Atomic plasma excitations in the field of a soft x-ray laser

The interaction of atoms with short-wavelength radiation at ultra-high intensities is described by plasma excitation. In contrast to former works on optical radiation and ponderomotive motion of quasi-free electrons, the excitation of correlated and bound electrons is considered here. The ponderomotive motion of a free electron is included as a special case. Values for the energy transfer from the radiation field to an atom are obtained in fair agreement with the unexpectedly high charge states of xenon recently observed at the soft x-ray free-electron laser FLASH.