Anharmonic coupling effects among adsorbate vibrational modes: The model system Ru (001)–( $\sqrt{3}$× $\sqrt{3}$)R30°–CO revisited

Abstract.

Effects associated with the anharmonic coupling of vibrational modes of adsorbate species are reviewed. CO adsorbed on Ru (001) has been chosen as a model system to discuss the various aspects of anharmonicity. Among them are the dephasing of fundamental and overtone modes, the excitation of overtone and combination bands, as well as Fermi resonances. Special attention is given to lateral-coupling effects between adsorbate species leading to a spatial delocalization of vibrational modes. These dynamic interactions invalidate the simple analysis applied to gas-phase molecules. It is shown that even though the dynamic lateral interactions substantially influence the spectral appearance of anharmonically coupled modes, the underlying anharmonic parameters are independent of the layer density and lateral-interaction strength. With appropriate theoretical treatment, anharmonic parameters as well as parameters associated with the dynamical lateral interaction can be extracted from the spectral line shapes and line positions.