ADSORPTION OF CO ON RH(100) STUDIED BY INFRARED REFLECTION ABSORPTION-SPECTROSCOPY

The interaction of CO with a Rh(100) surface at 90 and 300 K has been investigated with infrared reflection-absorption spectroscopy (IRAS). Absorption bands due to the C-O stretch are found in both the linear and bridging regions at all coverages for both adsorption temperatures. For adsorption at 300 K, an ordered CO layer is formed at ∼0.4 monolayers (ML) as evidenced by a sharp, highly symmetrical, linear-CO band. At θCO > 0.45 ML, the reduction in intermolecular separation and increasing intermolecular repulsive forces in the CO adlayer are evidenced by the increased broadness of both the linear- and the bridged-CO bands. CO adsorption at 90 K yields a nonuniform adlayer dominated by island formation at θCO < 0.5 as indicated by peak splitting of the linear-CO band and the presence of a broad bridged-CO band. At θCO ∼0.5, a highly ordered CO adlayer is suggested by the appearance of a highly symmetrical linear-CO band as well as the formation of a sharp c(2x2) LEED pattern. The presence of multiple components in the linear-CO band along with the drastic variations in the relative absorption intensities for both CO bands at coverages between 0.5 and 0.7 ML imply that the CO adlayer undergoes a phase transition within this coverage regime. These IRAS data are in agreement with the observed transition of a sharp c(2x2) pattern at θCO = 0.5 to an ordered LEED pattern with a p(4√2x4√2)R 45° coincidence structure at θCO = 0.7. From a series of temperature-dependent IRAS measurement, it has been determined that the difference in binding energy of linear and bridged sites at θCO = 0.2-0.5 ranges from ∼0.1 to ∼0.4 kcal/mol with the bridged-bound CO being the more stable species. © 1990 American Institute of Physics.