. Moller, M. M. Nielsen, V. Sundström, G. Vankó, K. Wärmark, S. E. Canton, and K. Haldrup
We study the structural dynamics of photoexcited [Co(terpy)2]2+ in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ∼7 ps.
A Multi-MHz Single Shot Data Acquisition Scheme with High Dynamic Range: Pump-Probe X-Ray Experiments at Synchrotrons
A. Britz, T. Assefa, A. Galler, W. Gawelda, M. Diez, P. Zalden, D. Khakhulin, B. Fernandes, P. Gessler, H. Sotuodi, A. Beckmann, M. Harder, H. Yavas, and C. Bressler
The technical implementation of a multi-MHz data acquisition scheme for laser-X-ray pump-probe experiments with pulse limited temporal resolution (100 ps) is presented. Such techniques are very attractive to benefit from the high-repetition rates of X-ray pulses delivered from advanced synchrotron radiation sources. Exploiting a synchronized 3.9 MHz laser excitation source, experiments in 60-bunch mode (7.8 MHz) at beamline P01 of the PETRA III storage ring are performed. Hereby molecular systems in liquid solutions are excited by the pulsed laser source and the total X-ray fluorescence yield (TFY) from the sample is recorded using silicon avalanche photodiode detectors (APDs). The subsequent digitizer card samples the APD signal traces in 0.5 ns steps with 12-bit resolution. These traces are then processed to deliver an integrated value for each recorded single X-ray pulse intensity and sorted into bins according to whether the laser excited the sample or not. For each subgroup the recorded single-shot values are averaged over ~107 pulses to deliver a mean TFY value with its standard error for each data point, e.g. at a given X-ray probe energy. The sensitivity reaches down to the shot-noise limit, and signal-to-noise ratios approaching 1000 are achievable in only a few seconds collection time per data point. The dynamic range covers 100 photons pulse-1 and is only technically limited by the utilized APD.
Finite difference method accelerated with sparse solvers for structural analysis of the metal-organic complexes
A. A. Guda, S. A. Guda, M. A. Soldatov, K. A. Lomachenko, A. L. Bugaev, C. Lamberti, W. Gawelda, C. Bressler, G. Smolentsev, A. V. Soldatov, Y. Joly
Finite difference method (FDM) implemented in the FDMNES software [Phys. Rev. B, 2001, 63, 125120] was revised. Thorough analysis shows, that the calculated diagonal in the FDM matrix consists of about 96% zero elements. Thus a sparse solver would be more suitable for the problem instead of traditional Gaussian elimination for the diagonal neighbourhood. We have tried several iterative sparse solvers and the direct one MUMPS solver with METIS ordering turned out to be the best. Compared to the Gaussian solver present method is up to 40 times faster and allows XANES simulations for complex systems already on personal computers. We show applicability of the software for metal-organic [Fe(bpy)3]2+ complex both for low spin and high spin states populated after laser excitation.
Observing Solvation Dynamics with Simultaneous Femtosecond X-Ray Emission Spectroscopy and X-ray Scattering
K. Haldrup, W. Gawelda, R. Abela, R. Alonso-Mori, U. Bergmann, A. Bordage, M. Cammarata, S. Canton, A. O. Dohn, T. Brandt van Driel, D. M. Fritz, A. Galler, P. Glatzel, T. Harlang, K. S. Kjaer, H. T. Lemke, K. B. Moller, Z. Németh, M. Papai, N. Sas, J. Uhlig, D. Zhu, G. Vankó, V. Sundström, M. M. Nielsen, C. Bressler
In liquid phase chemistry dynamic solute–solvent interactions often govern the path, ultimate outcome, and efficiency of chemical reactions. These steps involve many-body movements on subpicosecond time scales and thus ultrafast structural tools capable of capturing both intramolecular electronic and structural changes, and local solvent structural changes are desired. We have studied the intra- and intermolecular dynamics of a model chromophore, aqueous [Fe(bpy)3]2+, with complementary X-ray tools in a single experiment exploiting intense XFEL radiation as a probe. We monitored the ultrafast structural rearrangement of the solute with X-ray emission spectroscopy, thus establishing time zero for the ensuing X-ray diffuse scattering analysis. The simultaneously recorded X-ray diffuse scattering patterns reveal slower subpicosecond dynamics triggered by the intramolecular structural dynamics of the photoexcited solute. By simultaneous combination of both methods only, we can extract new information about the solvation dynamic processes unfolding during the first picosecond (ps). The measured bulk solvent density increase of 0.2% indicates a dramatic change of the solvation shell around each photoexcited solute, confirming previous ab initio molecular dynamics simulations. Structural changes in the aqueous solvent associated with density and temperature changes occur with ∼1 ps time constants, characteristic for structural dynamics in water. This slower time scale of the solvent response allows us to directly observe the structure of the excited solute molecules well before the solvent contributions become dominant.
Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses
S. Canton, K. Kjaer, G. Vankó, T. van Driel, S. Adachi, A. Bordage, C. Bressler, P. Chabera, M. Christensen, A. Dohn, A. Galler, W. Gawelda, D. Gosztola, K. Haldrup, T. Harlang, Y. Liu, K. Moller, Z. Németh, S. Nozawa, M. Pápai, T. Sato, Ta. Sato, K. Suarez-Alcantara, T. Togashi, K. Tono, J. Uhlig, D. Vithanage, K. Wärnmark, M. Yabashi, J. Zhang, V. Sundström, and M. Nielsen
Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor–acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined.
Optimized finite difference method for the full –potential XANES simulations: application to molecular adsorption geometries in MOFs and metal-ligand intersystem crossing transients
S. Guda, A. Guda, M. Soldatov, K. Lomachenko, A. Bugaev, C. Lamberti, W. Gawelda, C. Bressler, G. Smolentsev, A. Soldatov, Y. Joly
Accurate modeling of the X-ray absorption near-edge spectra (XANES) is required to unravel the local structure of metal sites in complex systems and their structural changes upon chemical or light stimuli. Two relevant examples are reported here concerning the following: (i) the effect of molecular adsorption on 3d metals hosted inside metal–organic frameworks and (ii) light induced dynamics of spin crossover in metal–organic complexes. In both cases, the amount of structural models for simulation can reach a hundred, depending on the number of structural parameters. Thus, the choice of an accurate but computationally demanding finite difference method for the ab initio X-ray absorption simulations severely restricts the range of molecular systems that can be analyzed by personal computers. Employing the FDMNES code [ Phys. Rev. B, 2001, 63, 125120] we show that this problem can be handled if a proper diagonalization scheme is applied. Due to the use of dedicated solvers for sparse matrices, the calculation time was reduced by more than 1 order of magnitude compared to the standard Gaussian method, while the amount of required RAM was halved. Ni K-edge XANES simulations performed by the accelerated version of the code allowed analyzing the coordination geometry of CO and NO on the Ni active sites in CPO-27-Ni MOF. The Ni–CO configuration was found to be linear, while Ni–NO was bent by almost 90°. Modeling of the Fe K-edge XANES of photoexcited aqueous [Fe(bpy)3]2+ with a 100 ps delay we identified the Fe–N distance elongation and bipyridine rotation upon transition from the initial low-spin to the final high-spin state. Subsequently, the X-ray absorption spectrum for the intermediate triplet state with expected 100 fs lifetime was theoretically predicted.
Feasibility of Valence-to-Core X ray Emission Spectroscopy for Tracking Transient Species
A. M. March, T. A. Assefa, C. Bressler, G. Doumy, A. Galler, W. Gawelda, E. P. Kanter, Z. Németh, M. Pápai, S. H. Southworth, L. Young, G. Vankó
X-ray spectroscopies, when combined in laser-pump, X-ray-probe measurement schemes, can be powerful tools for tracking the electronic and geometric structural changes that occur during the course of a photoinitiated chemical reaction. X-ray absorption spectroscopy (XAS) is considered an established technique for such measurements, and X-ray emission spectroscopy (XES) of the strongest core-to-core emission lines (Kα and Kβ) is now being utilized. Flux demanding valence-to-core XES promises to be an important addition to the time-resolved spectroscopic toolkit. In this paper we present measurements and density functional theory calculations on laser-excited, solution-phase ferrocyanide that demonstrate the feasibility of valence-to-core XES for time-resolved experiments. We discuss technical improvements that will make valence-to-core XES a practical pump–probe technique.
Detailed Characterization of a Nanosecond-lived Excited State: X-ray and Theoretical Investigation of the Quintet State in Photoexcited [Fe(terpy)2]2+
G. Vankó, A. Bordage, M. Pápai, K. Haldrup, P. Glatzel, A. M. March, G. Doumy, A. Britz, A. Galler, T. A. Assefa, D. Cabaret, A. Juhin, T. B. van Driel, K. S. Kjaer, A. O. Dohn, K. B. Moller, H. T. Lemke, E. Gallo, M. Rovezzi, Z. Németh, E. Rozsàlyi, T. Rozgonyi, J. Uhlig, V. Sundström, M. M. Nielsen, L. Young, S. H. Southworth, C. Bressler, W. Gawelda
Theoretical predictions show that depending on the populations of the Fe 3dxy, 3dxz, and 3dyz orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)2]2+. The differences in the structure and molecular properties of these 5B2 and 5E quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)2]2+ 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission, and resonant emission spectra as well as X-ray diffuse scattering data clearly reflect the formation of the high-spin state of the [Fe(terpy)2]2+ molecule; moreover, extended X-ray absorption fine structure spectroscopy resolves the Fe–ligand bond-length variations with unprecedented bond-length accuracy in time-resolved experiments. With ab initio calculations we determine why, in contrast to most related systems, one configurational mode is insufficient for the description of the low-spin (LS)–high-spin (HS) transition. We identify the electronic structure origin of the differences between the two possible quintet modes, and finally, we unambiguously identify the formed quintet state as 5E, in agreement with our theoretical expectations.
Hydration shell effects in the relaxation dynamics of photoexcited Fe-II complexes in water
P. Nalbach, A. J. A. Achner, M. Frey, M. Grosser, C. Bressler, M. Thorwart
We study the relaxation dynamics of photoexcited Fe-II complexes dissolved in water and identify the relaxation pathway which the molecular complex follows in presence of a hydration shell of bound water at the interface between the complex and the solvent. Starting from a low-spin state, the photoexcited complex can reach the high-spin state via a cascade of different possible transitions involving electronic as well as vibrational relaxation processes. By numerically exact path integral calculations for the relaxational dynamics of a continuous solvent model, we find that the vibrational life times of the intermittent states are of the order of a few ps. Since the electronic rearrangement in the complex occurs on the time scale of about 100 fs, we find that the complex first rearranges itself in a high-spin and highly excited vibrational state, before it relaxes its energy to the solvent via vibrational relaxation transitions. By this, the relaxation pathway can be clearly identified. We find that the life time of the vibrational states increases with the size of the complex (within a spherical model), but decreases with the thickness of the hydration shell, indicating that the hydration shell acts as an additional source of fluctuations.
Solvation Dynamics Monitored by Combined X-Ray Spectroscopies and Scattering: Photoinduced Spin Transition in aqueous [Fe(bpy)3]2+
C. Bressler, W. Gawelda, A. Galler, M. M. Nielsen, V. Sundström, G. Doumy, A. M. March, S. H. Southworth, L. Young, G. Vankó
We have studied the photoinduced low spin (LS) to high spin (HS) conversion of aqueous Fe(bpy)3 with pulse-limited time resolution. In a combined setup permitting simultaneous X-ray diffuse scattering (XDS) and spectroscopic measurements at a MHz repetition rate we have unraveled the interplay between intramolecular dynamics and the intermolecular caging solvent response with 100 ps time resolution. On this time scale the ultrafast spin transition including intramolecular geometric structure changes as well as the concomitant bulk solvent heating process due to energy dissipation from the excited HS molecule are long completed. The heating is nevertheless observed to further increase due to the excess energy between HS and LS states released on a subnanosecond time scale. The analysis of the spectroscopic data allows precise determination of the excited population which efficiently reduces the number of free parameters in the XDS analysis, and both combined permit extraction of information about the structural dynamics of the first solvation shell.
Tracking excited-state charge and spin dynamics in iron coordination complexes
W. Zhang, R. Alonso-Mori, U. Bergmann, C. Bressler, M. Chollet, A. Galler, W. Gawelda, R. G. Hadt, R. W. Hartsock1, T. Kroll, K. S. Kjær, K. Kubicˇek, H. T. Lemke, H. W. Liang, D. A. Meyer, M. M. Nielsen, C. Purser, J. S. Robinson, et. al
Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons1, 2, 3, 4. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics5 and the flux limitations of ultrafast X-ray sources6, 7. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2′-bipyridine)3]2+, where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy6, 7, 8, 9, 10, 11, 12, 13, 14, 15. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2′-bipyridine)3]2+ on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.
