Rovibrational state-resolved studies of methane dissociation on nickel(111)

Using supersonic molecular beam techniques along with narrow-band infrared laser excitation, we have experimentally measured the state-resolved reaction probabilities for methane dissociation on a clean Ni(111) surface. In particular, we report the reactivity on this surface for methane in the rovibrational ground state, for methane excited to one quantum of the ν₃ antisymmetric stretching vibration (ν′ = 1, J ′ = 2), and for methane excited to three quanta of the ν ₄ asymmetric bending vibration (ν′ = 3, J′ = 2). We have found, for methane that has been excited to ν₃, an enhancement in reactivity on this surface of at least a factor of 200 greater than that of the vibrational ground state, depending upon the incident translational energy. Furthermore, we have found that the reactive efficacy of molecules excited to this mode is much larger than that of an equivalent amount of incident translational energy, a result which effectively rules out the possibility of this system being described by a statistical mechanism. In comparing this observed efficacy with that previously measured on a Ni(100) surface we have found that excitation of methane in the ν₃ mode is more effective, relative to translational energy, at overcoming the barrier to dissociation on Ni(111). This is interpreted in terms of the Ni(111) surface presenting a wider, yet on average higher, Gaussian distribution of barrier heights than the Ni(100) surface. We have also found a large increase in the reactivity for methane excited to 3ν ₄ relative to the vibrational ground state, though not to the same extent as excitation to ν₃. The ordering of the measured vibrational states was found to be consistent with that of recent theoretical predictions. Furthermore, we were able to use these data to construct a simple model that accurately describes the reactivity of a thermal distribution of methane molecules in a laser-off experiment. These results seem to indicate that, at modest translational energies and relatively common nozzle temperatures, all of the observed reactivity in a molecular beam experiment is due to the thermal population of the low energy bending states.