Nonadiabatic charge transfer processes of oxygen on metal surfaces

The dynamics of charge transfer processes of oxygen on metal surfaces is reviewed. Two theoretical frameworks, the adiabatic and the nonadiabatic, are compared with experiment. The O₂/Al system is chosen as a representative example. In the adiabatic approach there is no barrier to dissociation. This fact contradicts experimental observations of an increase of the dissociation probability with incident energy. In this study a nonadiabatic framework is formulated where the encounter takes place simultaneously on four electronic surfaces, each representing a different charged oxygen species. The dynamics, starting from an oxygen molecule in the gas phase, is followed by solving the multichannel time dependent Schrödinger equation. The transition from the diabatic to the adiabatic limit is explored by varying the nonadiabatic coupling terms. By so doing the dissociation probability dependence on incident energy changes from a strong monotonic increase in the diabatic case, to a flat dependence in the adiabatic case. The influence of electronic quenching is also studied, based on a numerical solution of the Liouville von Neumann equation. The dynamics subject to quenching shows a stronger initial dependence on incident kinetic energy leading to saturation. The general trend is quite similar to the dynamics without quenching.