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 12C18O and 13C16O 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.