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 g ood agreement between the dissociation probabilities computed with the two methods for molecules incident normally in the first four rotatio nal states. For the rotational ground state, the dissociation decrease s with increasing molecular translational energy because it becomes ha rder for the molecules to be steered into the most favourable dissocia tion geometry, as is demonstrated with swarms of classical trajectorie s. For the rotationally excited states, the dissociation is also affec ted by orientational hindering, resulting in different dissociation pr obabilities for different azimuthal quantum numbers, m(j). This depend ence on the orientation of the angular momentum is also faithfully rep roduced by the classical methods. Resonant trapping is also demonstrat ed both classically and quantum mechanically; in the latter case it gi ves rise to rapid oscillations in the dissociation probability. Other calculations have shown these to be much greater for H-2/Pd(100). It i s shown that this is due to the presence of a well which leads to sele ctive, adsorption-like resonances.