How far can classical mechanics be trusted when treating surface reactions?

The non-activated dissociation of H-2 on W(100) has been studied using quantum and classical methods. It is found that there is remarkably good agreement between the dissociation probabilities computed with the two methods for molecules incident normally in the first four rotational states. For the rotational ground state, the dissociation decreases with increasing molecular translational energy because it becomes harder for the molecules to be steered into the most favourable dissociation geometry, as is demonstrated with swarms of classical trajectories. For the rotationally excited states, the dissociation is also affected by orientational hindering, resulting in different dissociation probabilities for different azimuthal quantum numbers, m(j). This dependence on the orientation of the angular momentum is also faithfully reproduced by the classical methods. Resonant trapping is also demonstrated both classically and quantum mechanically; in the latter case it gives rise to rapid oscillations in the dissociation probability. Other calculations have shown these to be much greater for H-2/Pd(100). It is shown that this is due to the presence of a well which leads to selective, adsorption-like resonances.