DISSOCIATION THRESHOLDS AND THE VIBRATIONAL-EXCITATION PROCESS IN THESCATTERING OF H-2

We have used low (two- and three-) dimensional models to examine the i nterplay between vibrational effects in the dissociative adsorption of H-2 on Cu(111) and the vibrational excitation of the reflected molecu les. It is found that the vibrational non-adiabaticity required for th e latter causes the dissociation probabilities for vibrationally excit ed states to shift to lower energies, giving a vibrational efficacy of 1. To reduce this to the experimental value of approximately 0.5, it is necessary to force the vibrational excitation to high energy, in co ntradiction with the experimental findings. Introducing a rotational c oordinate, to give the possibility of rotational-vibrational coupling, does not produce better agreement. These results suggest that the pro cesses occur at different surface sites.