Laser-Cluster Interaction: X-Ray Production by Short Laser Pulses

Recent experiments on the interaction of large rare-gas clusters (number of atoms N $>$10000) with short intense infrared laser-pulses found characteristic x-ray emission at moderate ($\sim $10$^{15}$Wcm$^{-2})$ intensities. The ponderomotive energy of the electrons at these intensities is by far too low to create the K-shell vacancies (2 keV to 4 keV) which are the precursors of x-ray radiation. In order to investigate the mechanisms for efficient heating of quasi-free electrons in a cluster, we developed a mean-field classical transport simulation to describe the electronic dynamics. Our model takes into account the interaction of the electrons with the laser field, the build-up of an overall charge of the cluster due to electrons leaving the cluster, the resulting cluster expansion, polarization of the cluster due to collective electron motion, electron-impact ionization, and elastic scattering of the electrons by the ions inside the cluster. Elastic large-angle backscattering of electrons at ionic cores in the presence of a laser field is shown to be essential at low laser intensities. Results for the absolute x-ray yields are in surprisingly good quantitative agreement with experimental results.