Electron spectra are reported for the scattering of Ar2+ ions by a well defined Pb(111) single-crystal surface. The measurements are performed for beam energies in the range 0.6-10 keV at a grazing angle of 2-degrees. The spectra at these conditions are found to be dominated by contributions from "spontaneous" electron emission processes, i.e., from Auger-type transitions in which two electrons are involved. They show a rather rich structure, especially at the lowest beam energies. A rather detailed analysis in terms of a model is possible. The model describes the time evolution of the ion-surface system during the slow approach. The processes involved are resonant one-electron exchange processes and Auger-type processes. The first processes lead to the distance-dependent population of various singly and doubly excited projectile states, the second lead to electron emission with distance dependent probability and kinetic energy. The analysis in terms of this model yields: (1) an identification of the main electronic states and processes that play a role during the slow surface approach of the projectile, (2) the absolute distance-dependent probabilities for the main resonant electron exchange and Auger-type processes, (3) the distance-dependent population probabilities for the electronic states involved. For the Ar2+-Pb(111) system it is found that at the low primary energies, autoionization of Ar** and Penning ionization of Ar+* dominate the first neutralization step leading to the ground state of Ar+, whereas with increasing energy the direct Auger-capture process of Ar2+ becomes more important. The second neutralization step leading to the ground state of Ar is found to be always due to direct Auger-capture. In the mentioned primary beam energy range the neutralization to the ground state of Ar is found to be completed at the turning point of the trajectory.
