He−resonances and the post-collision interaction

Four resonant states of ${\mathrm{He}}^{-}$ with all three electrons in the $n=2\mathrm{}$ shell are accessible by electron impact from the He ground state. These resonances have the structure of an electron attached to an autoionizing state ${\mathrm{He}}^{**}$. A discussion is given of the influence of these resonances on electron scattering processes. A theoretical model is proposed that includes the recently postulated post-collision interaction as the second step of a two-step decay process for these resonances. This model is shown to account qualitatively for properties of excitation structures observed in the energy range 57-61 eV, including the apparent selectivity of $L$ values in Rydberg-state excitation. Energies of the ${\mathrm{He}}^{-}$ resonances are computed variationally and are found to be consistent with earlier results for the three lowest resonances. A fourth broad resonance is predicted to occur near 60.4 eV. Experimental evidence for this resonance is discussed.