Space charge effects in photoemission with a low repetition, high intensity femtosecond laser source

In this paper, we present experimental results on the effect of space charging in photoelectron spectroscopy from a surface using a pulsed and intense femtosecond light source. We particularly focus on a quantitative evaluation of the induced spectral broadening. Our results are compared with analytic calculations based on energy conservation considerations as well as with experimental results from measurements using picosecond pulses for the excitation process. As a measure of space charge effects, we monitored the angular and energy distributions of the photoemission from the occupied Shockley surface state of Cu(111) as a function of the total number N of the photoemitted electrons per laser pulse. Our results show that spectral distortions exist for the entire laser fluence regime probed. The energetic broadening of the surface state peak can be fitted with remarkable accuracy by a root N dependence, in agreement with the theoretical predictions and different from the experimental picosecond results, where a dominating linear dependence has been reported. In addition to a pure energetic broadening of the photoemission spectra, we also identify modifications in the angular distribution of the photoemitted electrons due to space charge effects. (c) 2006 American Institute of Physics.