Publication Details (including relevant citation information):
H.L. Abbott and I. Harrison, Journal of Chemical Physics, 125, 024704 (2006).
A simple picture of the hydrogen dissociation/associative desorption dynamics on Cu(111) emerges from a two-parameter, full dimensionality microcanonical unimolecular rate theory (MURT) model of the gas-surface reactivity. Vibrational frequencies for the reactive transition state were taken from density functional theory calculations of a six-dimensional potential energy surface (Hammer et al., Phys. Rev. Lett. 73, 1400 (1994)). The two remaining parameters required by the MURT were fixed by simulation of experiments. These parameters are the dissociation threshold energy, E0 =79 kJ/mol, and the number of surface oscillators involved in the localized H2/Cu(111) collision complex, s=1. The two-parameter MURT quantitatively predicts much of the varied behavior observed for the H2 and D2/Cu(111) reactive systems, including the temperature-dependent associative desorption angular distributions, mean translational energies of the associatively desorbing hydrogen as a function of rovibrational eigenstate, etc. The divergence of the statistical theory’s predictions from experimental results at low rotational quantum numbers, J<~5, suggests
that either (i) rotational steering is important to the dissociation dynamics at low J, an effect that washes out at high J, or (ii) molecular rotation is approximately a spectator degree of freedom to the dissociation dynamics for these low J states, the states that dominate the thermal reactivity. Surface vibrations are predicted to provide ~30% of the energy required to surmount the activation barrier to H2 dissociation under thermal equilibrium conditions. The MURT with s=1 is used to analytically confirm the experimental finding that d“Ea(Ts)”/dEt=−1 for eigenstate-resolved dissociative sticking at translational energies Et<E0−Ev−Er. Explicit treatment of the surface motion (i.e., surface not frozen at Ts=0 K) is a relatively novel aspect of the MURT theoretical approach.
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