The dissociative chemisorption of methane on Ni(111): the effects of molecular vibration and lattice motion.

We examine the dissociative chemisorption of methane on a Ni(111) surface, using a fully quantum approach based on the Reaction Path Hamiltonian that includes all 15 molecular degrees of freedom and the effects of lattice motion. The potential energy surface and all parameters in our model are computed from first principles. Vibrational excitation of the molecule is shown to significantly enhance the reaction probability, and the efficacy for this is explained in terms of the vibrationally non-adiabatic couplings, vibrational mode softening, and mode symmetry. Agreement with experimental data for molecules initially in the ground and 1ν3 state is good, and including lattice anharmonicity further improves our results. The variation of the dissociation probability with substrate temperature is well reproduced by the model, and is shown to result primarily from changes in the dissociation barrier height with lattice motion. The enhancement of dissociative sticking with substrate temperature is particularly strong for processes that would otherwise have insufficient energy to surmount the barrier. Our model suggests that vibrationally excited molecules are likely to dominate the "laser off" dissociative sticking at high nozzle temperatures.