Sixfold differential cross sections for atomic helium, magnesium, and calcium in (γ,2e) experiments

We have calculated sixfold differential (γ,2e) double-photoionization cross sections (6DCS) for helium and for the alkaline-earth atoms Mg and Ca. We use configuration-interaction wave functions for the initial state and two sets of wave functions for the final state: (1) smoothly joined final-state wave functions corresponding to the Coulomb potential at large r and to a pseudopotential at small r and (2) wave functions that are the same as (1) but multiplied by the factor [1-exp(-${\mathit{r}}_{12}$/${\mathit{r}}_{\mathit{C}}$)] to account for the singularity at ${\mathit{r}}_{12}$=0 and the short-range correlation in the final state. To see the effects of the initial-state correlation on the 6DCS, sample calculations are performed with Hartree-Fock initial-state wave functions. In contrast to the results of Le Rouzo and Dal Cappello [Phys. Rev. A 43, 318 (1991)], our 6DCS plots for cases in which both electrons escape with equal energy ɛ=ɛ’=15 eV differ significantly from the cross sections for the case in which the electrons escape with different energies ɛ=5 eV and ɛ’=25 eV. The results show that approximate correlation by means of inclusion of a Coulomb hole into the final-state wave function has a significant effect in some cases for Mg and Ca, but has little effect in the cases studied for He. Comparison with the results of a very recent experiment is made.