Electron capture by multiply charged ions on a helium target: a population mechanism for minor channels

This paper reports on differential cross sections at keV energies for electron capture by various ions of charge q<9 colliding with helium atoms. Whereas the cross section for single capture into the principal quantum number n, the dominant process, is peaked near zero degrees, the cross section for capture into n+1, a less exothermic process, is peaked at large scattering angle. It is shown that such a process involves a population scheme taking place at small internuclear distance. This mechanism was predicted by the 'electron promotion model'. (author)