CO on NaCl(100): Model system for investigating vibrational energy flow

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Abstract
Vibrational energy flow from CO physisorbed on NaCl(100) has been investigated. Infrared spectroscopy of monolayer CO reveals the narrowest linewidth reported for any surface‐bound molecule. The vibrational band at 2154.98 cm−1 has a full‐width at half‐maximum (FWHM) of 0.09 cm−1 at 4 K. We believe that this residual linewidth is due to imperfections of the crystal face. Nevertheless it places a lower limit for all vibrational relaxation processes of 60 ps. The linewidth increases above 10 K, a behavior which can be accounted for by a vibrational phase relaxation mechanism. Attempts to photodesorb CO from its substrate by laser excitation into the vibrational absorption band reveal an exceedingly inefficient quantum yield of Φ≤10−7. Finally laser excitations of a multilayer of CO on NaCl(100) at 22 K demonstrate that vibrational energy can be stored in the adsorbed CO long enough to allow fluorescence however relaxation to the substrate reduces the fluorescence lifetime from that of the bulk crystal. These measurements (bandwidth, vibrational desorption quantum yield, and fluorescence lifetime) of monolayer and multilayer CO on NaCl(100) indicate that the vibrational motions are poorly coupled to this alkali halide substrate.

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