Effect of symmetry on two-electron escape at threshold

Abstract

The threshold law for the escape of two electrons from a positive ion, first derived by Wannier (1953) for ¹S^e states, is examined for more general values of the quantum numbers S, L and pi of the final state. In all cases except ³S^e and ¹P^e, the threshold behaviour of the cross section has the same Wannier exponent as for ¹S³, namely sigma varies as E^1.127 for a singly charged ion. Only the ³S^e and ¹P^e symmetries exhibit a much larger exponent. In all other symmetries there is an analogue of the +/- quantum numbers familiar from the study of doubly excited states, with the + component always exhibiting the Wannier exponent. The relevance of these findings to recent measurements of spin-selected cross sections in e-atom ionisation is pointed out. The authors also discuss the dependence of the angular distribution of the ejected electrons on L and S.