State-Resolved Gas-Surface Reactivity of Methane in the Symmetric C-H Stretch Vibration on Ni(100)

The state-resolved reactivity of CH₄ in its totally symmetric C-H stretch vibration (ν₁) has been measured on a Ni(100) surface. Methane molecules were accelerated to kinetic energies of 49 and 63.5 kJ/mol in a molecular beam and vibrationally excited to ν₁ by stimulated Raman pumping before surface impact at normal incidence. The reactivity of the symmetric-stretch excited CH₄ is about an order of magnitude higher than that of methane excited to the antisymmetric stretch (ν₃) reported by Juurlink et al. [Phys. Rev. Lett. 83, 868 (1999)] and is similar to that we have previously observed for the excitation of the first overtone (2ν₃). The difference between the state-resolved reactivity for ν₁ and ν₃ is consistent with predictions of a vibrationally adiabatic model of the methane reaction dynamics and indicates that statistical models cannot correctly describe the chemisorption of CH₄ on nickel.