The connection between multi-state resonant charge transfer dynamics and many-electron states in atom-metal surface scattering

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Physics and Astronomy


For Li+ and Na+ scattering from clean and cesium-covered Cu(001) surfaces, we have measured probabilities to form different final electronic states in the scattered flux as a function of the Cs-induced work-function shift Delta phi. Specifically, positive and negative ion yields for 400 eV Li+ scattering, and the relative yields of excited neutral Li(2p) and Na(3p) for 400 and 100 eV Li+ and 1320 eV Na+ scattering were measured. As we lowered the work function from its clean (metal) surface value, we observed a monotonic decrease in the Li+ yield, a monotonic increase in the Li- yield, and a peak in the minority Li(2p) channel yield (and the Na(3p) yield). The major trends in the Li/Cu(001) data (and likewise in the Na/Cu(001) data) can be reproduced by use of a multi-state model, developed by Marston and co-workers, of resonant charge transfer. Here we present a new, straightforward explanation of these trends, based upon an examination of the many-electron states of the atom-metal system. Much of the charge transfer dynamics can be understood through the ground state of the interacting Li/Cu(001) system, since the 'atom probabilities' - the probabilities that the Li is a Li+, Li(2s), Li(2p), or Li- - tend to equilibrate towards their groundstate values throughout the atom's trajectory. In each of our experiments, the velocity is low enough that the Li/Cu(001) system electronically equilibrates close to the surface, where the atom-metal couplings are large; at small atom-metal separations z, the Li atom probabilities therefore track their ground-state values. As the atom moves along its outgoing trajectory, the couplings decrease exponentially, and eventually the atom probabilities lose track of their ground-state values. Qualitative arguments, based upon general principles of quantum mechanics, allow us to understand the dependence of the ground-state probabilities on the work function phi and z. To comprehend the origin of the Li(2p) peak we must consider both the ground-state probabilities and the conditions under which the dynamical probabilities lose track of their ground-state values. (C) 1998 Elsevier Science B.V.

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