Neutralization of Li+ ions scattered by the Cu (1Â 0Â 0) and (1Â 1Â 1) surfaces: A localized picture of the atom-surface interaction

Large and face dependent neutral fractions have been found recently in the scattering of Li+ by Cu(1 0 0) and Cu(1 1 1) surfaces. These results for high work function surfaces are unexpected within the 'traditional' picture of a Li+ ion departing from a jellium surface model. In the present work the Li+/Cu(1 0 0) and Li+/Cu(1 1 1) interacting systems are described by a previously developed bond-pair model based on the localized interactions between the projectile ion and the atoms of the surface, and on the extended features of the electronic band structure through the surface local density of states. By only including the resonant neutralization to the Li atom ground state we explained the face and energy dependences of the measured neutral fractions for large outgoing energy values. We found that the downward shift of the Li ionization level below the Fermi level caused by the short range chemical interactions, is the main responsible of a high neutralization by the resonant mechanism. The remaining differences between theory and experiment values can be explained in terms of the energy gaps and image potential states appearing in these surfaces. The calculated distance behaviours of the energy levels corresponding to the first excited (Li-1s22p) and the negative (Li-1s22s2) atomic configurations indicate that they can also participate in the ion-surface charge exchange process.