Effect of symmetry on two-electron escape at threshold

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Abstract
The threshold law for the escape of two electrons from a positive ion, first derived by Wannier (1953) for 1Se states, is examined for more general values of the quantum numbers S, L and pi of the final state. In all cases except 3Se and 1Pe, the threshold behaviour of the cross section has the same Wannier exponent as for 1S3, namely sigma varies as E1.127 for a singly charged ion. Only the 3Se and 1Pe 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.