Electron propagation from a photo-excited surface: implications for time-resolved photoemission
We perform time- and angle-resolved photoelectron spectroscopy on p-type GaAs(110). We observe an optically excited population in the conduction band, from which the time scales of intraband relaxation and surface photovoltage decay are both extracted. Moreover, the photovoltage shift of the valence band intriguingly persists for hundreds of picoseconds at negative delays. By comparing to a recent theoretical study, we reveal that the negative-delay dynamics reflects the interaction of the photoelectrons with a photovoltage-induced electric field outside the sample surface. We develop a conceptual framework to disentangle the intrinsic electron dynamics from this long-range field effect, which sets the foundation for understanding time-resolved photoemission experiments on a broad range of materials in which poor electronic screening leads to surface photovoltage. Finally, we demonstrate how the long-lasting negative-delay dynamics in GaAs can be utilized to conveniently establish the temporal overlap of pump and probe pulses in a time-resolved photoemission setup.