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

A non-adiabatic dynamical framework has been developed in which each i dentified chemical species on the surface is assigned a potential-ener gy 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 N 2 on Fe and on Re and also to the O2 on Ag system. The model for the N 2 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 fro m the impinging nitrogen. This recoil is found to reduce the available energy required for the non-adiabatic transition. A large non-monoton ic isotope effect as a function of the initial kinetic energy has been found. The O2 Ag system is studied by employing three non-adiabatic s urfaces. The scenario for dissociation starting from the gas phase enc ounters first the physisorption potential. From this potential a non-a diabatic transition leads to a chemisorbed molecular ion, from which a nother non-adiabatic transition leads to the dissociated state. The di ssociation probability, as a function of kinetic energy, shows a quali tative resemblance to a molecular beam experiment where the dissociati on probability first decreases and then increases. The implication of the non-adiabatic framework for multidimensional studies including cou pling to surface motion, is outlined.