A theory for the effect of a strong intra-atomic Coulomb repulsion U on the nonadiabatic transfer of charge between a metallic surface and a moving atomic species is presented. Using slave bosons and a nonequilibrium Green s-function technique, we solve the equations appropriate for the U = oo problem in the case when either the atom-surface hopping matrix element is small, or the number of degenerate atomic states is large. We generalize the earlier treatment of Langreth and Nordlander (LN) to include off-diagonal self-energies and present a general numerical scheme for the exact solution of the Dyson equations. We verify that our scheme gives the correct answer in several limiting cases where an exact solution is known, and give quantitative predictions of when deviations from these limits become important. These limits include (1) the simple master equation limit for low velocities and weak coupling, (2) the generalized master equation of LN for larger velocities and atom-surface coupling, (3) the approach to thermal equilibrium when the time dependence is removed, and (4) the maintenance of local thermal equilibrium when the energy parameters vary sufFiciently slowly. From a calculation of the instantaneous (nonequilibrium) spectral function of the level on the scattering atom, we are able to study the rate of formation of the Kondo and mixed valent resonances near the Fermi level. We find a slow formation rate for such resonances relative to that of the broader parts of the spectral density centered near the bare atom level positions.

A progress report is given of an extension of the density functional formalism to include long-range interactions such as van der Waals or dispersion forces. This is done by proving a general expression for the so-called exchange-correlation energy to contain and to describe such interactions just as well as any other treatment. The proper long-range forms of the interactions are derived explicitly in the cases of two neutral atoms, an atom outside a metal surface, and two parallel metal surfaces. The long-standing problem of treating the attractive and repulsive forces on the same footing in this way gets a solution. For practical calculations, an approximate form, based on an analysis in the weakly inhomogeneous limit and on a limiting form of the three-point function given by Rapcewicz and Ashcroft, is proposed and applied to some prototype cases. 0 1995 John Wiley & Sons, Inc.well-known R -6 form of London [4]. The asymptotic z p 3 form of the interaction potential between a neutral atom and a surface was first identified by , with subsequent refined treatments of the atom and surface polarizabilities [6,7]. For the interaction between solid bodies, general formulas have been derived [S], which for flat sur-

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Many recent ion-surface scattering experiments, e.g. , He , 'Pb, show dramatic oscillations of the neutralization probability as a function of the inverse ion velocity. These oscillations are qualitatively similar to the Stueckelberg oscillations that occur during quasiresonant charge exchange in atom-atom collisions. We present here a general many-body approach for the description of such charge-exchange processes near surfaces. We show that for the proper understanding of the oscillatory behavior it is crucial to include intra-atomic correlation effects. The interference between difFerent tunneling channels in8uences both the amplitude and the frequency of these oscillations.

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