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

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 (k, k, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å diameter in a binding-energy range of several eV, resolving about 2.5 × 10 data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe 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, and a system response function of 150 fs.