Surface oscillator models for dissociative sticking of molecular hydrogen at non-rigid surfaces

Effects of surface atom motion on the dissociation dynamics of hydrogen at a copper model surface are studied using time-dependent quantum dynamics, the (ordinary) surface oscillator (SO) model of Harris et al., and a modified surface oscillator (MSO) model with a microscopically motivated molecule-surface coupling. For both models the dependence of zero-coverage dissociative sticking probabilities on (1) the Einstein oscillator frequency, (2) on anharmonicities of the surface vibrations and (3) on isotopomer masses, is studied. The unmodified SO model predicts an almost perfect insensitivity on the surface oscillator frequency which explains, therefore, the well-known good agreement with the surface mass model of Luntz and Harris. In contrast, the MSO model depends on the oscillator frequency and seems more realistic if applied to solids with a rapidly varying frequency spectrum. A further qualitative difference between the SO and the MSO models is an increase (relative to the rigid-surface case) of the dissociative sticking probability at a given impact energy predicted by the latter. The SO model predicts the opposite. This is explained by the observation that in the MSO case, low-barrier reaction paths become available during surface atom motion. In contrast in the SO model the barrier remains stiff, and the loss of relative velocity between the colliding partners is dominant. Anharmonicities in the lattice vibrations are found to have negligible effects within both models. Isotopic substitution of H with D and T, leads to an expected increase of the magnitude of surface-related corrections. It is finally shown that the MSO approach accounts in a way similar to the SO model, for the experimentally observed broadening of the sticking curve with increasing surface temperature. (C) 1997 Elsevier Science B.V.