Comparing the vibrational properties of low-energy modes of a molecular and an atomic adsorbate: CO and O on Pt(111)

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Abstract
In this work we present a situation where it has been possible to compare the vibrational properties of the low-energy modes of a molecular and an atomic adsorbate on a metal surface. We have, by infrared spectroscopy, studied well-ordered structures of carbon monoxide chemisorbed in two different sites and atomic oxygen in a third site, all three on the same surface, with the same relative coverage and studied under identical conditions. We are able to present a unique test of the antiabsorption mechanism and its applicability to transition metals like platinum. For atomic oxygen we observe a dip in the absorptance spectra associated with the frustrated translation, while in contrast no such dip was observed for the frustrated rotations of CO. This emphasizes that the magnitude of the broadband decrease in reflectivity and the character of the adsorbate motion is very important for the strength of the effect. We also present the first study on the temperature dependence of the antiabsorption, the results being in agreement with theory. For the metal–adsorbate stretch vibrational modes we make relative comparisons between the observed integrated absorptance for the different species. We find a good correspondence between the relative values of the oscillating charge and the binding energy by making simple estimations on the bond geometry and the screening by the metal electrons.