Laser-pump/X-ray-probe experiments with electrons ejected from a Cu(111) target: space-charge acceleration

Abstract

A comprehensive investigation of the emission characteristics is presented for electrons induced by X-rays of a few hundred eV at grazing incidence angles on an atomically clean Cu(111) sample during laser excitation. Electron energy-spectra due to intense infra-red laser irradiation are investigated at the BESSY II slicing facility.Furthermore, the influence of the corresponding high degree of target excitation (high peak current of photoemission) on the properties of Auger and photo electrons liberated by a probe X-ray beam is investigated in time-resolved pump and probe measurements. Strong electron-energy shifts have been found and assigned to space-charge acceleration. The variation of the shift with laser power and electron energy is investigated and discussed on the basis of experimental as well as new theoretical results. PACS numbers:

The influence of x-ray pulse length on space-charge effects in optical pump/x-ray probe photoemission

“…intense 800 nm laser radiation, with kinetic energies exceeding 100 eV have been reported [12,21], the vast majority of electrons in the cloud has much lower kinetic energies. To explain this discrepancy, it should be emphasized that the dynamic of the energy shift of the probe electrons is governed mostly by E L for positive delays in the simple 1D space-charge model.…”

“…In this situation, the probe electron is interacting with the pump electron disc during the initial propagation process and effects like increasing disc size, redistribution of the kinetic energies of pump electrons or collisions of the probe electron with pump electrons might become more important. The authors of [12] and [21] have shown that it is nevertheless adequate to approximate the propagation of the pump electron disc with an effective cloud electron velocity v L z (see figure 1(b)). Some extensions to the 1D model have been developed:…”

“…(2) including a realistic energy distribution of the cloud electrons N(E kin ) and building partial sums of electron discs with energies according to this distribution [21];…”

“…(3) different distributions of emission angles (normal, isotropic, cosine) and an elliptical spot shape for cloud electrons [21,27].…”

“…With Monte-Carlo simulations we find that the detailed dynamics of the Au 4f XPS spectra can be explained by describing the SC-effects with an analytic meanfield model of the pump electron cloud. While this model is well known and has been successfully used to describe the space-charge induced energy shift in several pump-probe XPS studies [12,21], our high resolution Au 4f spectra enable to test its applicability on the detailed line shape, in particular line broadening and asymmetry. We find that all observations can be understood within the mean-field model without the need to account for local electron-electron collision processes -also called stochastic scattering-.…”