Time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser

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.

Observation of an Excitonic Mott Transition Through Ultrafast Core- cum -Conduction Photoemission Spectroscopy

“…In the absence of QFCs, AðωÞ reduces to a Lorentzian profile with width dictated by γ, while at a finite QFC density, the real and imaginary parts of the self-energy are responsible for the shift of the core energy and the asymmetric line shape. Qualitatively similar results of core-hole renormalization were observed for both W 4f and Se 3d states [35]. Although quantitative differences due to different screening interactions with excited carriers occur, the observed dynamics of these core-level states are consistent.…”

“…We performed core-cum-conduction trXPS experiments of WSe 2 using the FLASH free-electron laser and optical pump pulses tuned to the optical A-exciton resonance at 1.6 eV at room temperature. A time-of-flight (TOF) momentum microscope was used as the photoelectron analyzer, which enabled us to simultaneously probe a ∼40 eV-broad spectral window including the excited states, valence band, and the highest W and Se core levels [28,35]. Schematics of the setup and a model conduction-core energy level diagram are shown in the Figs.…”

Ultrafast molecular orbital tomography of a pentacene thin film using time-resolved momentum microscopy at a free-electron laser

“…Pump and probe pulses impinge on the sample at a polar angle of θ = 68 • and an azimuthal angle of φ = 64 • with respect to the [-1 1 0] direction of the Ag(110) surface, and are aligned to have spacial overlap. The photoelectrons emitted into the hemisphere above the sample are detected by a novel TOF momentum microscope 27,28 and recorded according to their wave vector and kinetic energy. The photon wavelength of the FEL was 35 nm and the average FEL pulse energy of 30 µJ was attenuated by nitrogen gas and thin film filter foils to acquire spectra similar to reference measurements in static experiments 11 .…”

Non-equilibrium Phenomena in Superconductors Probed by Femtosecond Time-Domain Spectroscopy

“…Moreover, photoexcitation of cuprate superconductors seems to give rise to filling-up of the gap instead of the suppression of its magnitude [69]. Here a major progress in understanding can be expected from systematic tr-ARPES studies with high energy resolution and large momentum coverage [160][161][162].…”