Non-adiabatic processes at metal surfaces

Abstract

When an atom or other particle is moving sufficiently rapidly near a metal surface the state of the electron gas may deviate from the adiabatic one which would obtain if the particle moved infinitely slowly along its trajectory. In fast ion scattering from surfaces the ion fraction in the scattered beam is non-adiabatic, and the simple semi-classical theory of this is explained. An example of a surface reaction occurring at thermal energies is surface chemiluminescence where the non-adiabatic emission of a photon occurs. The semi-classical theory of this is discussed. The more rigorous coherent or quantum mechanical approach is developed by considering the Heisenberg equations of motion for the field operators, and is necessary for example in sputtering (e.g. in SIMS) and in post ionization in field evaporation. The theory is applied to ionization of hyperthermal sodium atoms from a hot tungsten surface. Finally, the possibility of energy loss to electron hole pairs providing a trapping mechanism for adsorbing particles is considered. This problem is approached by solving the Heisenberg equations of motion for a slowly moving particle. It is found that while this mechanism is a significant one, there are several competing mechanisms capable of masking it in individual cases.