Time-resolved HAXPES at SACLA: probe and pump pulse-induced space-charge effects

Abstract

Time-resolved hard x-ray photoelectron spectroscopy (trHAXPES) is established using the x-ray free-electron laser SACLA. The technique extends timeresolved photoemission into the hard x-ray regime and, as a core-level spectroscopy, combines element and atomic-site specificity and sensitivity to the chemical environment with femtosecond time resolution and bulk (sub-surface) sensitivity. The viability of trHAXPES using 8 keV x-ray free-electron-laser radiation is demonstrated by a systematic investigation of probe and pump pulseinduced vacuum space-charge effects on the V 1s emission of VO 2 and the Ti 1s emission of SrTiO 3 . The time and excitation energy dependencies of the measured spectral shifts and broadenings are compared to the results of N-body numerical simulations and simple analytic (mean-field) models. Good agreement between the experimental and calculated results is obtained. In particular, the 9 Present address: characteristic temporal evolution of the pump pulse-induced spectral shift is shown to provide an effective means to determine the temporal overlap of pump and probe pulses. trHAXPES opens a new avenue in the study of ultrafast atomic-site specific electron and chemical dynamics in materials and at buried interfaces.Keywords: time-resolved photoelectron spectroscopy, x-ray free-electron laser, space-charge effects IntroductionSub-picosecond time-resolved solid-state photoemission spectroscopy has recently emerged as a powerful novel technique for studying the electronic properties of condensed matter. The power of the technique is that it provides direct access to the electronic structure dynamics in materials and at their surfaces on the time scales of the underlying elementary electronic and lattice processes, such as electron-electron scattering, electron screening and thermalization, coherent phonon vibrations, electron-phonon and phonon-phonon coupling, as well as substrate-adsorbate charge transfer or the buildup of surface photovoltages. Time-resolved photoemission spectroscopy generally combines frequency-domain information with subpicosecond time resolution through a pump-probe scheme in which typically an infrared (IR) pump pulse is used to excite the system whose dynamics is then probed at different time delays by detecting the photoelectrons emitted by ultrashort pulses in the ultraviolet (UV) to soft x-ray regime. When probe pulses in the UV to extreme ultraviolet (XUV) range are used and angular resolution is added, the technique is referred to as time-and angle-resolved photoemission spectroscopy (trARPES) and provides direct information on the momentum-resolved dynamics of valence electrons, including the temporal evolution of electronic populations, band structures, Fermi surfaces, and energy gaps [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. When the probe pulses have higher photon energies, in the extreme ultraviolet to soft x-ray range, the technique becomes time-resolved x-ray photoemission spectroscopy (trXPS), with element specificity, sensitivi...

Spin-resolved photoelectron spectroscopy using femtosecond extreme ultraviolet light pulses from high-order harmonic generation

“…This behavior is similar to results from FEL measurements reported by Oloff et al 62 Here, the authors argue that the NIR pump-induced electrons are created after the excitation of the electrons from the valence band by the FEL probe-pulses. Since most of the mainly "slow" NIR pump-induced electrons stay behind the faster valence band electrons, the latter always experience an accelerating repulsion.…”

Electron Dynamics Probed by Time-Resolved Hard X-ray Photoelectron Spectroscopy

“…As we can see in Fig.3(b), the general trend of the temporal evolution of the space-charge effect is well reproduced by the model calculation. Details of the simulation will be described elsewhere [14]. From the experimental and simulation results of the pump-laser induced spacecharge dynamics, we have found that the temporal evolution gives us the precise definition of time zero for the pump-probe measurement as is discussed in the recent paper [15].…”

“…These low-KE electrons are mostly emitted by the multiphoton absorption due to the non-linear processes. This energy distribution curve was incorporated into the simulation using an extended 2D mean-field model [14]. In the simulation, we used the following parameters : Ekin(probe) = 3035.9 eV, Ekin(pump) = measured spectra shown in Fig.…”

The influence of x-ray pulse length on space-charge effects in optical pump/x-ray probe photoemission

“…Considering the nowadays achievable energy resolution at synchrotron facilities using stateof-the-art electron spectrometers (ΔE≈1 meV), it is clear that SC-effects can become the bottleneck for photoemission experiments. Dramatic SC-effects with energy shifts and broadenings of several eV can occur at FELs with very high photon intensities and reported electron numbers of many thousands per pulse ( [10][11][12]) or in high-intensity HHG sources [13]. Several analytic [14] and numerical models, e.g.…”

“…With Monte-Carlo simulations we find that the detailed dynamics of the Au 4f XPS spectra can be explained by describing the SC-effects with an analytic meanfield model of the pump electron cloud. While this model is well known and has been successfully used to describe the space-charge induced energy shift in several pump-probe XPS studies [12,21], our high resolution Au 4f spectra enable to test its applicability on the detailed line shape, in particular line broadening and asymmetry. We find that all observations can be understood within the mean-field model without the need to account for local electron-electron collision processes -also called stochastic scattering-.…”