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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.

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Transition metal oxides are often used as the active components in heterogenous catalysis. Therefore the investigation of single crystal oxides as model systems is important to understand the reaction mechanisms on a microscopic level. The reactivity of a ͑111͒ surface of ionic rocksalt type structures seems to be rather high as has been established for NiO͑111͒. The ideal ͑111͒ surface is polar, and thus unstable, which means that stabilization mechanisms must exist. Here, high resolution XPS measurements of a thin epitaxial CoO͑111͒ film grown on CO͑0001͒ by exposing the surface to Ϸ10000 L O 2 are reported.

We report experimental and theoretical evidence of H 2 adsorbed in a confined quantum rotor state on a stepped copper surface. Rotational transitions of step-adsorbed para-H 2 and ortho-H 2 observed in electron-energy-loss measurements occur close to the energies expected for an ideal 2D rotor. Dramatic enrichment of adsorbed ortho-H 2 is observed at elevated surface temperatures and hydrogen background pressures. Calculations reveal a molecule residing above a step atom and confined to rotate in a twodimensional manner. PACS numbers: 68.45.Kg, 67.70. + n, 82.65.Pa Recently, spirited attention has been directed to rotorlike states of single molecules adsorbed on metal surfaces, stimulated by ideas regarding molecular-based mechanical and memory devices. Experimental visualization using scanning tunneling microscopy has been reported, concerning, for example, supramolecular bearings formed by large organic molecules on a copper surface [1] and tunneling-current-controlled rotation of small molecules adsorbed on platinum [2] and copper surfaces [3]. Confined metal dimer rotation on a closepacked metal surface has been theoretically predicted [4]. The rotary motion, driven either by thermal energy or inelastic electron tunneling is in these cases restricted by energy barriers and is diffusive.Free rotary motion confined to two dimensions (2D rotor) is also conceivable, at least for weakly adsorbed light molecules such as hydrogen. For example, the mechanism of preferential adsorption of metastable orthohydrogen species was long ago [5] suggested to be associated with a 2D rotor adsorption state [6]. Enrichment occurs readily on activated alumina. Silvera and Nielsen [7] observed, using inelastic neutron scattering, that the rotational spectrum of hydrogen adsorbed on alumina powder at low temperatures was greatly distorted from that of the free molecule. They found that their observations were more consistent with an axially constrained rotor state. In the extreme limit the confining potential will in this case lead to a loss of two rotational degrees of freedom and the molecule can only vibrate against the surface. Whereas hydrogen molecules physisorbed on flat crystal surfaces [8-10] experience only weak rotational hindering, for local adsorption at low-coordinated surface sites such as adatoms or step atoms, a much stronger hindering may be conceived.In this Letter we present electron-energy-loss measurements and density functional calculations, which provide evidence of hydrogen adsorbed as a confined rotor on a stepped copper surface. Our measurements reveal, for step-adsorbed molecules, rotational transitions of parahydrogen ( p-H 2 ) and orthohydrogen (o-H 2 ) at energies close to the values expected for an ideal 2D rotor. This adsorption state is readily identified in total energy calculations and corresponds to a molecule residing above a single atom at the step edge in an axially symmetric orientational potential barrier, which confines the molecule to rotate in a two-dimensional manner. Our measurement...