The formation of negative ions in the scattering of protons by a highly oriented pyrolytic graphite (HOPG) surface is theoretically and experimentally analyzed for a large scattering angle and compared with previous results obtained in the same system but for a forward scattering geometry. These experiments were motivated by the fact that the interaction of a hydrogen atom with a surface is the prototype system for studying the intra-atomic Coulomb repulsion in an s-like valence orbital localized in the atom. We tried to answer the open questions related to the electronic correlation effects and the influence of the detailed surface band structure by using appropriate theoretical models. The comparison with the experiment of theoretical results obtained by using different limit approximations of the electronic repulsion in the atomic state shows the expected validity ranges according to the ion velocity. However, the most remarkable conclusion obtained from this comparison is the nonvalidity of an adiabatic picture of the energy levels shifted by the interactions with the surface atoms, when the energy uncertainty introduced by the ion velocity becomes of the order of the electronic repulsion in the hydrogen ground state.

The very high neutral fractions measured in He + scattered by graphitelike surfaces, at intermediate incoming energies (1 keV < E in < 6 keV), cannot be explained only by the resonance of the He ionization level with the valence band states of the surface. Excited configurations (1s2s) and (1s2p) appear as possible resonant neutralization channels together with the ground state one (1s 2 ). We develop, in this work, a time-dependent quantum-mechanical calculation of the charge-transfer process in He + /HOPG collision, where the resonant neutralization to the ground and first excited states of He is taken into account. We use an Anderson Hamiltonian projected on the electronic configurations of the projectile atom which are energetically favorable for the charge-exchange process. Thus, an exhaustive analysis of different possible approximations to the neutralization of He + is performed: the typical neutralization to the ground state by either neglecting or not the electron spin and finally the one including excited configurations. Our results reproduce the observed experimental trends only when excited configurations (1s2s) and (1s2p) are involved in the charge exchange between the ion and the surface.