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

## TL;DR

This paper introduces a novel time- and momentum-resolved photoemission setup using free-electron laser pulses and a full-field imaging momentum microscope with time-of-flight detection, enabling detailed studies of ultrafast electronic dynamics.

## Contribution

It combines free-electron laser capabilities with a multidimensional photoemission measurement scheme, providing high-efficiency, full-zone momentum imaging of electronic states in real-time.

## Key findings

- Achieved 3D band structure mapping with high efficiency.
- Demonstrated ultrafast dynamics in WSe₂.
- Enabled full surface Brillouin zone imaging.

## Abstract

Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application 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 few 10 fs soft X-ray pulses with some 10 meV spectral 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 bandwidth 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 FEL 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 ($k_x$, $k_y$, $E$) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 {\AA} $^{-1}$ 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 the model van der Waals semiconductor WSe$_2$.