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

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Abstract
We have used supersonic molecular beam techniques to measure the initial dissociative chemisorption probability S0 of O2 on Ru(001) as a function of incident kinetic energy Ei, surface temperature Ts, and angle of incidence θi. We observe different behavior in the adsorption dynamics in two separate kinetic energy regimes: the value of S0 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 S0 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 α 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 ∼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 S0 approaches a value of unity for the highest incident energies studied. However, we report an unusual surface temperature dependence of S0 in the high energy regime that is inconsistent with a simple direct mechanism. Indeed, in this higher energy regime the value of S0 rises as the surface temperature is increased. We suggest a mechanism involving electron transfer from the ruthenium surface to account for this phenomena.

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