Guest-Host Interactions Investigated by Time-Resolved X-Ray Spectroscopies and Scattering at MHz rates: Solvation Dynamics and Photoinduced Spin Transition in Aquesous [Fe(bipy]3]2+
K. Haldrup, G. Vankó, W. Gawelda, A. Galler, G. Doumy, A. M. March, E. P. Kanter, A. Bordage, A. Dohn, T. B. van Driel, K. S. Kjaer, H. T. Lemke, S. E. Canton, J. Uhlig, V. Sundström, L. Young, S. Southworth, M. M. Nielsen, C. Bressler
We have studied the photoinduced low spin (LS) to high spin (HS) conversion of [Fe(bipy)(3)](2+) in aqueous solution. In a laser pump/X-ray probe synchrotron setup permitting simultaneous, time-resolved X-ray diffuse scattering (XDS) and X-ray spectroscopic measurements at a 3.26 MHz repetition rate, we observed the interplay between intramolecular dynamics and the intermolecular caging solvent response with better than 100 ps time resolution. On this time scale, the initial ultrafast spin transition and the associated intramolecular geometric structure changes are long completed, as is the solvent heating due to the initial energy dissipation from the excited HS molecule. Combining information from X-ray emission spectroscopy and scattering, the excitation fraction as well as the temperature and density changes of the solvent can be closely followed on the subnanosecond time scale of the HS lifetime, allowing the detection of an ultrafast change in bulk solvent density. An analysis approach directly utilizing the spectroscopic data in the XDS analysis effectively reduces the number of free parameters, and both combined permit extraction of information about the ultrafast structural dynamics of the caging solvent, in particular, a decrease in the number of water molecules in the first solvation shell is inferred, as predicted by recent theoretical work.
Project A5
Field-enabled quantum interference in atomic Auger decay
Murali Krishna Ganesa Subramanian, Roman Brannath, Ralph Welsch, Robin Santra, and Markus Drescher
We demonstrate that an external terahertz (THz) field enables the formation of interference between two distinct Auger pathways leading to the same final ionic state. The kinetic energy of Auger electrons ejected from either of two spin-orbit split one-hole states of magnesium cations is recorded. In the presence of the THz field, a clear oscillatory structure in the Auger spectrum emerges, which we find to be in very good agreement with an analytical model based on perturbation theory. For this interference to occur, the THz field has to chirp the energy of both Auger electrons and photoelectrons simultaneously, in order to create states with indistinguishable quantum properties.
Roadmap of ultrafast x-ray atomic and molecular physics
L. Young, K. Ueda, M. Gühr, P. H. Bucksbaum, M. Simon, S. Mukamel, N. Rohringer, K. C. Prince, C. Masciovecchio, M. Meyer, A. Rudenko, D. Rolles, C. Bostedt, M. Fuchs, D. A. Reis, R. Santra, H. Kapteyn, M. Murnane, H. Ibrahim, F. Légaré, et. al.
X-ray free-electron lasers (XFELs) and table-top sources of x-rays based upon high harmonic generation (HHG) have revolutionized the field of ultrafast x-ray atomic and molecular physics, largely due to an explosive growth in capabilities in the past decade. XFELs now provide unprecedented intensity (1020 W cm−2) of x-rays at wavelengths down to ~1 Ångstrom, and HHG provides unprecedented time resolution (~50 attoseconds) and a correspondingly large coherent bandwidth at longer wavelengths. For context, timescales can be referenced to the Bohr orbital period in hydrogen atom of 150 attoseconds and the hydrogen-molecule vibrational period of 8 femtoseconds; wavelength scales can be referenced to the chemically significant carbon K-edge at a photon energy of ~280 eV (44 Ångstroms) and the bond length in methane of ~1 Ångstrom. With these modern x-ray sources one now has the ability to focus on individual atoms, even when embedded in a complex molecule, and view electronic and nuclear motion on their intrinsic scales (attoseconds and Ångstroms). These sources have enabled coherent diffractive imaging, where one can image non-crystalline objects in three dimensions on ultrafast timescales, potentially with atomic resolution. The unprecedented intensity available with XFELs has opened new fields of multiphoton and nonlinear x-ray physics where behavior of matter under extreme conditions can be explored. The unprecedented time resolution and pulse synchronization provided by HHG sources has kindled fundamental investigations of time delays in photoionization, charge migration in molecules, and dynamics near conical intersections that are foundational to AMO physics and chemistry. This roadmap coincides with the year when three new XFEL facilities, operating at Ångstrom wavelengths, opened for users (European XFEL, Swiss-FEL and PAL-FEL in Korea) almost doubling the present worldwide number of XFELs, and documents the remarkable progress in HHG capabilities since its discovery roughly 30 years ago, showcasing experiments in AMO physics and other applications. Here we capture the perspectives of 17 leading groups and organize the contributions into four categories: ultrafast molecular dynamics, multidimensional x-ray spectroscopies; high-intensity x-ray phenomena; attosecond x-ray science.
Weak-field few-femtosecond VUV photodissociation dynamics of water isotopologues
A. Baumann, S. Bazzi, D. Rompotis, O. Schepp, A. Azima, M. Wieland, D. Popova-Gorelova, O. Vendrell, R. Santra, M. Drescher
We present a joint experimental and theoretical study of the VUV-induced dynamics of H2O and its deuterated isotopologues in the first excited state (˜A1B1) utilizing a VUV-pump VUV-probe scheme combined with ab initio classical trajectory calculations. 16-fs VUV pulses centered at 161 nm created by fifth-order harmonic generation are employed for single-shot pump-probe measurements. Combined with a precise determination of the VUV pulses' temporal profile, they provide the necessary temporal resolution to elucidate sub-10-fs dissociation dynamics in the 1+1 photon ionization time window. Ionization with a single VUV photon complements established strong-field ionization schemes by disclosing the molecular dynamics under perturbative conditions. Kinetic isotope effects derived from the pump-probe experiment are found to be in agreement with our by ab initio classical trajectory calculations, taking into account photoionization cross sections for the ground and first excited state of the water cation.
State-resolved attosecond reversible and irreversible dynamics in strong optical fields
M. Sabbar, H. Timmer, Y. Chen, A. K. Pymer, Z. Loh, S. G. Sayres, S. Pabst, R. Santra, S. R. Leone
Strong-field ionization (SFI) is a key process for accessing real-time quantum dynamics of electrons on the attosecond timescale. The theoretical foundation of SFI was pioneered in the 1960s, and later refined by various analytical models. While asymptotic ionization rates predicted by these models have been tested to be in reasonable agreement for a wide range of laser parameters, predictions for SFI on the sub-laser-cycle timescale are either beyond the scope of the models or show strong qualitative deviations from full quantum-mechanical simulations. Here, using the unprecedented state specificity of attosecond transient absorption spectroscopy, we follow the real-time SFI process of the two valence spin–orbit states of xenon. The results reveal that the irreversible tunnelling contribution is accompanied by a reversible electronic population that exhibits an observable spin–orbit-dependent phase delay. A detailed theoretical analysis attributes this observation to transient ground-state polarization, an unexpected facet of SFI that cannot be captured by existing analytical models that focus exclusively on the production of asymptotic electron/ion yields.
Time-dependent configuration-interaction-singles calculation of the 5p-subshell two-photon ionization cross section in xenon
A. Karamatskou and R. Santra
The 5p two-photon ionization cross section of xenon in the photon-energy range below the one-photon ionization threshold is calculated within the time-dependent configuration-interaction-singles (TDCIS) method. The TDCIS calculations are compared to random-phase-approximation calculations [Wendin et al., J. Opt. Soc. Am. B 4, 833 (1987)] and are found to reproduce the energy positions of the intermediate Rydberg states reasonably well. The effect of interchannel coupling is also investigated and found to change the cross section of the 5p shell only slightly compared to the intrachannel case.
Stability of the time-dependent configuration-interaction-singles method in the attosecond and strong-field regimes: A study of basis sets and absorption methods
S. Pabst, A. Sytcheva, O. Geffert, R. Santra
We investigate the behavior of several spatial grid methods and complex absorbers for strong-field and attosecond scenarios when using the time-dependent configuration-interaction singles method to solve the multi-electron time-dependent Schrödinger equation for atoms. We compare the pseudospectral grid, finite-element, and finite-element-discrete-variable-representation (DVR) methods with each other and discuss their advantages and disadvantages. Additionally, we study the performances of complex absorbing potential (CAP) and smooth exterior complex scaling (SES) to absorb the outgoing electron. We find that SES performs generally better than CAP for calculating high-harmonic generation spectra and XUV photoelectron spectra. In both of these cases, the DVR and even more the FEM grid representations show more reliable results—especially when using SES. Both absorbers show drawbacks when calculating photoelectron spectra in the strong-field regime.
Maximizing hole coherence in ultrafast photoionization of argon with an optimization by sequential parametrization update
R. Esteban Goetz, M. Merkel, A. Karamatskou, R. Santra, C. P. Koch
Photoionization with attosecond pulses populates hole states in the photoion. Superpositions of hole states represent ideal candidates for time-dependent spectroscopy, for example via pump-probe studies. The challenge consists in identifying pulses that create coherent superpositions of hole states while satisfying practical constraints. Here, we employ quantum optimal control to maximize the degree of coherence between these hole states. To this end, we introduce a derivative-free optimization method with sequential parametrization update (SPA optimization). We demonstrate the versatility and computational efficiency of SPA optimization for photoionization in argon by maximizing the coherence between the 3s and 3p0 hole states using shaped attosecond pulses. We show that it is possible to maximize the hole coherence while simultaneously prescribing the ratio of the final hole state populations.
Quantum optimal control of photoelectron spectra and angular distributions
R. E. Goetz, A. Karamatskou, R. Santra, C. P. Koch
Photoelectron spectra and photoelectron angular distributions obtained in photoionization reveal important information on, e.g., charge transfer or hole coherence in the parent ion. Here we show that optimal control of the underlying quantum dynamics can be used to enhance desired features in the photoelectron spectra and angular distributions. To this end, we combine Krotov's method for optimal control theory with the time-dependent configuration interaction singles formalism and a splitting approach to calculate photoelectron spectra and angular distributions. The optimization target can account for specific desired properties in the photoelectron angular distribution alone, in the photoelectron spectrum, or in both. We demonstrate the method for hydrogen and then apply it to argon under strong XUV radiation, maximizing the difference of emission into the upper and lower hemispheres, in order to realize directed electron emission in the XUV regime.
Sensitivity of nonlinear photoionization to resonance substructure in collective excitation
T. Mazza, A. Karamatskou, M. Ilchen, S. Bakhtiarzadeh, A. J. Rafipoor, P. O’Keeffe, T. J. Kelly, N. Walsh, J. T. Costello, M. Meyer, R. Santra
Collective behaviour is a characteristic feature in many-body systems, important for developments in fields such as magnetism, superconductivity, photonics and electronics. Recently, there has been increasing interest in the optically nonlinear response of collective excitations. Here we demonstrate how the nonlinear interaction of a many-body system with intense XUV radiation can be used as an effective probe for characterizing otherwise unresolved features of its collective response. Resonant photoionization of atomic xenon was chosen as a case study. The excellent agreement between experiment and theory strongly supports the prediction that two distinct poles underlie the giant dipole resonance. Our results pave the way towards a deeper understanding of collective behaviour in atoms, molecules and solid-state systems using nonlinear spectroscopic techniques enabled by modern short-wavelength light sources.
Multielectron dynamics in the tunneling ionization of correlated quantum systems
M. Hollstein and D. Pfannkuche
The importance of multielectron dynamics during the tunneling ionization of a correlated quantum system is investigated. By comparison of the solution of the time-dependent Schrödinger equation with the time-dependent configuration-interaction singles approach, we demonstrate the importance of a multielectron description of the tunneling ionization process especially for weakly confined quantum systems. Within this context, we observe that adiabatic driving by an intense light field can even enhance the correlations between still trapped electrons.
Wave-packet propagation based calculation of above-threshold ionization in the x-ray regime
M. Tilley, A. Karamatskou, R. Santra
We investigate the multi-photon process of above-threshold ionization for the light elements hydrogen, carbon, nitrogen, and oxygen in the hard x-ray regime. Numerical challenges are discussed and by comparing Hartree–Fock–Slater calculations with configuration–interaction–singles results we justify the mean-field potential approach in this regime. We present a theoretical prediction of two-photon above-threshold-ionization cross sections for the mentioned elements. Moreover, we study how the importance of above-threshold ionization varies with intensity. We find that for carbon, at x-ray intensities around ${{10}^{23}}\ {\rm W}\;{\rm c}{{{\rm m}}^{-2}}$, two-photon above-threshold ionization of the K-shell electrons is as probable as one-photon ionization of the L-shell electrons.
Theoretical characterization of the collective resonance states underlying the xenon giant dipole resonance
Yi-Jen Chen (陳怡蓁), Stefan Pabst, Antonia Karamatskou, Robin Santra
We present a detailed theoretical characterization of the two fundamental collective resonances underlying the xenon giant dipole resonance (GDR). This is achieved consistently by two complementary methods implemented within the framework of the configuration-interaction singles (CIS) theory. The first method accesses the resonance states by diagonalizing the many-electron Hamiltonian using the smooth exterior complex scaling technique. The second method involves a different application of the Gabor analysis to wave-packet dynamics. We identify one resonance at an excitation energy of 74 eV with a lifetime of 27 as and the second at 107eV with a lifetime of 11as. Our work provides a deeper understanding of the nature of the resonances associated with the GDR: a group of close-lying intrachannel resonances splits into two far-separated resonances through interchannel couplings involving the 4d electrons. The CIS approach allows a transparent interpretation of the two resonances as new collective modes. Due to the strong entanglement between the excited electron and the ionic core, the resonance wave functions are not dominated by any single particle-hole state. This gives rise to plasma-like collective oscillations of the 4d shell as a whole.
