Spin Blocking in the Correlated Double-Electron Capture from Metal Surfaces

The resonant capture of electrons from a metallic surface into the outer shell of a helium ion creates doubly excited states in a spin singlet (↑↓) or triplet (↑↑) configuration. Here it is shown that the capture of one or two electrons can be described in a simple quantitative model, and the capture of two electrons by He(++) proceeds in a single step. The double capture of electrons from the Fermi energy of the metal is dominated by the spin dependent electron correlation that blocks the occupation of triplet states in the ion, but creates a singlet two-hole final state at the surface, related to the concept of the exchange-correlation hole.