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 [∼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