Kinetics and dynamics of the initial dissociative chemisorption of oxygen on Ru(001)

We have used supersonic molecular beam techniques to measure the initial dissociative chemisorption probability S-0 of O-2 on Ru(001) as a function of incident kinetic energy E(i), surface temperature T-s, and angle of incidence theta(i). We observe different behavior in the adsorption dynamics in two separate kinetic energy regimes: the value of S-0 decreases with incident energy in the low kinetic energy regime, and the value increases with incident energy in a higher kinetic energy regime, In the low energy regime, we observe a large inverse dependence of S-0 on surface temperature which is consistent with a trapping-mediated mechanism. Moreover, adsorption in the low energy regime can be accurately modeled by a trapping-mediated mechanism, with a surface temperature independent trapping probability alpha into a physically adsorbed state followed by a temperature dependent kinetic competition between desorption and dissociation. The barrier to dissociation from the physically adsorbed state is similar to 28 meV below the barrier to desorption from this state as determined by analysis of kinetic data. In the high kinetic energy regime, values of the initial adsorption probability scale with normal kinetic energy, and S-0 approaches a value of unity for the highest incident energies studied. However, we report an unusual surface temperature dependence of S-0 in the high energy regime that is inconsistent with a simple direct mechanism. Indeed, in this higher energy regime the value of S-0 rises as the surface temperature is increased. We suggest a mechanism involving electron transfer from the ruthenium surface to account for this phenomena. (C) 1996 American Institute of Physics.