Vibrational effects in the dissociation and scattering of hydrogen at a Cu(111) surface

Abstract

We have determined dissociative chemisorption probabilities for D 2 on Cu(111) as a function of translational energy and angle of incidence for molecules in specific vibrational states. We find that the translational energy required for dissociation is reduced with increasing vibrational energy, falling from ≈0.6 eV for D 2(v=0) to ≈0.4 and 0.2 eV for molecules in v=1 and v=2 states, respectively. These results were obtained by combining molecular beam measurements of absolute adsorption probabilities with studies of the velocity distributions of molecules desorbed from the surface in specific quantum states. The observations indicate that vibrational and translational energy are strongly coupled by the potential energy surface for this system. This conclusion is supported by quantum-state-resolved scattering experiments that reveal direct vibrational excitation in collisions of H 2 and D 2 with a Cu(111) surface. The threshold for excitation of v=0 to v=1 is found to be similar for both molecules, and slightly higher than that for dissociation.