The interaction of He ϩ with a typical metal surface ͑Al or Pd͒ is described, analyzing in detail the different mechanisms that contribute to the neutralization of the projectile when backscattered from the surface. Auger and resonant neutralization processes are considered and analyzed including a detailed quantum-mechanical description of the He-metal interaction, for projectile energies between 100 eV and 3 keV. We show that the promotion of the He-1s level, due to its interaction with the metal-atom-core orbitals, is the crucial mechanism making resonant processes operative. We find, however, that resonant processes are much more important for Al than for Pd. In Al, both Auger and resonant processes are equally important for neutralization of the ion, while for Pd we find that Auger is the dominant mechanism, making the He/Pd system the ideal case for which Hagstrum's exponential law appears to be practically valid for all velocities. We also find qualitative agreement with experimental data, which we consider a satisfactory result in view of the fact that our theory is a complex ab initio calculation free of adjustable parameters.

A bond-pair model Hamiltonian developed previously for systems consisting of interacting atoms is applied to describe atom-surface interactions. By proposing a mixed basis set involving localized adatom orbitals ͕ ␣ ͖ and extended surface states ͕ k ͖, and by application of a mean-field approximation, the Hamiltonian is reduced to the form of the single-particle Anderson model. The resulting model Hamiltonian is free from adjustable parameters. These parameters include both the effects of electronic interactions between the atom and the solid and those arising from the lack of orthogonality between the adsorbate and substrate orbitals. The nonlocal exchange contributions are treated consistently within the Hartree-Fock method, while valencelike and corelike band states are also taken into account. This model is applied to consider the interaction of hydrogen with metals ͑Al, Li, and Na͒. The results for chemisorption are in good agreement with those obtained by other theoretical approaches based on either the density functional theory or embedding cluster methods, as well as with existing experimental data. In addition, the calculation of the shifts and widths of the adsorbate levels in an ample range of separation distances are also in good agreement with those obtained by using atomic physics techniques. ͓S0163-1829͑98͒10031-0͔

We develop a theory of the Auger neutralization rate of ions on solid surfaces in which the matrix elements for the transition are calculated by means of a linear combination of atomic orbitals technique. We apply the theory to the calculation of the Auger rate of He + on unreconstructed Al͑111͒, ͑100͒, and ͑110͒ surfaces, assuming He + to approach these surfaces on high symmetry positions and compare them with the results of the jellium model. Although there are substantial differences between the Auger rates calculated with both kinds of approaches, those differences tend to compensate when evaluating the integral along the ion trajectory and, consequently, are of minor influence in some physical magnitudes like the ion survival probability for perpendicular energies larger than 100 eV. We find that many atoms contribute to the Auger process and small effects of lateral corrugation are registered.

This work is mainly devoted to the description of processes that involve the interaction between an atom and a surface, in which a strong Coulomb repulsion on the atomic site ͑U͒ limits the charge exchange to one electron ͑infinite-U limit͒. In this limit, the Anderson Hamiltonian for a many-fold ͑N͒ of states localized on the atomic site can be represented in terms of auxiliary bosons and physical operators in the mixed bosonelectron space can be defined. In this work the Hamiltonian is solved by defining appropriate Green's functions for physical operators. Then we solved the equations of motion of these Green's functions, up to a second order in the atom-surface coupling, either for the stationary case or for a real time-dependent problem. We show that our approach reproduces the known exact results in the nondegenerate ͑N =1͒ case, and for N ՝ 1 gives excellent agreement with exact calculations and approximations valid for large N ͑the 1 / N expansion͒. Finally, the accurate description of dynamical processes is shown by the comparison with the exact results available for a small four-level system. In this case we also compare with results obtained by using the noncrossing approximation and with the usual spinless model calculation.

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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.

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