Dynamics of the C-O stretch overtone vibration of CO/Ru(001)

The vibrational spectra of CO adsorbed on Ru(001) have been reinvestigated by infrared absorption spectroscopy at considerably improved sensitivity. A localized two-phonon bound state of the internal stretching mode has been identified. The anharmonicities of the adsorbate vibration have been determined and found to be 18.5 cm(-1) for (CO)-C-12-O-16 and about 17.7 cm(-1) for (CO)-C-12-O-18 and (CO)-C-13-O-16, which is about 40% higher than found for the gas phase species. Rapid broadening of the overtone band with increasing temperature is attributed to the thermally activated decay of the two-phonon bound state into single phonon states.