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

We have used low (two- and three-) dimensional models to examine the interplay between vibrational effects in the dissociative adsorption of H-2 on Cu(111) and the vibrational excitation of the reflected molecules. It is found that the vibrational non-adiabaticity required for the latter causes the dissociation probabilities for vibrationally excited 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 contradiction with the experimental findings. Introducing a rotational coordinate, to give the possibility of rotational-vibrational coupling, does not produce better agreement. These results suggest that the processes occur at different surface sites.