Abstract
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 $18.5{\mathrm{cm}}^{\ensuremath{-}1}$ for ${}^{12}{\mathrm{C}}^{16}\mathrm{O}$ and about $17.7{\mathrm{cm}}^{\ensuremath{-}1}$ for ${}^{12}{\mathrm{C}}^{18}\mathrm{O}$ and ${}^{13}{\mathrm{C}}^{16}\mathrm{O}$, 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.
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