Studies of post-collision interactions for electrons ejected from autoionising levels of helium excited by low-energy electrons

The ejected-electron energy distributions resulting from collisions of low -energy electrons with helium atoms have been measured. Four autoionising states (2s2)1S, (2s2p)3P, (2p2)1D and (2s2p)1P have been studied and, at impact energies within 0.15 to 2.5 eV of the threshold for exciting the states, oscillations have been observed in the ejected-electron spectra. These can be attributed to a post-collision interaction involving the two outgoing electrons and the residual ion. The spectra of the 1S and 3P states have been fitted using a semiclassical model, giving values for the parameters describing the excitation of the autoionising states. (author)