Spin conservation in electron-capture collisions

The coupling between electron spin and collisional angular momentum has been studied for ${\mathrm{He}}^{++}$-He, Ne, ${\mathrm{H}}_2$, and ${\mathrm{O}}_2$ collisions (4-600 eV) by observing radiation resulting from double electron capture into excited states of Hei and comparing the triplet and singlet emissions. The two-electron targets provide a rigid test of the spin-conservation rule, but of these only ${\mathrm{H}}_2$ yields data in accord with the rule. The apparently anomalous ${\mathrm{He}}^{++}$-He results are attributed to the domination of short-range effects. These short-range effects are not restricted to double-electron capture, but can apply in any process which involves spin flip.