AVS 47th International Symposium
    Surface Science Wednesday Sessions
       Session SS3-WeM

Invited Paper SS3-WeM3
Gas-Surface Dynamics of State-Selected Reagents

Wednesday, October 4, 2000, 9:00 am, Room 210

Session: Gas/Surface Dynamics
Presenter: A.L. Utz, Tufts University
Authors: A.L. Utz, Tufts University
L.B.F. Juurlink, Tufts University
R.R. Smith, Tufts University
C.L. DiCologero, Tufts University
Correspondent: Click to Email
We use supersonic molecular beams of rovibrationally state-selected molecules to probe methane dissociation dynamics on Ni(100). These studies reveal key features of the gas-surface potential energy surface governing activated dissociative chemisorption. Excitation of the @nu@@sub 3@ antisymmetric C-H stretching vibration enhances reactivity by a factor of 1600 relative to molecules in v=0, which points to the important role of C-H stretch excitation in methane activation. The Coriolis-coupled sublevels of the triply degenerate @nu@@sub 3@ mode differ considerably in the relative motion of the four excited C-H oscillators, but they share a similar efficacy for promoting dissociation. This observation suggests that the reactivity of @nu@@sub 3@ is likely derived from its C-H stretching character in the one C-H bond that is properly oriented for dissociative chemisorption, but not from the collective motion of all four C-H oscillators in the molecule. Rotational-state-resolved studies show that @nu@@sub 3@ reactivity varies by less than a factor of two from J=0 to 3 and provide tentative support for dynamical steering effects. Studies of other vibrational states in methane provide a dramatic example of vibrational mode specificity in a gas-surface reaction. We find that the second overtone of the @nu@@sub 4@ triply degenerate bending mode, 3@nu@@sub 4@, is at least five times less reactive than @nu@@sub 3@, despite its containing 30% more internal energy. Mode specificity establishes that a vibrationally excited molecule retains enough memory of its initial state to influence its reactivity on a metal surface. The lack of reactivity in the @nu@@sub 4@ coordinate, coupled with the absolute reactivities we measure for the @nu@@sub 3@ state, suggests that combination vibrations containing both stretch and bend excitation may be the most reactive vibrational states in a thermal sample of methane.