Quantum-mechanical rates for gas-surface processes

Abstract

We determine exact quantum-mechanical rates for reactive scattering of hydrogen and its isotopes at and from metal surfaces. To this end, we compute cumulative reaction probabilities according to Miller, Schwartz and Tromp [J. Chem. Phys. 79 (1983) 4889], in the discrete variable representation with absorbing boundary conditions formulation of Seideman and Miller [J. Chem. Phys. 96 (1992) 4412]. In particular, canonical rates for dissociative adsorption and associative desorption of H 2 and isotopomers, as well as rates and diffusion coefficients for the diffusive motion of single hydrogen atoms on transition-metal surfaces are considered. For the adsorption/desorption processes we use various two- and three-dimensional model potential energy surfaces, which are thought to be typical for the interaction of H 2 with first-row transition metals. First for uncorrugated and rigid surfaces, the dependence of the rates on temperature(s) and potential parameters are studied. Second, we allow for non-rigidity of the substrate using the so-called surface-mass and surface-oscillator models of Luntz and Harris [Surf. Sci. 258 (1991) 397]. Further, we find that the inclusion of surface corrugation increases desorption rates, characterized by larger Arrhenius-preexponential frequencies. In our study of rates and diffusion coefficients for the hopping motion of atoms on metal substrates we focus on hydrogen on Ni(100). One- and two-dimensional realistic potential energy surfaces are used to elucidate the role of quantum effects, isotope effects, and dimensionality. We compare our results to simple quasi-classical and classical forms of transition-state theory and to experiment.