Adsorption Geometry of CO versus Coverage on TiO2(110) from s- and p-Polarized Infrared Spectroscopy

The adsorption of CO on reduced, rutile TiO2(110) is investigated using IR reflection-absorption spectroscopy and temperature-programmed desorption. Experiments using s- and p-polarized IR light incident along the [001] and [1 (1) over bar0] azimuths give detailed information on the adsorption geometry of the CO as a function of the CO coverage, theta(CO). The results indicate that for theta(CO) <= 1 ML, CO adsorbs oriented perpendicular to the surface at Ti-5c sites. For 1 < theta(CO) <= 1.5 ML, the bonding geometry of the CO adsorbed at Ti-5c sites is unchanged, whereas the additional CO molecules adsorb at O-b sites parallel to the surface and parallel to the [1<(1)over bar>0] azimuth. The results do not support previous suggestions that CO at Ti-5c sites tilt similar to 20 degrees from normal at high coverages. The results demonstrate the utility, of polarization-resolved infrared reflection-absorption spectroscopy for elucidating adsorption geometries on dielectric substrates.