Spin–orbit effects in atomic high-harmonic generation
S. Pabst, R. Santra
Spin–orbit interactions lead to small energy gaps between the outermost p1/2 and p3/2 shells of noble gas atoms. Strong-field pulses tunnel-ionize an electron out of either shell resulting in spin–orbit-driven hole motion. These hole dynamics affect the high-harmonic generation (HHG) yield. However, the spectral shape as well as the angular distribution of the HHG emission is not influenced by spin–orbit coupling. We demonstrate the spin–orbit effect on atomic krypton by solving the multi-electron Schrödinger equation with the time-dependent configuration-interaction singles approach. We also provide pulse parameters where this effect can be identified in experiments through an enhancement in the HHG yield as the wavelength of the strong-field pulse increases.
Controlling the 2p hole alignment in neon via the 2s-3p Fano resonance
E. Heinrich-Josties, S. Pabst, R. Santra
We study the state-resolved production of neon ion after resonant photoionization of Ne via the 2s-3p Fano resonance. We find that by tuning the photon energy across the Fano resonance, a surprisingly high control over the alignment of the final 2p hole along the polarization direction can be achieved. In this way, hole alignments can be created that are otherwise very hard to achieve. The mechanism responsible for this hole alignment is the destructive interference of the direct and indirect (via the autoionizing 2s−13p state) ionization pathways of 2p. By changing the photon energy, the strength of the interference varies and 2p hole alignments with ratios up to 19:1 between 2p0 and 2p±1 holes can be created—an effect normally only encountered in tunnel ionization using strong-field ir pulses. The inclusion of spin-orbit interaction does not change the qualitative feature and leads only to a reduction in the alignment by 2/3. Our study is based on a time-dependent configuration-interaction singles approach, which solves the multichannel time-dependent Schrödinger equation.
Calculation of photoelectron spectra within the time-dependent configuration-interaction singles scheme
A. Karamatskou, S. Pabst, Y.-J. Chen, R. Santra
We present an extension of the time-dependent configuration-interaction singles (TDCIS) method to the computation of the electron kinetic-energy spectrum in photoionization processes. Especially for strong and long ionizing light pulses, the detection of the photoelectron poses a computational challenge because propagating the outgoing photoelectron wave packet requires large grid sizes. Two different methods that allow for the extraction of the asymptotic photoelectron momentum are compared regarding their methodological and computational performance. The first method follows the scheme of Tong et al. [X. M. Tong, K. Hino, and N. Toshima, Phys. Rev. A 74, 031405(R) (2006)], where the photoelectron wave function is absorbed by a real splitting function. The second method following that presented by Tao and Scrinzi [L. Tao and A. Scrinzi, New J. Phys. 14, 013021 (2012)], measures the flux of the electron wave packet through a surface at a fixed radius. With both methods the full angle- and energy-resolved photoelectron spectrum is obtained. Combined with the TDCIS scheme, it is possible to analyze the dynamics of the outgoing electron in a channel-resolved way and, additionally, to study the dynamics of the bound electrons in the parent ion. As an application, one-photon and above-threshold ionization of argon following strong XUV irradiation are studied via energy- and angle-resolved photoelectron spectra.
Introducing many-body physics using atomic spectroscopy
D. Krebs, S. Pabst, R. Santra
Atoms constitute relatively simple many-body systems, making them suitable objects for developing an understanding of basic aspects of many-body physics. Photoabsorption spectroscopy is a prominent method to study the electronic structure of atoms and the inherent many-body interactions. In this article, the impact of many-body effects on well-known spectroscopic features, such as Rydberg series, Fano resonances, Cooper minima, and giant resonances, is studied and related many-body phenomena in other fields are outlined. To calculate photoabsorption cross sections, the time-dependent configuration interaction singles (TDCIS) model is employed. The conceptual clearness of TDCIS in combination with the compactness of atomic systems allows for a pedagogical introduction to many-body phenomena.
Strong-Field Many-Body Physics and the Giant Enhancement in the High-Harmonic Spectrum of Xenon
S. Pabst, R. Santra
We resolve an open question about the origin of the giant enhancement in the high-harmonic generation spectrum of atomic xenon around 100 eV. By solving the many-body time-dependent Schrödinger equation with all 4d, 5s, and 5p orbitals active, we truly demonstrate the enhancement results from the collective many-body excitation induced by the returning photoelectron via two-body interchannel interactions. Without the many-body interactions, which promote a 4d electron into the 5p vacancy created by strong-field ionization, no collective excitation and no enhancement in the high-harmonic generation spectrum exist.
Atomic and molecular dynamics triggered by ultrashort light pulses on the atto- to picosecond time scale
S. Pabst
Time-resolved investigations of ultrafast electronic and molecular dynamics were not possible until recently. The typical time scale of these processes is in the picosecond to attosecond realm. The tremendous technological progress in recent years made it possible to generate ultrashort pulses, which can be used to trigger, to watch, and to control atomic and molecular motion. This tutorial focuses on experimental and theoretical advances which are used to study the dynamics of electrons and molecules in the presence of ultrashort pulses. In the first part, the rotational dynamics of molecules, which happens on picosecond and femtosecond time scales, is reviewed. Well-aligned molecules are particularly suitable for angle-dependent investigations like x-ray diffraction or strong-field ionization experiments. In the second part, the ionization dynamics of atoms is studied. The characteristic time scale lies, here, in the attosecond to few-femtosecond regime. Although a one-particle picture has been successfully applied to many processes, many-body effects do constantly occur. After a broad overview of the main mechanisms and the most common tools in attosecond physics, examples of many-body dynamics in the attosecond world (e.g., in high-harmonic generation and attosecond transient absorption spectroscopy) are discussed.
Adiabaticity and diabaticity in strong-field ionization
A. Karamatskou, S. Pabst, R. Santra
If the photon energy is much less than the electron binding energy, ionization of an atom by a strong optical field is often described in terms of electron tunneling through the potential barrier resulting from the superposition of the atomic potential and the potential associated with the instantaneous electric component of the optical field. In the strict tunneling regime, the electron response to the optical field is said to be adiabatic, and nonadiabatic effects are assumed to be negligible. Here, we investigate to what degree this terminology is consistent with a language based on the so-called adiabatic representation. This representation is commonly used in various fields of physics. For electronically bound states, the adiabatic representation yields discrete potential-energy curves that are connected by nonadiabatic transitions. When applying the adiabatic representation to optical strong-field ionization, a conceptual challenge is that the eigenstates of the instantaneous Hamiltonian form a continuum; i.e., there are no discrete adiabatic states. This difficulty can be overcome by applying an analytic-continuation technique. In this way, we obtain a rigorous classification of adiabatic states and a clear characterization of (non)adiabatic and (non)diabatic ionization dynamics. Moreover, we distinguish two different regimes within tunneling ionization and explain the dependence of the ionization probability on the pulse envelope.
Real time tracing of valence-shell electronic coherences with attosecond transient absorption spectroscopy
A. Wirth, R. Santra, E. Goulielmakis
The chemical properties of atoms, molecules and of more complex systems such as clusters, nanoparticles or condensed matter systems are determined by valence electrons. Real-time control of these properties requires the capability of tracing as well as of driving valence electrons on their native temporal scale of motion, that is, within tens to thousands of attoseconds. Here we detail the technique of attosecond transient absorption spectroscopy. It combines the extreme sensitivity of core-level spectroscopy with the unprecedented temporal resolution offered by the tools of attosecond technology. We use the technique to demonstrate real-time tracing and complete characterization of coherent electron motion triggered by single, double or multiple ionization of atoms exposed to intense, few-cycle pulses. Our work opens the door to high fidelity, time-domain studies and control of electron dynamics in the microcosm.
Real time tracing of valence-shell electronic coherences with attosecond transient absorption spectroscopy
A. Wirth, R. Santra, E. Goulielmakis
The chemical properties of atoms, molecules and of more complex systems such as clusters, nanoparticles or condensed matter systems are determined by valence electrons. Real-time control of these properties requires the capability of tracing as well as of driving valence electrons on their native temporal scale of motion, that is, within tens to thousands of attoseconds. Here we detail the technique of attosecond transient absorption spectroscopy. It combines the extreme sensitivity of core-level spectroscopy with the unprecedented temporal resolution offered by the tools of attosecond technology. We use the technique to demonstrate real-time tracing and complete characterization of coherent electron motion triggered by single, double or multiple ionization of atoms exposed to intense, few-cycle pulses. Our work opens the door to high fidelity, time-domain studies and control of electron dynamics in the microcosm.
Theory of attosecond transient-absorption spectroscopy of krypton for overlapping pump and probe pulses
S. Pabst, A. Sytcheva, A. Moulet, A. Wirth, E. Goulielmakis, R. Santra
We present a fully ab initio calculations for attosecond transient absorption spectroscopy of atomic krypton with overlapping pump and probe pulses. Within the time-dependent configuration interaction singles (TDCIS) approach, we describe the pump step (strong-field ionization using a near-infrared pulse) as well as the probe step (resonant electron excitation using an extreme-ultraviolet pulse) from first principles. We extend our TDCIS model and account for the spin-orbit splitting of the occupied orbitals. We discuss the spectral features seen in a recent attosecond transient absorption experiment [ A. Wirth et al. Science 334 195 (2011)]. Our results support the concept that the transient absorption signal can be directly related to the instantaneous hole population even during the ionizing pump pulse. Furthermore, we find strong deformations in the absorption lines when the overlap of pump and probe pulses is maximum. These deformations can be described by relative phase shifts in the oscillating ionic dipole. We discuss possible mechanisms contributing to these phase shifts. Our finding suggests that the nonperturbative laser dressing of the entire N-electron wave function is the main contributor.
Impact of multichannel and multipole effects on the Cooper minimum in the high-order-harmonic spectrum of argon
S. Pabst, L. Greenman, D. A. Mazziotti, R. Santra
We investigate the relevance of multiple-orbital and multipole effects during high-harmonic generation (HHG). The time-dependent configuration interaction singles (TDCIS) approach is used to study the impact of the detailed description of the residual electron-ion interaction on the HHG spectrum. We find that the shape and position of the Cooper minimum in the HHG spectrum of argon changes significantly whether or not interchannel interactions are taken into account. The HHG yield can be underestimated by up to 2 orders of magnitude in the energy region of 30–50 eV. We show that the argument of low ionization probability is not sufficient to justify ignoring multiple-orbital contributions. Additionally, we find the HHG yield is sensitive to the nonspherical multipole character of the electron-ion interaction.
Project A6
Formation of Spontaneous Density-Wave Patterns in dc Driven Lattices
H. P. Zahn, V. P. Singh, M. N. Kosch, L. Asteria, L. Freystatzky, K. Sengstock, L. Mathey, and C. Weitenberg
Driving a many-body system out of equilibrium induces phenomena such as the emergence and decay of transient states, which can manifest itself as pattern and domain formation. The understanding of these phenomena expands the scope of established thermodynamics into the out-of-equilibrium domain. Here, we experimentally and theoretically study the out-of-equilibrium dynamics of a bosonic lattice model subjected to a strong dc field, realized as ultracold atoms in a strongly tilted triangular optical lattice. We observe the emergence of pronounced density-wave patterns—which spontaneously break the underlying lattice symmetry—using a novel single-shot imaging technique with two-dimensional single-site resolution in three-dimensional systems, which also resolves the domain structure. Our study suggests that the short-time dynamics arises from resonant pair tunneling processes within an effective description of the tilted Hubbard model. More broadly, we establish the far out-of-equilibrium regime of lattice models subjected to a strong dc field, as an exemplary and paradigmatic scenario for transient pattern formation.
Tailoring quantum gases by Floquet engineering
Christof Weitenberg & Juliette Simonet
Floquet engineering is the concept of tailoring a system by a periodic drive, and it is increasingly employed in many areas of physics. Ultracold atoms in optical lattices offer a particularly large toolbox to design a variety of driving schemes. A strong motivation for developing these methods is the prospect to study the interplay between topology and interactions in a system where both ingredients are fully tunable. We review the recent successes of Floquet engineering in realizing new classes of Hamiltonians in quantum gases, such as Hamiltonians including artificial gauge fields, topological band structures and density-dependent tunnelling. The creation of periodically driven systems also gives rise to phenomena without static counterparts such as anomalous Floquet topological insulators. We discuss the challenges facing the field, particularly the control of heating mechanisms, which currently limit the preparation of many-body phases, as well as the potential future developments as these obstacles are overcome.
