Electron impact ionization of and photoionization of Be at the K edge

Abstract

We present new R-matrix calculations for ionization processes involving the beryllium K-shell, namely electron impact ionization of and single and double photoionization of Be. These are modelled near inner-shell thresholds in a unified way, coupling direct and indirect processes, enabling comparison of the respective autoionization features. Below the K edge, we predict that direct double photoionization contributes 4% to the total cross section background for photoionizing Be; the onset of K-shell excitation - autoionization (EA), an indirect mechanism, increases the background by a factor 20. By contrast, we find that EA in ionization increases its background by only 4%; this is less than previous estimates, but in line with experiment. We also study high-energy behaviour for photoionizing excited states, e.g. , where we find a background dominated by ionization. This process is normally omitted in R-matrix schemes used in opacity and recombination calculations, leading to orders-of-magnitude underestimation at high energies of excited-state photoionization, which also exhibits a large core resonance above the K edge.