He- resonances and the post-collision interaction

Abstract

Four resonant states of He- with all three electrons in the n=2 shell are accessible by electron impact from the He ground state. These resonances have the structure of an electron attached to an autoionizing state 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 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. © 1976 The American Physical Society.