Mode-selective chemistry on metal surfaces: The dissociative chemisorption of CH₄ on Pt(111)

A quantum approach based on an expansion in vibrationally adiabatic eigenstates is used to explore CH₄ dissociation on Pt(111). Computed sticking probabilities for molecules in the ground, 1v₃ and 2v₃, states are in very good agreement with the available experimental data, reproducing the variation in reactivity with collision energy and vibrational state. As was found in similar studies on Ni(100) and Ni(111), exciting the 1v₁ symmetric stretch of CH₄ is more effective at promoting the dissociative chemisorption of CH₄ than exciting the 1v₃ antisymmetric stretch. This behavior is explained in terms of symmetry, mode-softening, and nonadiabatic transitions between vibrationally adiabatic states. We find that the efficacies of the bending modes for promoting reaction are reasonably large, and similar to the 1v₃ state. The vibrational efficacies for promoting reaction on Ni(111) are larger than for reaction on Pt(111), due to the larger nonadiabatic couplings. Our computed sticking probabilities are in good agreement with results from recent ab initio molecular dynamics and reactive force field studies.