State-resolved reactivity of CH4(2nu3) on Pt(111) and Ni(111): effects of barrier height and transition state location.

Quantum state-resolved sticking coefficients on Pt(111) and Ni(111) surfaces have been measured for CH4 excited to the first overtone of the antisymmetric C-H stretch (2nu3) at well-defined kinetic energies in the range of 10-90 kJ/mol. The ground-state reactivity of CH4 is approximately 3 orders of magnitude lower on Ni(111) than on Pt(111) for kinetic energies in the range of 10-64 kJ/mol, reflecting a difference in barrier height of 28+/-6 kJ/mol. 2nu3 excitation of CH4 increases its reactivity by more than 4 orders of magnitude on Ni(111), whereas on Pt(111) the reactivity increase is lower by 2 orders of magnitude. We discuss the observed differences in the state-resolved reactivity for the ground state and 2nu3 excited state of methane in terms of a difference in barrier height and transition state location for the dissociation reaction on the two metal surfaces.