AVS 53rd International Symposium
    Surface Science Tuesday Sessions
       Session SS-TuP

Paper SS-TuP2
Vibrational Energy Flow in a Gas-Surface Reaction

Tuesday, November 14, 2006, 6:00 pm, Room 3rd Floor Lobby

Session: Surface Science Poster Session
Presenter: D.R. Killelea, Tufts University
Authors: D.R. Killelea, Tufts University
A.L. Utz, Tufts University
Correspondent: Click to Email
Energy flow plays a central role in the activation of surface reactions. We measure how energy selectively deposited into the translational, rotational, and vibrational coordinates of methane influences S@sub 0@ on Ni(100) and Ni(111) to answer key questions about energy flow in this prototypical gas-surface reaction. Infrared laser excitation of CH@sub 4@ in a supersonic molecular beam prepares a microcanonical ensemble of reagents with well-defined internal and translational energies. We quantify reactivity for the state-selected reagents and observe non-statistical patterns of activation, including vibrational mode specificity. For example, the @nu@@sub 3@ C-H stretch is more reactive than the second overtone of the umbrella-bending mode, 3@nu@@sub 4@, even though 3@nu@@sub 4@ contains 30% more energy. On Ni(111) energy deposited in @nu@@sub 3@ activates dissociative chemisorption more than an equivalent amount of translational energy. To understand these results, we propose a framework for describing energy flow among the energetic degrees of freedom available to the molecule-surface reaction complex. Our model draws on and is consistent with descriptions of intramolecular vibrational energy flow in both the gas-phase and in liquids, but accounts for features unique to gas-surface reactivity. We focus on the characteristic timescales for energy flow and the molecule-surface interaction time. We conclude that for direct dissociative chemisorption of CH@sub 4@, there is insufficient time for full energy randomization among the energetic degrees of freedom in the molecule-surface reaction complex. Our model not only predicts vibrational mode-specific behavior - it also suggests a consistent explanation of surface temperature effects, explains the success of statistical theories to model ensemble-averaged reactivity even when energy flow is not statistical, and provides a framework for assessing the extent of energy flow in other gas-surface reactions.