First 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 a huge potential. Real-time recording of non-equilibrium 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 analysis requires fs soft X-ray pulses with some 10~meV resolution, which are currently available at high repetition rate free-electron lasers. The PG2 beamline at FLASH (DESY, Hamburg) provides a high pulse rate of 5000~pulses/s, 60~fs pulse duration and 40~meV spectral resolution in an energy range of 25--830~eV with a photon beam size down to 50~microns in diameter. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines the superior source parameters of the beamline with a revolutionary 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 the entire 3D band structure in the ($k_x$, $k_y$, $E$) parameter space with unprecedented efficiency. Our instrument can image the full surface Brillouin zone with up to 7~\angstrom$^{-1}$ diameter in a binding-energy range of several~eV, resolving about $2.5\times10^5$ data voxels. As an example, we present results for the ultrafast excited state dynamics in a model van der Waals semiconductor WSe$_2$.