Identifying quantum phase transitions using artificial neural networks on experimental data
Benno S. Rem, Niklas Käming, Matthias Tarnowski, Luca Asteria, Nick Fläschner, Christoph Becker, Klaus Sengstock, and Christof Weitenberg
Quantum point spread function for imaging trapped few-body systems with a quantum gas microscope
M. Pyzh, S. Krönke, C. Weitenberg, P. Schmelcher
Quantum gas microscopes, which image the atomic occupations in an optical lattice, have opened a new avenue to the exploration of many-body lattice systems. Imaging trapped systems after freezing the density distribution by ramping up a pinning lattice leads, however, to a distortion of the original density distribution, especially when its structures are on the scale of the pinning lattice spacing. We show that this dynamics can be described by a filter, which we call in analogy to classical optics a quantum point spread function. Using a machine learning approach, we demonstrate via several experimentally relevant setups that a suitable deconvolution allows for the reconstruction of the original density distribution. These findings are both of fundamental interest for the theory of imaging and of immediate importance for current quantum gas experiments.
Spectral properties and breathing dynamics of a few-body Bose–Bose mixture in a 1D harmonic trap
M. Pyzh, S. Krönke, C. Weitenberg, P. Schmelcher
We investigate a few-body mixture of two bosonic components, each consisting of two particles confined in a quasi one-dimensional harmonic trap. By means of exact diagonalization with a correlated basis approach we obtain the low-energy spectrum and eigenstates for the whole range of repulsive intra- and inter-component interaction strengths. We analyse the eigenvalues as a function of the inter-component coupling, covering hereby all the limiting regimes, and characterize the behaviour in-between these regimes by exploiting the symmetries of the Hamiltonian. Provided with this knowledge we study the breathing dynamics in the linear-response regime by slightly quenching the trap frequency symmetrically for both components. Depending on the choice of interactions strengths, we identify 1 to 3 monopole modes besides the breathing mode of the centre of mass coordinate. For the uncoupled mixture each monopole mode corresponds to the breathing oscillation of a specific relative coordinate. Increasing the inter-component coupling first leads to multi-mode oscillations in each relative coordinate, which turn into single-mode oscillations of the same frequency in the composite-fermionization regime.
Superfluidity and relaxation dynamics of a laser-stirred 2D Bose gas
Singh V. P., Weitenberg C., Dalibard J., Mathey L.
We investigate the superfluid behavior of a two-dimensional (2D) Bose gas of 87Rb atoms using classical field dynamics. In the experiment by R. Desbuquois et al. [Nat. Phys. 8, 645 (2012)], a 2D quasicondensate in a trap is stirred with a blue-detuned laser beam along a circular path around the trap center. Here, we study this experiment from a theoretical perspective. The heating induced by stirring increases rapidly above a velocity vc, which we define as the critical velocity. We identify the superfluid, the crossover, and the thermal regime by a finite, a sharply decreasing, and a vanishing critical velocity, respectively. We demonstrate that the onset of heating occurs due to the creation of vortex-antivortex pairs. A direct comparison of our numerical results to the experimental ones shows a good agreement, if a systematic shift of the critical phase-space density is included. We relate this shift to the absence of thermal equilibrium between the condensate and the thermal wings, which were used in the experiment to extract the temperature. We expand on this observation by studying the full relaxation dynamics between the condensate and the thermal cloud.
Implementing supersymmetric dynamics in ultracold-atom systems
M. Lahrz, C. Weitenberg, L. Mathey
Supersymmetric systems derive their properties from conserved supercharges which form a supersymmetric algebra. These systems naturally factorize into two subsystems, which, when considered as individual systems, have essentially the same eigenenergies, and their eigenstates can be mapped onto each other. We propose a Mach-Zehnder interference experiment to detect supersymmetry in quantum-mechanical systems, which can be realized with current technology. To demonstrate this interferometric technique, we first propose a one-dimensional ultracold-atom setup to realize a pair of supersymmetric systems. Here, a single-atom wave packet evolves in a superposition of the subsystems and gives an interference contrast that is sharply peaked if the subsystems form a supersymmetric pair. Second, we propose a two-dimensional setup that implements supersymmetric dynamics in a synthetic gauge field.
Observation of topological Bloch-state defects and their merging transition
M. Tarnowski, M. Nuske, N. Fläschner, B. Rem, D. Vogel, L. Freystatzky, K. Sengstock, L. Mathey, C. Weitenberg
Topological defects in Bloch bands, such as Dirac points in graphene, and their resulting Berry phases play an important role for the electronic dynamics in solid state crystals. Such defects can arise in systems with a two-atomic basis due to the momentum-dependent coupling of the two sublattice states, which gives rise to a pseudo-spin texture. The topological defects appear as vortices in the azimuthal phase of this pseudo-spin texture. Here, we demonstrate a complete measurement of the azimuthal phase in a hexagonal optical lattice
employing a versatile method based on time-of-flight imaging after off-resonant lattice modulation. Furthermore we map out the merging transition of the two Dirac points induced by beam imbalance. Our work paves the way to accessing geometric properties in general multi-band systems also with spin-orbit coupling and interactions.
Phys. Rev. Lett. 118, 240403 (2017)
https://arxiv.org/abs/1703.02813
Emulating molecular orbitals and electronic dynamics with ultracold atoms
D.-S. Lühmann, C. Weitenberg, K. Sengstock
In the recent years, ultracold atoms in optical lattices have proven their great value as quantum simulators for studying strongly-correlated phases and complex phenomena in solid-state systems. Here we reveal their potential as quantum simulators for molecular physics and propose a technique to image the three-dimensional molecular orbitals with high resolution. The outstanding tunability of ultracold atoms in terms of potential and interaction offer fully-adjustable model systems for gaining deep insight into the electronic structure of molecules. We study the orbitals of an artificial benzene molecule and discuss the effect of tunable interactions in its conjugated pi electron system with special regard to localization and spin order. The dynamical timescale of ultracold atom simulators are on the order milliseconds which allow for the time-resolved monitoring of a broad range of dynamical processes. As an example, we compute the hole dynamics in the conjugated pi system of the artificial benzene molecule.
Project A7
Shining light on the microscopic resonant mechanism responsible for cavity-mediated chemical reactivity
Schäfer, C., Flick, J., Ronca, E. et al.
Strong light–matter interaction in cavity environments is emerging as a promising approach to control chemical reactions in a non-intrusive and efficient manner. The underlying mechanism that distinguishes between steering, accelerating, or decelerating a chemical reaction has, however, remained unclear, hampering progress in this frontier area of research. We leverage quantum-electrodynamical density-functional theory to unveil the microscopic mechanism behind the experimentally observed reduced reaction rate under cavity induced resonant vibrational strong light-matter coupling. We observe multiple resonances and obtain the thus far theoretically elusive but experimentally critical resonant feature for a single strongly coupled molecule undergoing the reaction. While we describe only a single mode and do not explicitly account for collective coupling or intermolecular interactions, the qualitative agreement with experimental measurements suggests that our conclusions can be largely abstracted towards the experimental realization. Specifically, we find that the cavity mode acts as mediator between different vibrational modes. In effect, vibrational energy localized in single bonds that are critical for the reaction is redistributed differently which ultimately inhibits the reaction.
Effect of spin-orbit coupling on the high harmonics from the topological Dirac semimetal Na3Bi
Tancogne-Dejean, N., Eich, F.G. & Rubio, A.
In this work, we performed extensive first-principles simulations of high-harmonic generation in the topological Diract semimetal Na3Bi using a first-principles time-dependent density functional theory framework, focusing on the effect of spin-orbit coupling (SOC) on the harmonic response. We also derived an analytical model describing the microscopic mechanism of strong-field dynamics in presence of spin-orbit coupling, starting from a locally U(1) × SU(2) gauge-invariant Hamiltonian. Our results reveal that SOC: (i) affects the strong-field excitation of carriers to the conduction bands by modifying the bandstructure of Na3Bi, (ii) makes each spin channel reacts differently to the driven laser by modifying the electron velocity (iii) changes the emission timing of the emitted harmonics. Moreover, we show that the SOC affects the harmonic emission by directly coupling the charge current to the spin currents, paving the way to the high-harmonic spectroscopy of spin currents in solids.
Unconventional excitonic states with phonon sidebands in layered silicon diphosphide
Zhou, L., Huang, J., Windgaetter, L. et al.
Complex correlated states emerging from many-body interactions between quasiparticles (electrons, excitons and phonons) are at the core of condensed matter physics and material science. In low-dimensional materials, quantum confinement affects the electronic, and subsequently, optical properties for these correlated states. Here, by combining photoluminescence, optical reflection measurements and ab initio theoretical calculations, we demonstrate an unconventional excitonic state and its bound phonon sideband in layered silicon diphosphide (SiP2), where the bound electron–hole pair is composed of electrons confined within one-dimensional phosphorus–phosphorus chains and holes extended in two-dimensional SiP2 layers. The excitonic state and emergent phonon sideband show linear dichroism and large energy redshifts with increasing temperature. Our ab initio many-body calculations confirm that the observed phonon sideband results from the correlated interaction between excitons and optical phonons. With these results, we propose layered SiP2 as a platform for the study of excitonic physics and many-particle effects.
Probing phonon dynamics with multidimensional high harmonic carrier-envelope-phase spectroscopy
Ofer Neufeld, Jin Zhang, Umberto De Giovannini, and Angel Rubio
High harmonic generation (HHG) has recently been established as a powerful method for probing ultrafast electron dynamics in solids. However, it remains unknown if HHG can be similarly applied for probing lattice distortions such as phonons. Specifically, it is unclear if the extreme nonlinearity of HHG can contribute to enhanced temporal resolution or sensitivity for probing lattice dynamics (compared to other, perturbative, methods). Here, we theoretically explore HHG in solids with active phonons. We present a pump-probe and multidimensional spectroscopy approach that relies on carrier-envelope-phase-sensitivity, in which HHG is highly sensitive for phonon dynamics. Strikingly, the predicted temporal resolution is ∼1 femtosecond, much below the probe pulse duration, owing to the subcycle nature of the approach.
Ultrafast dynamics of adenine following XUV ionization
Erik P Månsson, Simone Latini, Fabio Covito, Vincent Wanie, Mara Galli, Enrico Perfetto, Gianluca Stefanucci, Umberto De Giovannini, Mattea C Castrovilli, Andrea Trabattoni, Fabio Frassetto, Luca Poletto, Jason B Greenwood, François Légaré, Mauro Nisoli, Angel Rubio, Francesca Calegari
The dynamics of biologically relevant molecules exposed to ionizing radiation contains many facets and spans several orders of magnitude in time and energy. In the extreme ultraviolet (XUV) spectral range, multi-electronic phenomena and bands of correlated states with inner-valence holes must be accounted for in addition to a plethora of vibrational modes and available dissociation channels. The ability to track changes in charge density and bond length during ultrafast reactions is an important endeavor toward more general abilities to simulate and control photochemical processes, possibly inspired by those that have evolved biologically. By using attosecond XUV pulses extending up to 35 eV and few-femtosecond near-infrared pulses, we have previously time-resolved correlated electronic dynamics and charge migration occurring in the biologically relevant molecule adenine after XUV-induced sudden ionization. Here, using additional experimental data, we comprehensively report on both electronic and vibrational dynamics of this nucleobase in an energy range little explored to date with high temporal resolution. The time-dependent yields of parent and fragment ions in the mass spectra are analyzed to extract exponential time constants and oscillation periods. Together with time-dependent density functional theory and ab-initio Green’s function methods, we identify different vibrational and electronic processes. Beyond providing further insights into the XUV-induced dynamics of an important nucleobase, our work demonstrates that yields of specific dissociation outcomes can be influenced by sufficiently well-timed ultrashort pulses, therefore providing a new route for the control of the multi-electronic and dissociative dynamics of a DNA building block.
Making ab initio QED functional(s): Nonperturbative and photon-free effective frameworks for strong light–matter coupling
Schäfer, C., Buchholz, F., Penz, M., Ruggenthaler, M., & Rubio, A.
Strong light–matter coupling provides a promising path for the control of quantum matter where the latter is routinely described from first principles. However, combining the quantized nature of light with this ab initio tool set is challenging and merely developing as the coupled light–matter Hilbert space is conceptually different and computational cost quickly becomes overwhelming. In this work, we provide a nonperturbative photon-free formulation of quantum electrodynamics (QED) in the long-wavelength limit, which is formulated solely on the matter Hilbert space and can serve as an accurate starting point for such ab initio methods. The present formulation is an extension of quantum mechanics that recovers the exact results of QED for the zero- and infinite-coupling limit and the infinite-frequency as well as the homogeneous limit, and we can constructively increase its accuracy. We show how this formulation can be used to devise approximations for quantum-electrodynamical density-functional theory (QEDFT), which in turn also allows us to extend the ansatz to the full minimal-coupling problem and to nonadiabatic situations. Finally, we provide a simple local density–type functional that takes the strong coupling to the transverse photon degrees of freedom into account and includes the correct frequency and polarization dependence. This QEDFT functional accounts for the quantized nature of light while remaining computationally simple enough to allow its application to a large range of systems. All approximations allow the seamless application to periodic systems.
Making ab initio QED functional(s): Nonperturbative and photon-free effective frameworks for strong light–matter coupling
Schäfer, C., Buchholz, F., Penz, M., Ruggenthaler, M., & Rubio, A.
