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Abstract

Palladium deposits on a single crystalline Cr2O3(0001) film grown on Cr(110) have been studied under ultrahigh vacuum conditions. The growth mode at two different temperatures (90 and 300 K) was investigated using infrared reflection absorption spectroscopy of the probe molecule CO. The reduction of the amount of CO adsorbed on the oxide surface measured by the decrease of the CO stretching signal intensity was used to extract information about the surface area covered by the palladium deposits. The shape, fine structure, and half-width of the infrared absorption bands in the stretching frequency region of CO molecules bound to the palladium aggregates provides information about the morphology of the deposits. Additionally. the palladium induced attenuation of the phonon signals of the Cr2O3(0001) film has been studied applying HREELS. At 90 K, the palladium grows in two-dimensional islands which cover the entire oxide surface after deposition of one monolayer equivalent. If the palladium is deposited at a sample temperature of 300 K. the islands are three-dimensional but with a low aspect ratio. The changes of the structure of the deposits with increasing temperature are studied by infrared reflection absorption spectroscopy using the probe molecule CO and thermal desorption spectroscopy of CO. Upon an increase in the temperature, the crystallinity of the disordered particles grown at 90 K increases. Between 300 and 500 K, no structural changes are observed. At temperatures above 600 K, the palladium-induced signal vanishes in the infrared spectra. We suppose that the palladium diffuses into the substrate.