Cluster-laser interaction: fast production of hot electrons by short laser pulses

The surprisingly low laser intensity threshold of I ≃ 1015 W cm−2 for production of characteristic X-rays in the interaction with large rare gas clusters (N > 106 atoms) is investigated. We propose an efficient production mechanism of fast electrons involving the interplay of the acceleration in the laser field and backscattering at ionic cores resembling the Fermi shuttle. We show with the help of a Monte Carlo simulation that this mechanism provides an efficient heating mechanism for moderate laser intensities resulting in an X-ray production threshold in approximate agreement with the experiment.