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.
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)
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.J. Phys. B 49, 165003 (2016)
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.Nature Photonics 10, 176 (2016)
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.
New J. Phys. 17, 043054 (2015)
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. Nature Communications 5, Article number: 364 (2014)
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.
Phys. Rev. Lett. 108, 253003 (2012)