Dynamics of space-charge acceleration of X-ray generated electrons emitted from a metal surface

The so-called vacuum space-charge acceleration of emitted electrons triggered by absorption of soft X-rays (a few 100 eV) under intense near-infrared laser excitation is studied experimentally and theoretically. The influence of high excitation densities on the properties of Auger and photo electrons liberated by a probe X-ray beam is investigated for grazing-incidence photons interacting with an atomically clean Cu (111) surface as a model system. Transient electron spectra have been taken in a pump-probe setup at the BESSY II storage ring using a newly developed compact electrostatic retarding Bessel-box spectrometer. Strong electron-energy shifts have been found and assigned to space-charge acceleration. Model calculations based on experimental input parameters are in good agreement with experimentally obtained energy-gain values indicating ultimate limits of ultrafast X-ray experiments with photo or Auger electrons as a probe.