Electron spectra from keV collisions of He2+ with a single crystal Cu(110) surface are reported. The spectra are obtained for well defined angles of incidence and detection. From the spectra and from their variation with detection angle the following conclusions are drawn. At vertical velocities of the order of 0.01 atomic units the He2+ neutralizes in two successive steps while it approaches the surface. In the first step He2+ is converted to He+ in the ground state by two competing processes, namely, double capture followed by autoionization or Penning ionization, and direct Auger capture. In the second step He+ is neutralized by Auger capture into ground state He. A model is developed that allows one to calculate electron spectra due to those processes using model functions for distance-dependent transition rates and transition energies. It is shown that the main features of the observed spectra can be well reproduced by the model calculations if it is assumed that the metal does not respond adiabatically to the sudden changes induced upon the emission of fast electrons in the various spontaneous ionization processes.
Zeijlmans van Emmichoven, P. A., Wouters, P. A. A. F., & Niehaus, A. (1988). Angle resolved electron spectra from grazing incidence He2+/Cu(110) collisions. Surface Science, 195(1-2), 115-132. https://doi.org/10.1016/0039-6028(88)90784-4