Project A1

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.

J. Chem. Phys. Lett. 14, 24 (2023)                                               

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.

Phys. Rev. Research 4, 033231 (2022)

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.

Optica 9, 4, 439 (2022)

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.

Appl. Sci. 11, 9791 (2021)

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.

New J. Phys. 23, 043046 (2021)

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.

Optics Express 29 (23), 37429 (2021)

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.

Nature Physics 17, 512 (2021)

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.

Phys. Rev. A 103, 022832 (2021)

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.

Optics Express Vol. 29, Issue 21, pp. 32739-32754 (2021)

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.

J. Phys. B 53, 244006 (2020)

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 timescales^1,2,3. The reproducible generation and characterization of attosecond waveforms has been demonstrated so far only through the process of high-order harmonic generation^4,5,6,7. Several methods for shaping attosecond waveforms have been proposed, including the use of metallic filters^8,9, 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 millijoules^12,13. Recent experiments have shown that they can generate subfemtosecond spikes, but with temporal characteristics that change shot-to-shot^14,15,16. 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.

Nature 578, 386–391(2020)

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.

Phys. Rev. X 10, 031070 (2020)

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.

Meas. Sci. Technol. 31, 035901 (2020)

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.

J. Phys. B: At. Mol. Opt. Phys. 53, 184004 (2020)

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.

Physical Review A 102, 022807 (2020)

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.

Phys. Rev. Lett. 123, 213904 (2019)

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)

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.

Phys. Rev. A 100, 033404 (2019)

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.

J. Phys. B: At. Mol. Opt. Phys. 51 032003 (2018)

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.

Phys. Rev. Lett. 119, 073203 (2017)

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.

Appl. Sci. 7, 915 (2017)

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.

Phys. Rev. Lett. 118, 013002 (2017)

S. Düsterer, G. Hartmann, F. Babies, A. Beckmann et. al.

K.C. Prince, E. Allaria, C. Callegari, R. Cucini et. al

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.

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.

New J. Phys. 18, 113055 (2016)

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.

Nature Communications 6, Article no 6799 (2015)

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.

Electr. Spectr. Rel. Phenom. 204B, 313

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.

Phys. Rev. A 91, 063411

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.

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.

Nature Communications 6, 5938

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. Nature Communications 5, Article number: 364 (2014)

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.

J. Phys. B: At. Mol. Opt. Phys. 46, 192001 (2013)

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.
Phys. Rev. Lett. 108, 253003 (2012)