Reactivity of vibrationally excited methane on nickel surfaces

Four-dimensional variational calculations have been performed for modeling energy flow between methane (CH4) stretching vibrational energy states as the molecule adiabatically approaches a metallic surface. The model is based on a local mode Hamiltonian for an isolated CH4 molecule and a London-Eyring-Polanyi-Sato potential describing surface-molecule interactions. The results suggest the possibility of mode specific effects on chemical reactivity. Specifically, the symmetric A(1) stretch fundamental adiabatically correlates with the localized excitation in the unique CH bond pointing towards the surface. Conversely, the antisymmetric F-2 stretch fundamental excitation correlates with A and E vibrations in the CH3 radical, and therefore this degree of freedom is localized away from the reactive CH bond. Landau-Zener semiclassical analysis of nonadiabatic curve crossings predicts a significant velocity dependence to the state specific energy flow dynamics. Since excitation localized in active versus spectator bonds is expected to be more efficient in accelerating CH bond cleavage and adsorption reactivity, these results offer insight into interpreting velocity and vibrationally mediated reaction dynamics of CH4 on catalytic surfaces. (C) 2001 American Institute of Physics.