Identification of a surface reaction product by detection of a vibrational combination band

NCO molecules formed by the reaction of adsorbed CO and nitrogen atoms are unambiguously identified by means of infrared absorption spectroscopy. This identification of a surface reaction product is achieved by the detection of the combination band of two intramolecular modes. Isotopic substitution experiments using (CO)-C-12-O-18 and (CO)-C-13-O-16 are used to confirm the mode assignments through the unequal shifts of the various vibrational modes. The intermode anharmonicity is accurately determined to be 25.5-26 cm(-1) which establishes the first reliable number of this type for an adsorbate species.