VIBRATIONAL DEPHASING AT SURFACES - THE ROLE OF CUBIC ANHARMONICITY AND FERMI RESONANCES

We make a theoretical study of the vibrational contributions to the li ne shape of a top-bonded adsorbate. Dephasing of the adsorbate-substra te stretch occurs via anharmonic coupling to lower-frequency modes. We find the surprising result that central forces, which are often much larger than bond-bending forces, do not contribute to the linewidth, d ue to a cancellation between cubic and quartic terms in the effective coupling to the dephasing modes. This cancellation is complete wheneve r the probed mode has a frequency far above all others in the system, e.g., H/Si(111) or the C-O stretch in CO on metals. The C-metal stretc h in CO on metals is more complicated, as it lies well below the C-O s tretch. If the CO bond is treated as rigid, the cubic term introduces Fermi-resonance effects which greatly broaden the linewidth and also g ive rise to extra peaks in the absorption spectrum. However, we show t hat the internal motion of the molecule, no matter how small, causes a dramatic reduction in the magnitude of these effects, probably render ing them unobservable. We suggest that one must look to anharmonic ter ms in the noncentral (bending) interatomic forces to explain the magni tude of the experimental dephasing linewidth.