Promotion of dissociative chemisorption with vibrational and translational energy

Molecular‐beam techniques have been employed to probe the effects of translational and vibrational energy in promoting dissociative chemisorption. We review work on the three systems that we have examined to date: N₂/Fe(111), CH₄/W(110), and N₂/W(110). These two forms of energy are found to be roughly equivalent in their ability to promote dissociation, both being highly efficient. Specifically, results for the CH₄/W(110) system are found to be consistent with a model in which all vibrational modes are equally effective and vibrational energy and translational energy are approximately equivalent in promoting this highly activated process. In the N₂/Fe(111) case, we find that for kinetic energies in the range 0.3–0.6 eV that vibrational energy is on average only about one‐half as effective as translational energy. This is consistent with a proposed precursor mechanism, where access to the precursor state is blocked by a potential barrier that is more easily overcome with translational than with vibrational energy. Finally, for N₂ on W(110), we again find a rough equivalence between these two forms of energy, but here results are clouded by the unknown effect of high levels of rotational energy present in the effusive beams employed for that study.