State-selective double-electron capture inHe2++ He collisions at intermediate impact energies

We report calculations of cross sections for electron capture to individual doubly excited states for 150–500-keV ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$ ions colliding with helium atoms. By adopting an independent-electron approximation, the capture probability to each doubly excited state is calculated in terms of single-capture amplitudes to each substate which are properly weighted with configuration-interaction coefficients. The validity of such an approximation is checked by comparing the results with calculations in which the electron correlation was considered explicitly. The calculated final results for double capture to the (2$s^2$) ${}_{}{}^1{}_{}{}^{}S$, (2s2p${)}^1$P, and (2$p^2$ ${)}^1$D states are in fair agreement with the recent experiment of Zouros et al. [Phys. Rev. A 35, 1963 (1987)].