From quantum-state-specific dynamics to reaction rates: The dominant role of translational energy in promoting the dissociation of D₂ on Cu(111) under equilibrium conditions

We have calculated the rate of adsorption of isotropic D2 gas on a Cu(111) surface, using recently determined differential adsorption probabilities, as a function of translational energy, angle of incidence, and surface temperature for molecules in each vibrational-rotational state. If the D2 gas is at the same temperature, T, as the surface, the mean probability of dissociation per collision, 〈S0〉, is calculated to increase rapidly with temperature. Arrhenius plots of 〈S0〉 vs. 1/T are in good qualitative agreement with measurements for hydrogen dissociation on Cu, but display a distinct curvature over the range 300-1000 K. A detailed analysis of this temperature dependence reveals that the increase in 〈S0〉 with T is due almost entirely to the increase in translational energy of the incident molecules. Increases in the populations of vibrationally or rotationally excited molecules are relatively unimportant, as are the changes in the adsorption with surface temperature.