STUDIES OF BIMETALLIC SURFACES USING INFRARED REFLECTION ABSORPTION-SPECTROSCOPY

Well-defined bimetallic surfaces have been studied using carbon monoxide adsorption in conjunction with infrared reflection-absorption spectroscopy. The vibrational frequency of CO on low Cu coverages [approximately 0.1 monolayer (ML)] on Rh (100) and Mo (110) is blue-shifted relative to its gas phase stretching frequency of 2143 cm-1. This blue shift is explained as arising from CO adsorbed on well-dispersed Cu adatoms which are slightly positively charged due to polarization arising from the CO-Cu interaction. As the sample temperature is increased to 150-200 K, CO becomes mobile on these surfaces as evidenced by the migration of CO from submonolayer Pd overlayers to a W (110) substrate. Three-dimensional Cu clusters, well-ordered pseudomorphic two-dimensional Cu islands, and single Cu atoms are distinctively characterized by their CO infrared (IR) peaks. In addition, it is found that IR spectra of adsorbed CO show a remarkable sensitivity to surface structural phase transitions. Both order-order and disorder-order transitions are observed for Ni and Co overlayers on a Mo (110) substrate. It is further shown that localized segregation and ordering of C and O on Mo (110), B on Ru (0001) and Co on S/Mo (110) are observable.