State-resolved reactivity of CH4(2ν3) on Pt(111) and Ni(111):: Effects of barrier height and transition state location

Quantum state-resolved sticking coefficients on Pt(l I I) and Ni(l 11) surfaces have been measured for CH4 excited to the first overtone of the antisymmetric C-H stretch (2 nu 3) 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(1 11) than on Pt(I 11) for kinetic energies in the range of 10-64 kJ/mol, reflecting a difference in barrier height of 28 6 kJ/mol. 2 nu 3 excitation of CH4 increases its reactivity by more than 4 orders of magnitude on Ni(I 11), whereas on Pt(I 11) 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 2 nu 3 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.