Strong light–matter coupling provides a promising path for the control of quantum matter where the latter is routinely described from first principles. However, combining the quantized nature of light with this ab initio tool set is challenging and merely developing as the coupled light–matter Hilbert space is conceptually different and computational cost quickly becomes overwhelming. In this work, we provide a nonperturbative photon-free formulation of quantum electrodynamics (QED) in the long-wavelength limit, which is formulated solely on the matter Hilbert space and can serve as an accurate starting point for such ab initio methods. The present formulation is an extension of quantum mechanics that recovers the exact results of QED for the zero- and infinite-coupling limit and the infinite-frequency as well as the homogeneous limit, and we can constructively increase its accuracy. We show how this formulation can be used to devise approximations for quantum-electrodynamical density-functional theory (QEDFT), which in turn also allows us to extend the ansatz to the full minimal-coupling problem and to nonadiabatic situations. Finally, we provide a simple local density–type functional that takes the strong coupling to the transverse photon degrees of freedom into account and includes the correct frequency and polarization dependence. This QEDFT functional accounts for the quantized nature of light while remaining computationally simple enough to allow its application to a large range of systems. All approximations allow the seamless application to periodic systems.
Engineering Three-Dimensional Moiré Flat Bands
Xian, L. D., Fischer, A., Claassen, M., Zhang, J., Rubio, A., & Kennes, D. M.
Twisting two adjacent layers of van der Waals materials with respect to each other can lead to flat two-dimensional electronic bands which enables a wealth of physical phenomena. Here, we generalize this concept of so-called moiré flat bands to engineer flat bands in all three spatial dimensions controlled by the twist angle. The basic concept is to stack the material such that the large spatial moiré interference patterns are spatially shifted from one twisted layer to the next. We exemplify the general concept by considering graphitic systems, boron nitride, and WSe2, but the approach is applicable to any two-dimensional van der Waals material. For hexagonal boron nitride, we develop an ab initio fitted tight binding model that captures the corresponding three-dimensional low-energy electronic structure. We outline that interesting three-dimensional correlated phases of matter can be induced and controlled following this route, including quantum magnets and unconventional superconducting states.
Realization of nearly dispersionless bands with strong orbital anisotropy from destructive interference in twisted bilayer MoS2
Xian, L. D., Claassen, M., Kiese, D., Scherer, M. M., Trebst, S., Kennes, D. M., & Rubio, A.
Recently, the twist angle between adjacent sheets of stacked van der Waals materials emerged as a new knob to engineer correlated states of matter in two-dimensional heterostructures in a controlled manner, giving rise to emergent phenomena such as superconductivity or correlated insulating states. Here, we use an ab initio based approach to characterize the electronic properties of twisted bilayer MoS2. We report that, in marked contrast to twisted bilayer graphene, slightly hole-doped MoS2 realizes a strongly asymmetric px-py Hubbard model on the honeycomb lattice, with two almost entirely dispersionless bands emerging due to destructive interference. The origin of these dispersionless bands, is similar to that of the flat bands in the prototypical Lieb or Kagome lattices and co-exists with the general band flattening at small twist angle due to the moiré interference. We study the collective behavior of twisted bilayer MoS2 in the presence of interactions, and characterize an array of different magnetic and orbitally-ordered correlated phases, which may be susceptible to quantum fluctuations giving rise to exotic, purely quantum, states of matter.
Approximations based on density-matrix embedding theory for density-functional theories
Theophilou, I., Reinhard, T., Rubio, A., & Ruggenthaler, M.
Recently a novel approach to find approximate exchange–correlation functionals in density-functional theory was presented (Mordovina et al 2019 J. Chem. Theory Comput. 15 5209), which relies on approximations to the interacting wave function using density-matrix embedding theory (DMET). This approximate interacting wave function is constructed by using a projection determined by an iterative procedure that makes parts of the reduced density matrix of an auxiliary system the same as the approximate interacting density matrix. If only the diagonal of both systems are connected this leads to an approximation of the interacting-to-non-interacting mapping of the Kohn–Sham approach to density-functional theory. Yet other choices are possible and allow to connect DMET with other density-functional theories such as kinetic-energy density functional theory or reduced density-matrix functional theory. In this work we give a detailed review of the basics of the DMET procedure from a density-functional perspective and show how both approaches can be used to supplement each other. We do not present a specific realization of combining density-functional methods with DMET but rather provide common grounds to facilitate future developments that encompass both approaches. We do so explicitly for the case of a one-dimensional lattice system, as this is the simplest setting where we can apply DMET and the one that was originally presented. Among others we highlight how the mappings of density-functional theories can be used to identify uniquely defined auxiliary systems and projections in DMET and how to construct approximations for different density-functional theories using DMET inspired projections. Such alternative approximation strategies become especially important for density-functional theories that are based on non-linearly coupled observables such as kinetic-energy density-functional theory, where the Kohn–Sham fields are no longer obtainable by functional differentiation of an energy expression, or for reduced density-matrix functional theories, where a straightforward Kohn–Sham construction is not feasible.
Light-Driven Extremely Nonlinear Bulk Photogalvanic Currents
Neufeld, O., Tancogne-Dejean, N., de Giovannini, U., Hübener, H., & Rubio, A.
We predict the generation of bulk photocurrents in materials driven by bichromatic fields that are circularly polarized and corotating. The nonlinear photocurrents have a fully controllable directionality and amplitude without requiring carrier-envelope-phase stabilization or few-cycle pulses, and can be generated with photon energies much smaller than the band gap (reducing heating in the photoconversion process). We demonstrate with ab initio calculations that the photocurrent generation mechanism is universal and arises in gaped materials (Si, diamond, MgO, hBN), in semimetals (graphene), and in two- and three-dimensional systems. Photocurrents are shown to rely on sub-laser-cycle asymmetries in the nonlinear response that build-up coherently from cycle to cycle as the conduction band is populated. Importantly, the photocurrents are always transverse to the major axis of the co-circular lasers regardless of the material’s structure and orientation (analogously to a Hall current), which we find originates from a generalized time-reversal symmetry in the driven system. At high laser powers (∼1013 W/cm2) this symmetry can be spontaneously broken by vast electronic excitations, which is accompanied by an onset of carrier-envelope-phase sensitivity and ultrafast many-body effects. Our results are directly applicable for efficient light-driven control of electronics, and for enhancing sub-band-gap bulk photogalvanic effects.
All-optical generation of antiferromagnetic magnon currents via the magnon circular photogalvanic effect
Viñas Boström, E., Parvini, T. S., McIver, J. W., Rubio, A., Kusminskiy, S. V., & Sentef, M. A.
We introduce the magnon circular photogalvanic effect enabled by two-magnon Raman scattering. This provides an all-optical pathway to the generation of directed magnon currents with circularly polarized light in honeycomb antiferromagnetic insulators. The effect is the leading order contribution to magnon photocurrent generation via optical fields. Control of the magnon current by the polarization and angle of incidence of the laser is demonstrated. Experimental detection by sizable inverse spin Hall voltages in platinum contacts is proposed.
The 2021 ultrafast spectroscopic probes of condensed matter roadmap
Lloyd-Hughes, J., Oppeneer, P. M., Pereira dos Santos, T., Schleife, A., Meng, S., Sentef, M. A., Ruggenthaler, M., Rubio, A., Radu, I., Murnane, M., Shi, X., Kapteyn, H., Stadtmüller, B., Dani, K. M., da Jornada, F. H., Prinz, E., Aeschlimann, M., Milot, R. L., Burdanova, M., Boland, J., Cocker, T., & Hegmann, F.
In the 60 years since the invention of the laser, the scientific community has developed numerous fields of research based on these bright, coherent light sources, including the areas of imaging, spectroscopy, materials processing and communications. Ultrafast spectroscopy and imaging techniques are at the forefront of research into the light–matter interaction at the shortest times accessible to experiments, ranging from a few attoseconds to nanoseconds. Light pulses provide a crucial probe of the dynamical motion of charges, spins, and atoms on picosecond, femtosecond, and down to attosecond timescales, none of which are accessible even with the fastest electronic devices. Furthermore, strong light pulses can drive materials into unusual phases, with exotic properties. In this roadmap we describe the current state-of-the-art in experimental and theoretical studies of condensed matter using ultrafast probes. In each contribution, the authors also use their extensive knowledge to highlight challenges and predict future trends.
Light-Induced Charge Transfer from Transition-Metal-Doped Aluminum Clusters to Carbon Dioxide
Göbel, A., Rubio, A., & Lischner, J.
Charge transfer between molecules and catalysts plays a critical role in determining the efficiency and yield of photochemical catalytic processes. In this paper, we study light-induced electron transfer between transition-metal-doped aluminum clusters and CO2 molecules using first-principles time-dependent density-functional theory. Specifically, we carry out calculations for a range of dopants (Zr, Mn, Fe, Ru, Co, Ni, and Cu) and find that the resulting systems fall into two categories: Cu- and Fe-doped clusters exhibit no ground-state charge transfer, weak CO2 adsorption, and light-induced electron transfer into the CO2. In all other systems, we observe ground-state electron transfer into the CO2 resulting in strong adsorption and predominantly light-induced electron back-transfer from the CO2 into the cluster. These findings pave the way toward a rational design of atomically precise aluminum photocatalysts.
Down-conversion processes in ab initio nonrelativistic quantum electrodynamics
Welakuh, D., Ruggenthaler, M., Tchenkoue Djouom, M.-L., Appel, H., & Rubio, A.
The availability of efficient photon sources with specific properties is important for quantum-technological applications. However, the realization of such photon sources is often challenging and hence alternative perspectives that suggest different means to enhance desired properties while suppressing detrimental processes are valuable. In this work we highlight that ab initio simulations of coupled light-matter systems can provide such alternative avenues. We show for a simple model of a quantum ring that by treating light and matter on equal footing, we can create and enhance pathways for down-conversion processes. By changing the matter subsystem as well as the photonic environment in experimentally feasible ways, we can engineer hybrid light-matter states that enhance at the same time the efficiency of the down-conversion process and the nonclassicality of the created photons. Furthermore, we show that this also leads to a faster down-conversion, potentially avoiding detrimental decoherence effects.
Nonlinear electric conductivity and THz-induced charge transport in graphene
Sato, S., & Rubio, A.
Based on the quantum master equation approach, the nonlinear electric conductivity of graphene is investigated under static electric fields for various chemical potential shifts. The simulation results show that, as the field strength increases, the effective conductivity is firstly suppressed, reflecting the depletion of effective carriers due to the large displacement in the Brillouin zone caused by the strong field. Then, as the field strength exceeds 1 MV m−1, the effective conductivity increases, overcoming the carrier depletion via the Landau–Zener tunneling process. Based on the nonlinear behavior of the conductivity, the charge transport induced by few-cycle THz pulses is studied to elucidate the ultrafast control of electric current in matter.
Strong chiral dichroism and enantiopurification in above-threshold ionization with locally chiral light
Neufeld, O., Hübener, H., Rubio, A., & de Giovannini, U.
We derive here a highly selective photoelectron-based chirality-sensing technique that utilizes “locally chiral” laser pulses. We show that this approach results in strong chiral discrimination, where the standard forwards/backwards asymmetry of photoelectron circular dichroism (PECD) is lifted. The resulting dichroism is larger and more robust than conventional PECD (especially in the high-energy part of the spectrum), is found in all hemispheres, and is not symmetric or antisymmetric with respect to any symmetry operator. Remarkably, chiral dichroism of up to 10% survives in the angularly integrated above-threshold ionization (ATI) spectra, and chiral dichroism of up to 5% survives in the total ionization rates. We demonstrate these results through ab initio calculations in the chiral molecules bromochlorofluoromethane, limonene, fenchone, and camphor. We also explore the parameter space of the locally chiral field and show that the observed dichroism is strongly correlated to the degree of chirality of the light, validating it as a measure for chiral-interaction strengths. Our results pave the way for highly selective probing of ultrafast chirality in ATI and motivate the use of locally chiral light for enhancing ultrafast spectroscopies. Most importantly, the technique can be implemented to achieve all-optical enantiopurification of chiral samples.
Ultrafast dynamical Lifshitz transition
Beaulieu, S., Dong, S., Tancogne-Dejean, N., Dendzik, M., Pincelli, T., Maklar, J., Xian, R. P., Sentef, M. A., Wolf, M., Rubio, A., Rettig, L., & Ernstorfer, R.
Fermi surface is at the heart of our understanding of metals and strongly correlated many-body systems. An abrupt change in the Fermi surface topology, also called Lifshitz transition, can lead to the emergence of fascinating phenomena like colossal magnetoresistance and superconductivity. While Lifshitz transitions have been demonstrated for a broad range of materials by equilibrium tuning of macroscopic parameters such as strain, doping, pressure, and temperature, a nonequilibrium dynamical route toward ultrafast modification of the Fermi surface topology has not been experimentally demonstrated. Combining time-resolved multidimensional photoemission spectroscopy with state-of-the-art TDDFT+U simulations, we introduce a scheme for driving an ultrafast Lifshitz transition in the correlated type-II Weyl semimetal Td-MoTe2. We demonstrate that this nonequilibrium topological electronic transition finds its microscopic origin in the dynamical modification of the effective electronic correlations. These results shed light on a previously unexplored ultrafast scheme for controlling the Fermi surface topology in correlated quantum materials.
