Autoionization of Doubly Excited Helium Atoms Formed by Electron Tunneling at Metal Surfaces

It is shown that the principal process of de-excitation of the He⁺(2s) ion at a metal surface is its conversion to doubly excited He⁰ by resonance capture of an electron from the solid, followed by autoionization to produce He⁺(1s) and a fast electron. The ejected electron is faster when produced near a metal surface than when produced in free space. The energetics of this two-stage process provides a detailed corroboration of previously studied interactions of atomic particles with metal surfaces.