Mitigating space charge in time-resolved photoelectron spectroscopy to study laser-heated copper dynamics in the high fluence regime

The performance of time-resolved photoelectron spectroscopy for the study of subpicosecond dynamics of laser-heated solids is often limited by space charge effects. The consequent shift and distortion of the photoelectron spectrum induced by electrons emitted by the ultrashort pump pulse is studied here using a fully coherent approach based on experimental measurements and space charge calculations. The temporal dynamics of the valence band of a copper sample is recorded before and after an 800 nm laser pump excitation at a fluence of 7504pt0exmJ/cm². The probe pulse is produced using a laboratory-based high-harmonics source delivering 25 fs pulses up to 100 eV photon energy. We extract the laser-heating contribution by comparing these measurements with space charge calculations based on particle-in-cell simulations of the pump and probe electron clouds mutual interaction on their way to the detector. The deduced picosecond dynamics associated to the electronic density of states shift is attributed to lattice changes with the help of hydrodynamic simulations including the two-temperature model.