Abstract
Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical, and structural analyses requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines free-electron laser capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in (kx, ky, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å-1 diameter in a binding-energy range of several eV, resolving about 2.5 × 105 data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe2 measured at photon energies of 36.5 eV and 109.5 eV, we demonstrate an experimental energy resolution of 130 meV, a momentum resolution of 0.06 Å-1, and a system response function of 150 fs.
Highlights
X-ray free-electron lasers (XFELs) are a remarkable development in the range of tools for scientific experimentation
Momentum-resolved photoemission spectroscopy (PES) using momentum microscopy with time-of-flight (TOF) energy recording12,13 has been combined in static measurements with parallel spin detection,14–16 circular dichroism in the angular distribution CDAD (LDAD),17,18 and X-ray photoelectron diffraction (XPD)
In our series of experiments, we optically excite the samples with 775 nm light pulses and probe by mapping the electronic band structure via photoemission with 36.5 eV soft X-ray pulses delivered at the FLASH PG2 beamline
Summary
X-ray free-electron lasers (XFELs) are a remarkable development in the range of tools for scientific experimentation They are characterized by much enhanced peak brightness, many orders of magnitude higher than other X-ray sources, pulse durations on the order of a few tens of femtoseconds, the possibility for polarization control and wavelength tuning in a broad energy range, and multicolour operation mode. Adding temporal information on the subpicosecond timescale opens a new path to direct determination of the couplings between the electronic, spin, and lattice degrees of freedom Such studies are of central importance to quantum materials, since the couplings are relevant to nonequilibrium dynamics, and determine the materials’ ground-state properties
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