Vibrational excitation, harpooning, and sticking in molecule--surface collisions

The problem of vibrational excitation of a diatomic molecule scattering from a metal surface is considered for encounters in which the molecular electron affinity level crosses the surface Fermi level, thus allowing for electron transfer back and forth between metal and molecule during the scattering process. The problem is formulated within a diabatic representation in terms of a Landau--Zener--Tully--Preston curve hopping at the location where charge transfer or harpooning occurs, following related theory for other surface charge transfer processes. Account is taken of both the time dependence of the affinity level position and width due to the translational motion of the molecule. Vibrational excitation probability distributions for scattered molecules are calculated. Under certain circumstances, these are obtained in analytic form using a semiclassical wave packet dynamics model. A novel mechanism in which the energy redistribution from translational to internal vibrational modes gives rise to sticking of the undissociated molecule on the surface is presented