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Quantum state resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH in three different vibrational symmetry components, A, E, and F, of the 2ν antisymmetric stretch overtone vibration as well as in the ν+ν symmetric plus antisymmetric C-H stretch combination band of F symmetry. The quantum state specific dissociation probability S (sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH dissociation by Auger electron spectroscopy. We observe strong mode specificity, where S for the most reactive state ν+ν is an order of magnitude higher than for the least reactive, more energetic 2ν-E state. Our first principles quantum scattering calculations show that as molecules in the ν state approach the surface, the vibrational amplitude becomes localized on the reacting C-H bond, making them very reactive. This behavior results from the weakening of the reacting C-H bond as the molecule approaches the surface, decoupling its motion from the three non-reacting C-H stretches. Similarly, we find that overtone normal mode states with more ν character are more reactive: S(2ν) > S(ν + ν) > S(2ν). The 2ν eigenstates excited in the experiment can be written as linear combinations of these normal mode states. The highly reactive 2ν and ν + ν normal modes, being of A and F symmetry, can contribute to the 2ν-A and 2ν-F eigenstates, respectively, boosting their reactivity over the E component, which contains no ν character due to symmetry.

Quantum-state-resolved reactivity of overtone excited CH4 on Ni(111): Comparing experiment and theory