Photoionization cross sections for Fe XVIII

Abstract

A sophisticated R-matrix calculation is performed for the photoionization of the fine-structure levels of the ground state of Fe XVIII. High resolution total and partial cross sections are obtained with the later restricted to the cases of the Fe XVIII ion being left in one of the fine-structure levels corresponding to the energetically lowest 11 LS states of Fe XIX after ionization. Both sets of cross sections are obtained using the Breit-Pauli R-matrix approximation which allows for the possibility of fine structure splitting in both resonances and thresholds. We find extensive resonance structure in the 99 to 113 Ryd photon energy range, a high percentage of which arises from 2p photoionization leaving the ion in the 2s2 2p4 3 P2 state while the background cross section is constructed almost exclusively of 2p and 2s photoionization with the former dominant in both photoionization cases. Relativistic effects are found to be important in obtaining accurate threshold positions as well as being responsible for a significant drop in the background cross section in comparison with various calculations done in LS coupling. We believe that this is the first relativistic calculation for this ion.