NONADIABATIC TRANSITIONS ON METAL-SURFACES AS A MECHANISM OF DISSOCIATION OF ADSORBED MOLECULES

A non-adiabatic dynamical framework has been developed in which each identified chemical species on the surface is assigned a potential-energy function. Transitions between the various potentials are induced by non-adiabatic coupling terms. The numerical scheme based on this non-adiabatic framework is summarized and applied to the dissociation of N2 on Fe and on Re and also to the O2 on Ag system. The model for the N2 dissociation on Fe and Re, is based on two non-adiabatic surfaces in three dimensions. The emphasis is on the recoil of the metal atom from the impinging nitrogen. This recoil is found to reduce the available energy required for the non-adiabatic transition. A large non-monotonic isotope effect as a function of the initial kinetic energy has been found. The O2 Ag system is studied by employing three non-adiabatic surfaces. The scenario for dissociation starting from the gas phase encounters first the physisorption potential. From this potential a non-adiabatic transition leads to a chemisorbed molecular ion, from which another non-adiabatic transition leads to the dissociated state. The dissociation probability, as a function of kinetic energy, shows a qualitative resemblance to a molecular beam experiment where the dissociation probability first decreases and then increases. The implication of the non-adiabatic framework for multidimensional studies including coupling to surface motion, is outlined.