Identification of the Mott Insulating Charge Density Wave State in 1T−TaS2
Shin, D., Tancogne-Dejean, N., Zhang, J., Okyay, M. S., Rubio, A., & Park, N.
We investigate the low-temperature charge density wave (CDW) state of bulk TaS2 with a fully self-consistent density-functional theory with the Hubbard U potential, over which the controversy has remained unresolved regarding the out-of-plane metallic band. By examining the innate structure of the Hubbard U potential, we reveal that the conventional use of atomic-orbital basis could seriously misevaluate the electron correlation in the CDW state. By adopting a generalized basis, covering the whole David star, we successfully reproduce the Mott insulating nature with the layer-by-layer antiferromagnetic order. Similar consideration should be applied for description of the electron correlation in molecular solid.
Single and double charge transfer in the Ne2+ + He collision within time-dependent density-functional theory
Yu, W., Gao, C.-Z., Sato, S., Castro, A., Rubio, A., & Wei, B.
We calculate the charge-transfer cross sections for the Ne2++He collision. To this end, we employ Ehrenfest molecular dynamics with time-dependent density-functional theory. The active electrons of the projectile are handled by applying an initial velocity to the Kohn-Sham orbitals via a Galilean boost. The dynamical calculations are performed in an inverse collision framework—the reference frame considers Ne2+ to be initially at rest, which ensures numerically converged final-time scattering states. The charge-transfer probabilities are extracted by extending the particle number projection technique to be able to handle the degenerate Ne2+ ion. Compared with experimental data available at 10–3000 keV, a fairly good agreement is found for the calculated single- and double-charge transfer cross sections, superior to other theoretical calculations for this Ne2++He collision. A time-resolved analysis of the charge-transfer probabilities finds that ionization to the continuum also takes place after the charge transfer has occurred. To account for it, the final scattering states should be followed for a long time, approximately 350 fs, until they stabilize.
Simulating Vibronic Spectra without Born–Oppenheimer Surfaces
Lively, K., Albareda Piquer, G., Sato, S., Kelly, A., & Rubio, A.
We show how linear vibronic spectra in molecular systems can be simulated efficiently using first-principles approaches without relying on the explicit use of multiple Born–Oppenheimer potential energy surfaces. We demonstrate and analyze the performance of mean-field and beyond-mean-field dynamics techniques for the H2 molecule in one dimension, in the later case capturing the vibronic structure quite accurately, including quantum Franck–Condon effects. In a practical application of this methodology we simulate the absorption spectrum of benzene in full dimensionality using time-dependent density functional theory at the multitrajectory Ehrenfest level, finding good qualitative agreement with experiment and significant spectral reweighting compared to commonly used single-trajectory Ehrenfest dynamics. These results form the foundation for nonlinear spectral calculations and show promise for future application in capturing phenomena associated with vibronic coupling in more complex molecular and potentially condensed phase systems.
Higher-Order Band Topology in Twisted Moiré Superlattice.
Liu, B., Xian, L. D., Mu, H., Zhao, G., Liu, Z., Rubio, A., & Wang, Z.
The two-dimensional (2D) twisted bilayer materials with van der Waals coupling have ignited great research interests, paving a new way to explore the emergent quantum phenomena by twist degree of freedom. Generally, with the decreasing of twist angle, the enhanced interlayer coupling will gradually flatten the low-energy bands and isolate them by two high-energy gaps at zero and full filling, respectively. Although the correlation and topological physics in the low-energy flat bands have been intensively studied, little information is available for these two emerging gaps. In this Letter, we predict a 2D second-order topological insulator (SOTI) for twisted bilayer graphene and twisted bilayer boron nitride in both zero and full filling gaps. Employing a tight-binding Hamiltonian based on first-principles calculations, three unique fingerprints of 2D SOTI are identified, that is, nonzero bulk topological index, gapped topological edge state, and in-gap topological corner state. Most remarkably, the 2D SOTI exists in a wide range of commensurate twist angles, which is robust to microscopic structure disorder and twist center, greatly facilitating the possible experimental measurement. Our results not only extend the higher-order band topology to massless and massive twisted moiré superlattice, but also demonstrate the importance of high-energy bands for fully understanding the nontrivial electronics.
Hydrated Alkali Atoms on Copper(111): A Density Functional Theory Study
Pérez Paz, A., & Rubio, A.
We present a systematic computational study of submonolayer coverage of alkali atoms (Na, K, Cs) on Cu(111) surface hydrated from 1 to 6 water molecules. Our calculations show that water molecules preferentially bind to the adsorbed alkali ion and that a gradual detachment of the alkali from the Cu(111) surface is found as the hydration increases. This decoupling of the alkali from the Cu(111) surface results in a linear decrease of the charge transfer to the substrate. The orientation of the water dipoles pointing toward the surface leads to a gradual increase of the work function of the substrate as the number of coordinated water molecules increases from 1 to 4. Beyond 5 coordinated water molecules, the alkali adatom becomes saturated, and water adsorption sets in on the Cu(111) surface with the expected decrease in the work function of the system, as measured in two-photon photoemission spectroscopy (2PPE) experiments. From the detailed analysis of the orientation of the water electric dipoles, we were able to understand the experimentally observed initial increase of work function upon hydration and its subsequent decrease after saturation of alkali sites with water molecules. From the calculated energetics, we gauge the relative strengths of the alkali–Cu(111), alkali–water, and water–Cu(111) interactions as we move across the alkaline group. We found an excellent linear correlation between experimental water desorption temperatures and our computed water–alkali binding energies on Cu(111).
Real-time observation of a correlation-driven sub 3 fs charge migration in ionised adenine
Erik P. Månsson, Simone Latini, Fabio Covito, Vincent Wanie, Mara Galli, Enrico Perfetto, Gianluca Stefanucci, Hannes Hübener, Umberto De Giovannini, Mattea C. Castrovilli, Andrea Trabattoni, Fabio Frassetto, Luca Poletto, Jason B. Greenwood, François Légaré, Mauro Nisoli, Angel Rubio & Francesca Calegari
Sudden ionisation of a relatively large molecule can initiate a correlation-driven process dubbed charge migration, where the electron density distribution is expected to rapidly move along the molecular backbone. Capturing this few-femtosecond or attosecond charge redistribution would represent the real-time observation of electron correlation in a molecule with the enticing prospect of following the energy flow from a single excited electron to the other coupled electrons in the system. Here, we report a time-resolved study of the correlation-driven charge migration process occurring in the nucleic-acid base adenine after ionisation with a 15–35 eV attosecond pulse. We find that the production of intact doubly charged adenine – via a shortly-delayed laser-induced second ionisation event – represents the signature of a charge inflation mechanism resulting from many-body excitation. This conclusion is supported by first-principles time-dependent simulations. These findings may contribute to the control of molecular reactivity at the electronic, few-femtosecond time scale.
Moiréless correlations in ABCA graphene
Kerelsky, A., Rubio-Verdú, C., Xian, L. D., Kennes, D. M., Halbertal, D., Finney, N., Song, L., Turkel, S., Wang, L., Watanabe, K., Taniguchi, T., Hone, J., Dean, C., Basov, D., Rubio, A., & Pasupathy, A. N.
Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform toward achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. In this work we demonstrate the formation of emergent correlated phases in multilayer rhombohedral graphene––a simple material that also exhibits a flat electronic band edge but without the need of having a moiré superlattice induced by twisted van der Waals layers. We show that two layers of bilayer graphene that are twisted by an arbitrary tiny angle host large (micrometer-scale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independently studied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp van Hove singularity of 3–5-meV half-width. We demonstrate that when this van Hove singularity straddles the Fermi level, a correlated many-body gap emerges with peak-to-peak value of 9.5 meV at charge neutrality. Mean-field theoretical calculations for model with short-ranged interactions indicate that two primary candidates for the appearance of this broken symmetry state are a charge-transfer excitonic insulator and a ferrimagnet. Finally, we show that ABCA graphene hosts surface topological helical edge states at natural interfaces with ABAB graphene which can be turned on and off with gate voltage, implying that small-angle twisted double-bilayer graphene is an ideal programmable topological quantum material.
Vibrational coherent control of localized d–d electronic excitation
Alexandre Marciniak, Stefano Marcantoni, Francesca Giusti, Filippo Glerean, Giorgia Sparapassi, Tobia Nova, Andrea Cartella, Simone Latini, Francesco Valiera, Angel Rubio, Jeroen van den Brink, Fabio Benatti & Daniele Fausti
Addressing the role of quantum coherence in the interplay between the different matter constituents (electrons, phonons and spin) is a critical step towards understanding transition metal oxides and designing complex materials with new functionalities. Here we use coherent vibrational control of on-site d–d electronic transitions in a model edge-sharing insulating transition metal oxide (CuGeO3) to single out the effects of vibrational coherence in electron–phonon coupling. By comparing time-domain experiments based on high- and low-photon-energy ultrashort laser excitation pulses with a fully quantum description of phonon-assisted absorption, we could distinguish the processes associated with incoherent thermal lattice fluctuations from those driven by the coherent motion of the atoms. In particular, while thermal fluctuations of the phonon bath uniformly increase the electronic absorption, the resonant excitation of phonon modes also results in light-induced transparency that is coherently controlled by the vibrational motion.
Moiré metrology of energy landscapes in van der Waals heterostructures
Dorri Halbertal, Nathan R. Finney, Sai S. Sunku, Alexander Kerelsky, Carmen Rubio-Verdú, Sara Shabani, Lede Xian, Stephen Carr, Shaowen Chen, Charles Zhang, Lei Wang, Derick Gonzalez-Acevedo, Alexander S. McLeod, Daniel Rhodes, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Cory R. Dean, James C. Hone, Abhay N. Pasupathy, Dante M. Kennes, Angel Rubio & D. N. Basov
The emerging field of twistronics, which harnesses the twist angle between two-dimensional materials, represents a promising route for the design of quantum materials, as the twist-angle-induced superlattices offer means to control topology and strong correlations. At the small twist limit, and particularly under strain, as atomic relaxation prevails, the emergent moiré superlattice encodes elusive insights into the local interlayer interaction. Here we introduce moiré metrology as a combined experiment-theory framework to probe the stacking energy landscape of bilayer structures at the 0.1 meV/atom scale, outperforming the gold-standard of quantum chemistry. Through studying the shapes of moiré domains with numerous nano-imaging techniques, and correlating with multi-scale modelling, we assess and refine first-principle models for the interlayer interaction. We document the prowess of moiré metrology for three representative twisted systems: bilayer graphene, double bilayer graphene and H-stacked MoSe2/WSe2. Moiré metrology establishes sought after experimental benchmarks for interlayer interaction, thus enabling accurate modelling of twisted multilayers.
High-order harmonic generation in graphene: nonlinear coupling of intra and interband transitions
Shunsuke A. Sato, Hideki Hirori, Yasuyuki Sanari, Yoshihiko Kanemitsu, Angel Rubio
We investigate high-order harmonic generation (HHG) in graphene with a quantum master equation approach. The simulations reproduce the observed enhancement in HHG in graphene under elliptically polarized light [N. Yoshikawa et al, Science 356, 736 (2017)]. On the basis of a microscopic decomposition of the emitted high-order harmonics, we find that the enhancement in HHG originates from an intricate nonlinear coupling between the intraband and interband transitions that are respectively induced by perpendicular electric field components of the elliptically polarized light. Furthermore, we reveal that contributions from different excitation channels destructively interfere with each other. This finding suggests a path to potentially enhance the HHG by blocking a part of the channels and canceling the destructive interference through band-gap or chemical potential manipulation.
Entangled photon assisted multidimensional nonlinear optics of exciton–polaritons
Debnath, A., & Rubio, A.
We present a theoretical formulation of the frequency domain multidimensional pump-probe analog spectroscopy, which utilizes the spectral–temporal entanglement features of the biphoton sources. It has been shown, via a compact multi-time, convolutional Green’s function expression and the accompanying numerical simulations, that utilizing the correlation properties of non-classical sources offers a viable scheme for the exploration of dissipative kinetics of the cavity confined quantum aggregates. The cooperative and competitive modifications brought in by the photonic cavity mode and the auxiliary vibrational modes into the scattering and dephasing properties of the exciton–polaritons have been explored via their signatures in the multidimensional correlation maps. The study offers a new parameter window for the investigation of the dynamical polariton characteristics and warrants the usage of multi-mode entanglement properties of the external photonic sources in future studies.
Journal of Applied Physics 128, 113102 (2020)
High-harmonic generation from spin-polarised defects in solids
Mrudul, M. S., Tancogne-Dejean, N., Rubio, A., & Dixit, G.
The generation of high-order harmonics in gases enabled to probe the attosecond electron dynamics in atoms and molecules with unprecedented resolution. Extending these techniques to solids, which were originally developed for atomic and molecular gases, requires a fundamental understanding of the physics that has been partially addressed theoretically. Here, we employ time-dependent density-functional theory to investigate how the electron dynamics resulting in high-harmonic emission in monolayer hexagonal boron nitride is affected by the presence of vacancies. We show how these realistic spin-polarised defects modify the harmonic emission and demonstrate that important differences exist between harmonics from a pristine solid and a defected solid. In particular, we found that the different spin channels are affected differently by the presence of the spin-polarised point defect. Moreover, the localisation of the wavefunction, the geometry of the defect, and the electron–electron interaction are all crucial ingredients to describe high-harmonic generation in defected solids.
