Electron-impact inner-shell excitation of Fe XVI

Abstract

We report R-matrix calculations of electron-impact collision strengths for excitation from the inner L-shell into doubly excited states of Fe XVI. The basic close-coupling expansion used includes 59 LS terms of the target ion dominated by the configurations 2p63s, 2p63p, 2p63d, 2p53s2, 2p53s3p, 2p53s3d, 2p53p2 and 2p53p3d. We investigate the convergence of the collision strengths with the number of partial waves included in the expansion, the contributions from relativistic effects and the convergence of the close-coupling expansion by using pseudo-orbitals that try to account for the effect of the infinite number of levels that lie between the bound and doubly excited states. The present collision strengths exhibit dense packs of high resonances that enhance the effective collision strengths by up to three orders of magnitude. Also important are relativistic effects which alter both the resonance structures and background cross sections, changing the effective collision strengths by up to several orders of magnitude. In addition, a large number of carefully optimized pseudo-orbitals are necessary to achieve convergence in the target expansion. By taking into account the bound states not explicitly included in the expansion, the collision strengths into the doubly excited levels are reduced by up to more than a decade in some cases.