Infrared absorption study of physisorbed carbon monoxide on graphite

We have applied infrared reflection absorption spectroscopy (IRRAS) to the study of carbon monoxide monolayers adsorbed on a single surface of graphite (HOPG). Concurrent monitoring by laser ellipsometry determines the time of appearance of the first and second layer and of bulk CO. CO is one of the few adsorbates previously studied on graphite by IRRAS. Our spectra agree with the previously published results at 35 K and extend the temperature range to 20–40 K, including all four known solid monolayer phases in this range, and unambiguously establish which spectrum corresponds to the bilayer. Integrated absorption strength provides estimates of the average tilt of the molecular axes with respect to the surface in the nominally flat phases. Frequency shifts due to dynamic dipole coupling may contain information on the short-range correlations of tilt azimuths. We find evidence for specific tilt correlations in the commensurate phases.