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Abstract:
Vapour deposition of Pd with CO background pressure onto an ordered alumina film kept at 90 K results in the formation of a palladium carbonyl-like compound. The growth and decomposition of this Pdx(CO)y species is investigated using SPA-LEED (spot-profile analysis), thermal desorption spectroscopy (TDS) and photoemission of both core and valence levels. It is found that the nucleation and growth process is drastically modified in the presence of CO. The TD spectrum for the carbonyl-like species exhibits a sharp and dominant CO desorption peak at 190 K, which is not observed for Pd deposits grown under UHV conditions. This feature appears for coverages ranging from fractions of a monolayer to multilayers and is associated with decomposition of the compound. The binding energy shifts observed for increasing amounts of Pdx(CO)y indicate that metal-metal bonds are formed during the growth. Upon heating, two different processes occur. (I) The compound decomposes under formation of compact, metallic Pd clusters which are present up to 500 K. (II) Heating to higher temperatures results in diffusion of the Pd atoms into the oxide film. It is demonstrated how these processes can be directly monitored by recording the intensity of the oxide LEED pattern as a function of temperature. Finally, we briefly discuss the data in terms of possible growth mechanisms of the Pdx(CO)y species.
