THE ROLE OF NONADIABATIC MECHANISMS IN THE DISSOCIATION DYNAMICS OF O-2 ON SILVER SURFACES

The dissociation dynamics of oxygen on silver surfaces is studied theo retically. The method is based on a quantum-mechanical time-dependent non-adiabatic picture. A universal functional form for the potential e nergy surfaces is employed. The diabatic potentials describing the seq uence of events leading to dissociation begin from the physisorption p otential crossing over to a charged molecular chemisorption potential and crossing over again to the dissociated atomic-surface potential. W ithin such a potential surface topology, two different surfaces leadin g to dissociation are studied: the empirical potential of Spruit and t he ab-initio potential of Nakatsuji. It is found that the system is ca ptured by the molecular chemisorption well for a considerable length o f time, long enough for thermalization. Thus the calculation is split into two parts: the calculation of ''direct'' dissociation probability and the calculation of nonadiabatic dissociative tunneling rate from the thermalized chemisorbed molecular state. For the direct probabilit ies, the Fourier method with the Chebychev polynomial expansion of the evolution operator is used to solve the time-dependent Schrodinger eq uation. For the tunneling rate calculation, a similar expansion of Gre en's operator is used. The output of the direct-reaction calculation i s the dissociation probability as a function of the initial energy con tent, while the tunneling calculation yields the dissociation rate. Th e dependence of the direct dissociation probability on the initial kin etic energy is found to be non-monotonic. A strong isotope effect has been found, favoring the dissociation of the light species.