Double photoionization of atomic helium and its isoelectronic partners at x-ray energies

Abstract

A simple and accurate procedure for calculating the rate of double photoionization of ``two-electron'' systems at very high photon energies is presented. Arguments are given to support the validity of the method used. In particular, we show that the many-body perturbation theory diagrams depend asymptotically on the choice of the gauge for the electric dipole operator. The ratio of double-to-single ionization is calculated to be 1.68% in agreement with the recent synchrotron measurements. For ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{Li}}^{+}$, we predict ratios of 1.51% and 0.89%, respectively.