Excitation — Ionization and Excitation — Autoionization of Helium

Abstract

In this article we survey some new theoretical results on (e, 2e) excitation — ionization and excitation — autoionization of ground state helium. A fuller account may be found in references [1–3]. Our interest lies in the high energy regime and in geometries for which the ejected electron is very much slower than the incident and scattered electrons. Under these conditions we may think of the collision as exciting a continuum state ψ _f of the isolated atom. In the case of excitation — ionization this state consists of the ejected electron, momentum κ, moving in the field of an excited He⁺ ion, for example, the system {He⁺(2s or 2p) + e⁻ (κ)}. In excitation — autoionization the helium atom makes a transition to an unstable doubly excited bound state He**(nl, n′l′) (nl ≠ ls, n′l′ ≠ 1s), ie, a resonant state of the atom, which then ejects an electron leaving a residual He⁺ ion. The particular case we consider in this article is where this ion is left in its ground state He⁺(1s). The autoionization reaction then interferes with ordinary direct ionization to He⁺(1s) since both processes lead to the same final state {He⁺(1s) + e⁻(κ)}