Ab initiocalculation of electron-impact-ionization cross sections for ions in exotic electron configurations

Abstract

Atomic or molecular assemblies irradiated with intense hard x-ray pulses, such as those emitted from x-ray free-electron lasers (XFELs), are subject to a strong ionization, which also releases electrons from atomic inner shells. The resulting core-hole states relax via various channels, including fluorescence and Auger decay. The latter is the predominant relaxation channel for light elements and typically occurs on a time scale of 1--10 fs. In dense samples, the core-hole ions may already undergo electron-impact ionizations during this time due to the abundance of highly energetic photoelectrons and Auger electrons. In this study we perform an ab initio calculation of the electron-impact-ionization cross sections of ions with an arbitrary electronic configuration at zero temperature. This allows us to evaluate and compare impact-ionization cross sections for ions in ground and ``exotic'' electronic states (e.g., with a few core holes), which may be formed during their interaction with intense x-ray pulses. We show that the impact-ionization cross sections for ions of the same charge, but with varying electronic configurations, may significantly differ. This finding has to be taken into account in any modeling tool treating the relaxation of atoms after high-energy-impact collision, e.g., simulations dedicated for coherent x-ray diffraction imaging of nanocrystals and single biological macromolecules, or laser-created plasma studies. Our computationally efficient ab initio calculation scheme can be easily incorporated in such simulation schemes.