Surface-induced bond anharmonicity probed through a two-phonon bound state: Methoxide adsorbed on Mo(110)

The ${\nu }_{0\to 2}$ excitation of the C-O stretch in methoxide adsorbed on Mo(110) is analyzed by infrared absorption spectroscopy. Using isotopic dilutions we conclusively assign this vibrational band to a two-phonon bound state. The influence of the surface on the intramolecular potential can thereby be directly probed. A threefold increase of the anharmonicity is found for the adsorbed species compared to gaseous methanol. The effect of both dipole-dipole coupling and adsorbate inhomogeneity on the two-phonon bound state is experimentally investigated. © 1996 The American Physical Society.