Electron–phonon-driven three-dimensional metallicity in an insulating cuprate
Baldini, E., Sentef, M. A., Acharya, S., Brumme, T., Sheveleva, E., Lyzwa, F., et al.
The role of the crystal lattice for the electronic properties of cuprates and other high-temperature superconductors remains controversial despite decades of theoretical and experimental efforts. While the paradigm of strong electronic correlations suggests a purely electronic mechanism behind the insulator-to-metal transition, recently the mutual enhancement of the electron–electron and the electron–phonon interaction and its relevance to the formation of the ordered phases have also been emphasized. Here, we combine polarization-resolved ultrafast optical spectroscopy and state-of-the-art dynamical mean-field theory to show the importance of the crystal lattice in the breakdown of the correlated insulating state in an archetypal undoped cuprate. We identify signatures of electron–phonon coupling to specific fully symmetric optical modes during the buildup of a three-dimensional (3D) metallic state that follows charge photodoping. Calculations for coherently displaced crystal structures along the relevant phonon coordinates indicate that the insulating state is remarkably unstable toward metallization despite the seemingly large charge-transfer energy scale. This hitherto unobserved insulator-to-metal transition mediated by fully symmetric lattice modes can find extensive application in a plethora of correlated solids.
Correlated electronic phases in twisted bilayer transition metal dichalcogenides
Wang, L., Shih, E.-M., Ghiotto, A., Xian, L. D., Rhodes, D. A., Tan, C., Claassen, M., Kennes, D. M., Bai, Y., Kim, B., Watanabe, K., Taniguchi, T., Zhu, X., Hone, J., Rubio, A., Pasupathy, A., & Dean, C. R.
In narrow electron bands in which the Coulomb interaction energy becomes comparable to the bandwidth, interactions can drive new quantum phases. Such flat bands in twisted graphene-based systems result in correlated insulator, superconducting and topological states. Here we report evidence of low-energy flat bands in twisted bilayer WSe2, with signatures of collective phases observed over twist angles that range from 4 to 5.1°. At half-band filling, a correlated insulator appeared that is tunable with both twist angle and displacement field. At a 5.1° twist, zero-resistance pockets were observed on doping away from half filling at temperatures below 3 K, which indicates a possible transition to a superconducting state. The observation of tunable collective phases in a simple band, which hosts only two holes per unit cell at full filling, establishes twisted bilayer transition metal dichalcogenides as an ideal platform to study correlated physics in two dimensions on a triangular lattice.
Light–Matter Hybrid-Orbital-Based First-Principles Methods: The Influence of Polariton Statistics
Florian Buchholz, Iris Theophilou, Klaas J. H. Giesbertz, Michael Ruggenthaler, and Angel Rubio
A detailed understanding of strong matter–photon interactions requires first-principle methods that can solve the fundamental Pauli–Fierz Hamiltonian of nonrelativistic quantum electrodynamics efficiently. A possible way to extend well-established electronic-structure methods to this situation is to embed the Pauli–Fierz Hamiltonian in a higher-dimensional light–matter hybrid auxiliary configuration space. In this work we show the importance of the resulting hybrid Fermi–Bose statistics of the polaritons, which are the new fundamental particles of the “photon-dressed” Pauli–Fierz Hamiltonian for systems in cavities. We show that violations of these statistics can lead to unphysical results. We present an efficient way to ensure the correct statistics by enforcing representability conditions on the dressed one-body reduced density matrix. We further present a general prescription how to extend a given first-principles approach to polaritons and as an example introduce polaritonic Hartree–Fock theory. While being a single-reference method in polariton space, polaritonic Hartree–Fock is a multireference method in the electronic space, i.e., it describes electronic correlations. We also discuss possible applications to polaritonic QEDFT. We apply this theory to a lattice model and find that, the more delocalized the bound-state wave function of the particles is, the stronger it reacts to photons. The main reason is that within a small energy range, many states with different electronic configurations are available as opposed to a strongly bound (and hence energetically separated) ground-state wave function. This indicates that under certain conditions coupling to the quantum vacuum of a cavity can indeed modify ground state properties.
Coupled Cluster Theory for Molecular Polaritons: Changing Ground and Excited States
Tor S. Haugland, Enrico Ronca, Eirik F. Kjønstad, Angel Rubio, and Henrik Koch
We present an ab initio correlated approach to study molecules that interact strongly with quantum fields in an optical cavity. Quantum electrodynamics coupled cluster theory provides a nonperturbative description of cavity-induced effects in ground and excited states. Using this theory, we show how quantum fields can be used to manipulate charge transfer and photochemical properties of molecules. We propose a strategy to lift electronic degeneracies and induce modifications in the ground-state potential energy surface close to a conical intersection.
Floquet dynamics in light-driven solid
M. Nuske, L. Broers, B. Schulte, G. Jotzu, S. A. Sato, A. Cavalleri, A. Rubio, J. W. McIver, L. Mathey
We demonstrate how the properties of light-induced electronic Floquet states in solids impact natural physical observables, such as transport properties, by capturing the environmental influence on the electrons. We include the environment as dissipative processes, such as inter-band decay and dephasing, often ignored in Floquet predictions. These dissipative processes determine the Floquet band occupations of the emergent steady state, by balancing out the optical driving force. In order to benchmark and illustrate our framework for Floquet physics in a realistic solid, we consider the light-induced Hall conductivity in graphene recently reported by J.~W.~McIver, et al., Nature Physics (2020). We show that the Hall conductivity is estimated by the Berry flux of the occupied states of the light-induced Floquet bands, in addition to the kinetic contribution given by the average band velocity. Hence, Floquet theory provides an interpretation of this Hall conductivity as a geometric-dissipative effect. We demonstrate this mechanism within a master equation formalism, and obtain good quantitative agreement with the experimentally measured Hall conductivity, underscoring the validity of this approach which establishes a broadly applicable framework for the understanding of ultrafast non-equilibrium dynamics in solids.
Role of intraband dynamics in the generation of circularly polarized high harmonics from solids
N. Klemke, O. D. Mücke, A. Rubio, F. X. Kärtner, and N. Tancogne-Dejean
Recent studies have demonstrated that the polarization states of high harmonics from solids can differ from those of the driving pulses. To gain insights on the microscopic origin of this behavior, we perform one-particle intraband-only calculations and reproduce some of the most striking observations. For instance, our calculations yield circularly polarized harmonics from elliptically polarized pulses that sensitively depend on the driving conditions. Furthermore, we perform experiments on ZnS and find characteristics partly similar to those reported from silicon. Comparison to our intraband-only calculations shows reasonable qualitative agreement for a below-band-gap harmonic. We show that intraband dynamics predict depolarization effects that gain significance with higher field strengths and we observe such effects in the experimental data. For harmonics above the band gap, interband dynamics become important and the high-harmonic response to elliptical excitation looks systematically different. Our work proposes a method to distinguish between different high-harmonic generation mechanisms and it could pave the way to compact solid-state high-harmonic sources with controllable polarization states.
Room Temperature Terahertz Electroabsorption Modulation by Excitons in Monolayer Transition Metal Dichalcogenides
Shi, J., Baldini, E., Latini, S., Sato, S., Zhang, Y., Pein, B. C., Shen, P.-C., Kong, J., Rubio, A., Gedik, N., & Nelson, K. A.
The interaction between off-resonant laser pulses and excitons in monolayer transition metal dichalcogenides is attracting increasing interest as a route for the valley-selective coherent control of the exciton properties. Here, we extend the classification of the known off-resonant phenomena by unveiling the impact of a strong THz field on the excitonic resonances of monolayer MoS2. We observe that the THz pump pulse causes a selective modification of the coherence lifetime of the excitons, while keeping their oscillator strength and peak energy unchanged. We rationalize these results theoretically by invoking a hitherto unobserved manifestation of the Franz–Keldysh effect on an exciton resonance. As the modulation depth of the optical absorption reaches values as large as 0.05 dB/nm at room temperature, our findings open the way to the use of semiconducting transition metal dichalcogenides as compact and efficient platforms for high-speed electroabsorption devices.
Light-Induced Renormalization of the Dirac Quasiparticles in the Nodal-Line Semimetal ZrSiSe
G. Gatti, A. Crepaldi, M. Puppin, N. Tancogne-Dejean, L. Xian, U. De Giovannini, S. Roth, S. Polishchuk, Ph. Bugnon, A. Magrez, H. Berger, F. Frassetto, L. Poletto, L. Moreschini, S. Moser, A. Bostwick, Eli Rotenberg, A. Rubio, M. Chergui, and M. Grioni
In nodal-line semimetals, linearly dispersing states form Dirac loops in the reciprocal space with a high degree of electron-hole symmetry and a reduced density of states near the Fermi level. The result is reduced electronic screening and enhanced correlations between Dirac quasiparticles. Here we investigate the electronic structure of ZrSiSe, by combining time- and angle-resolved photoelectron spectroscopy with ab initio density functional theory (DFT) complemented by an extended Hubbard model (DFT+U+V) and by time-dependent DFT+U+V. We show that electronic correlations are reduced on an ultrashort timescale by optical excitation of high-energy electrons-hole pairs, which transiently screen the Coulomb interaction. Our findings demonstrate an all-optical method for engineering the band structure of a quantum material.
Virial Relations for Electrons Coupled to Quantum Field Modes
Theophilou, I., Penz, M., Ruggenthaler, M., & Rubio, A.
In this work, we present a set of virial relations for many electron systems coupled to both classical and quantum fields, described by the Pauli–Fierz Hamiltonian in dipole approximation and using length gauge. Currently, there is growing interest in solutions of this Hamiltonian because of its relevance for describing molecular systems strongly coupled to photonic modes in cavities and in the possible modification of chemical properties of such systems compared to the ones in free space. The relevance of such virial relations is demonstrated by showing a connection to mass renormalization and by providing an exact way to obtain total energies from potentials in the framework of quantum electrodynamical density functional theory.
Effect of many modes on self-polarization and photochemical suppression in cavities
Hoffmann, N., Lacombe, L., Rubio, A., & Maitra, N. T.
The standard description of cavity-modified molecular reactions typically involves a single (resonant) mode, while in reality, the quantum cavity supports a range of photon modes. Here, we demonstrate that as more photon modes are accounted for, physicochemical phenomena can dramatically change, as illustrated by the cavity-induced suppression of the important and ubiquitous process of proton-coupled electron-transfer. Using a multi-trajectory Ehrenfest treatment for the photon-modes, we find that self-polarization effects become essential, and we introduce the concept of self-polarization-modified Born–Oppenheimer surfaces as a new construct to analyze dynamics. As the number of cavity photon modes increases, the increasing deviation of these surfaces from the cavity-free Born–Oppenheimer surfaces, together with the interplay between photon emission and absorption inside the widening bands of these surfaces, leads to enhanced suppression. The present findings are general and will have implications for the description and control of cavity-driven physical processes of molecules, nanostructures, and solids embedded in cavities.
Parameter-free hybridlike functional based on an extended Hubbard model: DFT+U+V
Tancogne-Dejean, N., & Rubio, A.
In this paper, we propose an energy functional at the level of DFT+U+V that allows us to compute self-consistently the values of the onsite interaction, Hubbard U and Hund J, as well as the intersite interaction V. This functional extends the previously proposed ACBN0 functional [L. A. Agapito et al., Phys. Rev. X 5, 011006 (2015)] including both onsite and intersite interactions. We show that this ab initio self-consistent functional yields improved electronic properties for a wide range of materials, ranging from sp materials to strongly correlated materials. This functional can also be seen as an alternative general and systematic way to construct parameter-free hybrid functionals, based on the extended Hubbard model and a selected set of Coulomb integrals, and might be used to develop novel approximations. By extending the DFT+U method to materials where strong local and nonlocal interactions are relevant, this work opens the door to the ab initio study the electronic, ionic, and optical properties of a larger class of strongly correlated materials in and out of equilibrium.
Dynamical amplification of electric polarization through nonlinear phononics in 2D SnTe
Shin, D., Sato, S., Hübener, H., de Giovannini, U., Park, N., & Rubio, A.
Ultrafast optical control of ferroelectricity using intense terahertz fields has attracted significant interest. Here we show that the nonlinear interactions between two optical phonons in SnTe, a two-dimensional in-plane ferroelectric material, enables a dynamical amplification of the electric polarization within subpicoseconds time domain. Our first-principles time-dependent simulations show that the infrared-active out-of-plane phonon mode, pumped to nonlinear regimes, spontaneously generates in-plane motions, leading to rectified oscillations in the in-plane electric polarization. We suggest that this dynamical control of ferroelectric material, by nonlinear phonon excitation, can be utilized to achieve ultrafast control of the photovoltaic or other nonlinear optical responses.
