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(111) and Ni(111) surfaces have been measured for CH4 excited to the first overtone of the antisymmetric C-H stretch (2ν3) 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. 2ν3 excitation of CH4 increases its reactivity by more than four orders of magnitude on Ni(111), whereas on Pt(111) the reactivity increase is lower by two orders of magnitude. We discuss the observed differences in the state-resolved reactivity for the ground state and 2ν3 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.