Double photoionization of excited1Sand3Sstates of the helium isoelectronic sequence

We present results for the photoionization of ground and excited S states of two-electron systems by x-ray photons. The method uses the asymptotic formulation of Dalgarno and Stewart [Proc. Phys. Soc. London 76, 49 (1960)] with highly correlated Frankowski-Pekeris–type wave functions for N ${}_{}{}^1{}_{}{}^{}\mathrm{S}$ and N ${}_{}{}^3{}_{}{}^{}\mathrm{S}$ initial states. The calculations show a smaller ratio of double-to-single photoionization R for triplet states due to the spatial antisymmetry of the wave function near the nucleus. We find an asymmetric population of photoion satellite states and provide expressions for the asymptotic single and total photoionization, and R as a function of nuclear charge. We also make predictions for the differential double photoionization cross sections in the extreme energy sharing case when one electron is ejected with zero energy.