Ultrafast Real-Time Dynamics of CO Oxidation over an Oxide Photocatalyst
Wagstaffe, M., Wenthaus, L., Dominguez-Castro, A., Chung, S., Lana Semione, G. D., Palutke, S., Mercurio, G., Dziarzhytski, S., Redlin, H., Klemke, N., Yang, Y., Frauenheim, T., Dominguez, A., Kärtner, F., Rubio, A., Wurth, W., Stierle, A., & Noei, H.
Femtosecond X-ray laser pulses synchronized with an optical laser were employed to investigate the reaction dynamics of the photooxidation of CO on the anatase TiO2(101) surface in real time. Our time-resolved soft X-ray photoemission spectroscopy results provide evidence of ultrafast timescales and, coupled with theoretical calculations, clarify the mechanism of oxygen activation that is crucial to unraveling the underlying processes for a range of photocatalytic reactions relevant to air purification and self-cleaning surfaces. The reaction takes place between 1.2 and 2.8 (±0.2) ps after irradiation with an ultrashort laser pulse leading to the formation of CO2, prior to which no intermediate species were observed on a picosecond time scale. Our theoretical calculations predict that the presence of intragap unoccupied O2 levels leads to the formation of a charge-transfer complex. This allows the reaction to be initiated following laser illumination at a photon energy of 1.6 eV (770 nm), taking place via a proposed mechanism involving the direct transfer of electrons from TiO2 to physisorbed O2.
Charge-Transfer Plasmon Polaritons at Graphene/α-RuCl3 Interfaces
Daniel J. Rizzo, Bjarke S. Jessen, Zhiyuan Sun, Francesco L. Ruta, Jin Zhang, Jia-Qiang Yan, Lede Xian, Alexander S. McLeod, Michael E. Berkowitz, Kenji Watanabe, Takashi Taniguchi, Stephen E. Nagler, David G. Mandrus, Angel Rubio, Michael M. Fogler, Andrew J. Millis, James C. Hone, Cory R. Dean, and D. N. Basov
Nanoscale charge control is a key enabling technology in plasmonics, electronic band structure engineering, and the topology of two-dimensional materials. By exploiting the large electron affinity of α-RuCl3, we are able to visualize and quantify massive charge transfer at graphene/α-RuCl3 interfaces through generation of charge-transfer plasmon polaritons (CPPs). We performed nanoimaging experiments on graphene/α-RuCl3 at both ambient and cryogenic temperatures and discovered robust plasmonic features in otherwise ungated and undoped structures. The CPP wavelength evaluated through several distinct imaging modalities offers a high-fidelity measure of the Fermi energy of the graphene layer: EF = 0.6 eV (n = 2.7 × 1013 cm–2). Our first-principles calculations link the plasmonic response to the work function difference between graphene and α-RuCl3 giving rise to CPPs. Our results provide a novel general strategy for generating nanometer-scale plasmonic interfaces without resorting to external contacts or chemical doping.
Photoelectron spectroscopy of large water clusters ionized by an XUV comb
Andrea Trabattoni, Lorenzo Colaizzi, Loren Ban, Vincent Wanie, Krishna Saraswathula, Erik P Månsson, Philipp Rupp, Qingcao Liu, Lennart Seiffert, Elisabeth A Herzig, Andrea Cartella, Bruce L Yoder, François Légaré, Matthias F Kling, Thomas Fennel, Ruth Signorell and Francesca Calegari
Detailed knowledge about photo-induced electron dynamics in water is key to the understanding of several biological and chemical mechanisms, in particular for those resulting from ionizing radiation. Here we report a method to obtain photoelectron spectra from neutral water clusters following ionization by an extreme-ultraviolet (XUV) attosecond pulse train, representing a first step towards a time-resolved analysis. Typically, a large background signal in the experiment arises from water monomers and carrier gas used in the cluster source. We report a protocol to quantify this background in order to eliminate it from the experimental spectra. We disentangle the accumulated XUV photoionization signal into contributions from the background species and the photoelectron spectra from the clusters. This proof-of-principle study demonstrates feasibility of background free photoelectron spectra of neutral water clusters ionized by XUV combs and paves the way for the detailed time-resolved analysis of the underlying dynamics.
Relevance of the quadratic diamagnetic and self-polarization terms in cavity quantum electrodynamics
Christian Schäfer, Michael Ruggenthaler, Vasil Rokaj, Angel Rubio
Experiments at the interface of quantum optics and chemistry have revealed that strong coupling between light and matter can substantially modify the chemical and physical properties of molecules and solids. While the theoretical description of such situations is usually based on nonrelativistic quantum electrodynamics, which contains quadratic light–matter coupling terms, it is commonplace to disregard these terms and restrict the treatment to purely bilinear couplings. In this work, we clarify the physical origin and the substantial impact of the most common quadratic terms, the diamagnetic and self-polarization terms, and highlight why neglecting them can lead to rather unphysical results. Specifically, we demonstrate their relevance by showing that neglecting these terms leads to the loss of gauge invariance, basis set dependence, disintegration (loss of bound states) of any system in the basis set limit, unphysical radiation of the ground state, and an artificial dependence on the static dipole. Besides providing important guidance for modeling of strongly coupled light–matter systems, the presented results also indicate conditions under which those effects might become accessible.
Octopus, a computational framework for exploring light-driven phenomena and quantum dynamics in extended and finite systems
Nicolas Tancogne-Dejean, Micael J. T. Oliveira, Xavier Andrade, Heiko Appel, Carlos H. Borca, Guillaume Le Breton, Florian Buchholz, Alberto Castro, Stefano Corni, Alfredo A. Correa, Umberto De Giovannini, Alain Delgado, Florian G. Eich, Johannes Flick, Gabriel Gil, Adrián Gomez, Nicole Helbig, Hannes Hübener, René Jestädt, Joaquim Jornet-Somoza, Ask H. Larsen, Irina V. Lebedeva, Martin Lüders, Miguel A. L. Marques, Sebastian T. Ohlmann, Silvio Pipolo, Markus Rampp, Carlo A. Rozzi, David A. Strubbe, Shunsuke A. Sato, Christian Schäfer, Iris Theophilou, Alicia Welden, and Angel Rubio
Over the last few years, extraordinary advances in experimental and theoretical tools have allowed us to monitor and control matter at short time and atomic scales with a high degree of precision. An appealing and challenging route toward engineering materials with tailored properties is to find ways to design or selectively manipulate materials, especially at the quantum level. To this end, having a state-of-the-art ab initio computer simulation tool that enables a reliable and accurate simulation of light-induced changes in the physical and chemical properties of complex systems is of utmost importance. The first principles real-space-based Octopus project was born with that idea in mind, i.e., to provide a unique framework that allows us to describe non-equilibrium phenomena in molecular complexes, low dimensional materials, and extended systems by accounting for electronic, ionic, and photon quantum mechanical effects within a generalized time-dependent density functional theory. This article aims to present the new features that have been implemented over the last few years, including technical developments related to performance and massive parallelism. We also describe the major theoretical developments to address ultrafast light-driven processes, such as the new theoretical framework of quantum electrodynamics density-functional formalism for the description of novel light–matter hybrid states. Those advances, and others being released soon as part of the Octopus package, will allow the scientific community to simulate and characterize spatial and time-resolved spectroscopies, ultrafast phenomena in molecules and materials, and new emergent states of matter (quantum electrodynamical-materials).
Light-matter interactions within the Ehrenfest–Maxwell–Pauli–Kohn–Sham framework: fundamentals, implementation, and nano-optical applications
René Jestädt, Michael Ruggenthaler, Micael J. T. Oliveira, Angel Rubio &Heiko Appel
In recent years significant experimental advances in nano-scale fabrication techniques and in available light sources have opened the possibility to study a vast set of novel light-matter interaction scenarios, including strong coupling cases. In many situations nowadays, classical electromagnetic modeling is insufficient as quantum effects, both in matter and light, start to play an important role. Instead, a fully self-consistent and microscopic coupling of light and matter becomes necessary. We provide here a critical review of current approaches for electromagnetic modeling, highlighting their limitations. We show how to overcome these limitations by introducing the theoretical foundations and the implementation details of a density-functional approach for coupled photons, electrons, and effective nuclei in non-relativistic quantum electrodynamics. Starting point of the formalism is a generalization of the Pauli–Fierz field theory for which we establish a one-to-one correspondence between external fields and internal variables. Based on this correspondence, we introduce a Kohn-Sham construction which provides a computationally feasible approach for ab-initio light-matter interactions. In the mean-field limit, the formalism reduces to coupled Ehrenfest–Maxwell–Pauli–Kohn–Sham equations. We present an implementation of the approach in the real-space real-time code Octopus using the Riemann–Silberstein formulation of classical electrodynamics to rewrite Maxwell's equations in Schrödinger form. This allows us to use existing very efficient time-evolution algorithms developed for quantum-mechanical systems also for Maxwell's equations. We show how to couple the time-evolution of the electromagnetic fields self-consistently with the quantum time-evolution of the electrons and nuclei. This approach is ideally suited for applications in nano-optics, nano-plasmonics, (photo) electrocatalysis, light-matter coupling in 2D materials, cases where laser pulses carry orbital angular momentum, or light-tailored chemical reactions in optical cavities just to name but a few.
Nature of Symmetry Breaking at the Excitonic Insulator Transition: Ta2NiSe5
Giacomo Mazza, Malte Rösner, Lukas Windgätter, Simone Latini, Hannes Hübener, Andrew J. Millis, Angel Rubio, and Antoine Georges
Ta2NiSe5 is one of the most promising materials for hosting an excitonic insulator ground state. While a number of experimental observations have been interpreted in this way, the precise nature of the symmetry breaking occurring in Ta2NiSe5, the electronic order parameter, and a realistic microscopic description of the transition mechanism are, however, missing. By a symmetry analysis based on first-principles calculations, we uncover the discrete lattice symmetries which are broken at the transition. We identify a purely electronic order parameter of excitonic nature that breaks these discrete crystal symmetries and contributes to the experimentally observed lattice distortion from an orthorombic to a monoclinic phase. Our results provide a theoretical framework to understand and analyze the excitonic transition in Ta2NiSe5 and settle the fundamental questions about symmetry breaking governing the spontaneous formation of excitonic insulating phases in solid-state materials.
Force balance approach for advanced approximations in density functional theories
Tchenkoue Djouom, M.-L., Penz, M., Theophilou, I., Ruggenthaler, M., & Rubio, A.
We propose a systematic and constructive way to determine the exchange-correlation potentials of density-functional theories including vector potentials. The approach does not rely on energy or action functionals. Instead, it is based on equations of motion of current quantities (force balance equations) and is feasible both in the ground-state and the time-dependent settings. This avoids, besides differentiability and causality issues, the optimized-effective-potential procedure of orbital-dependent functionals. We provide straightforward exchange-type approximations for different density functional theories that for a homogeneous system and no external vector potential reduce to the exchange-only local-density and Slater Xα approximations.
Microscopic theory for the light-induced anomalous Hall effect in graphene
S. A. Sato, J. W. McIver, M. Nuske, P. Tang, G. Jotzu, B. Schulte, H. Hübener, U. De Giovannini, L. Mathey, M. A. Sentef, A. Cavalleri, and A. Rubio
We employ a quantum Liouville equation with relaxation to model the recently observed anomalous Hall effect in graphene irradiated by an ultrafast pulse of circularly polarized light. In the weak-field regime, we demonstrate that the Hall effect originates from an asymmetric population of photocarriers in the Dirac bands. By contrast, in the strong-field regime, the system is driven into a nonequilibrium steady state that is well described by topologically nontrivial Floquet-Bloch bands. Here, the anomalous Hall current originates from the combination of a population imbalance in these dressed bands together with a smaller anomalous velocity contribution arising from their Berry curvature. This robust and general finding enables the simulation of electrical transport from light-induced Floquet-Bloch bands in an experimentally relevant parameter regime and creates a pathway to designing ultrafast quantum devices with Floquet-engineered transport properties.
Light–Matter Response in Nonrelativistic Quantum Electrodynamics
Johannes Flick, Davis M. Welakuh, Michael Ruggenthaler, Heiko Appel, and Angel Rubio
We derive the full linear-response theory for nonrelativistic quantum electrodynamics in the long wavelength limit and provide a practical framework to solve the resulting equations by using quantum-electrodynamical density-functional theory. We highlight how the coupling between quantized light and matter changes the usual response functions and introduces cross-correlated light-matter response functions. These cross-correlation responses lead to measurable changes in Maxwell’s equations due to the quantum-matter-mediated photon–photon interactions. Key features of treating the combined matter-photon response are that natural lifetimes of excitations become directly accessible from first-principles, changes in the electronic structure due to strong light-matter coupling are treated fully nonperturbatively, and self-consistent solutions of the back-reaction of matter onto the photon vacuum and vice versa are accounted for. By introducing a straightforward extension of the random-phase approximation for the coupled matter-photon problem, we calculate the ab initio spectra for a real molecular system that is coupled to the quantized electromagnetic field. Our approach can be solved numerically very efficiently. The presented framework leads to a shift in paradigm by highlighting how electronically excited states arise as a modification of the photon field and that experimentally observed effects are always due to a complex interplay between light and matter. At the same time the findings provide a route to analyze as well as propose experiments at the interface between quantum chemistry, nanoplasmonics and quantum optics.