Over-the-barrier ionization of multielectron atoms by intense VUV free-electron laser

Free-electron laser sources now under construction are intended to attain radiative intensities higher than at photon energies up to 30 eV. We investigate the over-the-barrier mechanism of ionization of heavy atoms by such a source. We find that, for valence electrons, this mode of ionization is suppressed in the regime where the electric field and photon energy of the laser are comparable, respectively, to atomic electric fields and binding energies. Ionization of inner-shell electrons is still possible, but some kind of `two-fluid' model is necessary to describe this phenomenon.