Double photoionization of helium using R-matrix methods

The eigenchannel R-matrix method for single photoionization is extended to describe a class of two-electron escape processes. We apply this approach to calculate the double photoionization of helium by single-photon absorption for photon energies in the range 80--280 eV. Pseudoresonances in the double continuum are eliminated by performing a Gailitis average and by averaging the final spectrum over the size of the R-matrix box. Calculations are presented in both the velocity and acceleration gauges. The ratio of double- and single-photoionization cross sections, a key parameter for characterizing electron correlations, is compared to existing theoretical and experimental values