Paper SS2-TuA4
The Role of Surface Atom Motion in Methane Activation on Ni(111)

Tuesday, October 21, 2008, 2:40 pm, Room 209

Quantum state resolved gas-surface reactivity measurements have yielded detailed insights into how a gas phase reagent's vibrational energy promotes reaction, but our understanding of how surface vibrations influence reactivity is less complete. Here, we show that state-selected reagents, with their precisely defined internal and translational energy, are powerful probes of the reaction dynamics along other important energetic degrees of freedom, including surface atom motion. We focus on how surface atom motion influences the distribution of energetic barriers experienced by methane molecules prepared in v=1, J=2 of the ν₃ C-H stretching state and incident on a clean Ni(111) surface. Recent theoretical work suggests when a nickel atom is displaced above the (111) surface plane, the energetic barrier for reaction decreases. Within a dynamical framework, this result suggests that increasing surface temperature increases the probability that an incident methane molecule will impact on a Ni atom in this energetically favorable geometry. Statistical models of reactivity assume that energy in phonon modes is pooled to activate the reaction complex; increased surface temperature increases the pool of energy available to activate the reaction. We use state-selected reagents, with their well-defined internal and translational energies, to quantify reaction probability as a function of incident kinetic energy at a series of surface temperatures. At each surface temperature, the state-resolved reaction probability curves we obtain reveal the effective distribution of barriers along the translational energy coordinate. Changes in the shape of the effective barrier height distribution function with respect to surface temperature reveal how surface atom motion impacts reaction energetics. Our ability to prepare and control the non-equilibrium distribution of energy among the many degrees of freedom in this system permits a test of the energy pooling assumption.