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We have investigated the adsorption of CO and CO2 on epitaxially grown Cr2O3(111) by means of EELS. LEED, ARUPS, NEXAFS and XPS. CO is found to adsorb on the oxide surface in an ordered (√3 × √3)R30° structure with the molecular axis oriented approximately parallel to the surface. CO2 on the other hand reacts with the chromium oxide to form a surface carbonate. Adsorption of CO, respectively reaction of CO2 only takes place on a clean, freshly flashed oxide surface. Preadsorption of oxygen leads to a surface which is rather inert to adsorption, likely due to electronic or steric reasons.

A polar Cr,0,(0001) surface is prepared as an epitaxial film on a Cr(ll0) substrate. The film is thick enough to represent the bulk surface. Applying a variety of surface sensitive techniques [thermal desorption spectroscopy (TDS), reflection absorption infrared spectroscopy (RAIRS), electron energy loss spectroscopy (EELS) and photoelectron spectroscopy (PES)] we have studied adsorption of molecular oxygen, ethene and sodium. ExperimentalA chromium oxide film with (0001) orientation was grown on a Cr(ll0) surface via oxidation and thermal treatment c y~l e s .~' -~~ The order of the film was checked via 295

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