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

Quantum state resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH₄ on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH₄ in three different vibrational symmetry components, A₁, E, and F₂, of the 2ν₃ antisymmetric stretch overtone vibration as well as in the ν₁+ν₃ symmetric plus antisymmetric C-H stretch combination band of F₂ symmetry. The quantum state specific dissociation probability S₀ (sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH₄ dissociation by Auger electron spectroscopy. We observe strong mode specificity, where S₀ for the most reactive state ν₁+ν₃ is an order of magnitude higher than for the least reactive, more energetic 2ν₃-E state. Our first principles quantum scattering calculations show that as molecules in the ν₁ 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 ν₁ character are more reactive: S₀(2ν₁) > S₀(ν₁ + ν₃) > S₀(2ν₃). The 2ν₃ eigenstates excited in the experiment can be written as linear combinations of these normal mode states. The highly reactive 2ν₁ and ν₁ + ν₃ normal modes, being of A₁ and F₂ symmetry, can contribute to the 2ν₃-A₁ and 2ν₃-F₂ eigenstates, respectively, boosting their reactivity over the E component, which contains no ν₁ character due to symmetry.