State-Resolved Reactivity of Methane (ν₂ + ν₄) on Ni(111)

Vibrational state-resolved experiments that probe methane dissociation on Ni(111) quantify the reactivity of CH4 excited to the ν2 + ν4 bending vibration. A comparison of these data and previous state-resolved measurements reveals that this bending vibration is significantly less reactive than the ν3 C-H stretching vibration in the same polyad of vibrations. Comparison with the 3ν4 bend overtone also suggests that the doubly degenerate bending state, ν2, is less effective than the triply degenerate bend (ν4) in promoting methane dissociative chemisorption on Ni(111). This observation of vibrational mode selectivity contradicts thermal statistical theories of gas-surface reactivity and provides direct experimental evidence of roles that different vibrational states can play in activating this gas-surface reaction.