MANY-BODY THEORY FOR CHARGE-TRANSFER IN ATOM-SURFACE COLLISIONS

A theory for the effect of a strong intra-atomic Coulomb repulsion U o n the nonadiabatic transfer of charge between a metallic surface and a moving atomic species is presented. Using slave bosons and a nonequil ibrium Green's-function technique, we solve the equations appropriate for the U = infinity problem in the case when either the atom-surface hopping matrix element is small, or the number of degenerate atomic st ates is large. We generalize the earlier treatment of Langreth and Nor dlander (LN) to include off-diagonal self-energies and present a gener al numerical scheme for the exact solution of the Dyson equations. We verify that our scheme gives the correct answer in several limiting ca ses where an exact solution is known, and give quantitative prediction s of when deviations from these limits become important. These limits include (1) the simple master equation limit for low velocities and we ak coupling, (2) the generalized master equation of LN for larger velo cities and atom-surface coupling, (3) the approach to thermal equilibr ium when the time dependence is removed, and (4) the maintenance of lo cal thermal equilibrium when the energy parameters vary sufficiently s lowly. From a calculation of the instantaneous (nonequilibrium) spectr al 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 th e Fermi level. We find a slow formation rate for such resonances relat ive to that of the broader parts of the spectral density centered near the bare atom level positions.