AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS3+MC-TuM

Paper SS3+MC-TuM5
Surface Vibrational Spectroscopy Beyond the Harmonic Approximation, Experiments and Calculations

Tuesday, October 3, 2000, 9:40 am, Room 210

Session: Technique Innovations: Experiment, Theory and Simulation
Presenter: P. Uvdal, Lund University, Sweden
Authors: P. Uvdal, Lund University, Sweden
M.P. Andersson, Lund University, Sweden
R. Asmundsson, Lund University, Sweden
A.D. MacKerell, Jr., University of Maryland
Correspondent: Click to Email
Vibrational analysis of surface adsorbates involves in general the assignment of fundamental normal modes. Based on such assignment conclusions about chemical identity and geometry can be drawn. It is however well known from fundamental textbooks on molecular vibrations that if one goes beyond the harmonic approximation of the intramolecular bond potential overtone and combination modes are allowed. Binary modes, i.e. excitation of one vibration with two quanta or excitation of two vibrations with one quanta, will be the most intense even though higher excitations are allowed. The presence or absence of binary modes will contain information about bond anharmonicity and coupling between different modes. The latter may reveal surface induced changes of activation barriers of importance for the understanding of e.g. heterogeneous catalysis. The former reveal dynamical properties of the system as the extent of coupling governs the dissipation and randomization, within the molecule, of the energy initially localized in a single bond excitation. Using methoxy and ethoxy adsorbed on Cu(100) and W(110) surfaces as model systems we will demonstrate binary intramolecular modes in the presence and absence of Fermi resonance coupling. We will also show a combination mode consisting of the C-O stretch and the hindered motion of methoxy with respect to the surface.