DISSOCIATIVE CHEMISORPTION OF OXYGEN ON THE RU(001) SURFACE - SPECTROSCOPIC IDENTIFICATION OF PRECURSOR INTERMEDIATES AT LOW SURFACE TEMPERATURES

The adsorption of oxygen on the Ru(001) surface at 30 K has been investigated by high-resolution electron energy loss spectroscopy. Dissociative chemisorption occurs up to the saturation atomic coverage [nu(perpendicular-to) (Ru-O) = 595 cm-1], after which a small amount of a molecularly chemisorbed peroxide species was isolated [nu(O-O) = 790 cm-1]. After saturation of these atomic and molecularly chemisorbed states, physically adsorbed oxygen with a vibrational frequency close to the gas phase value of 1555 cm-1 was observed. The physically adsorbed oxygen was found to exhibit a resonance in the electron scattering cross section at an electron beam energy of about 5.5 eV. Heating the surface to 55 K leads to desorption of the physically adsorbed overlayer. The peroxide species, on the other hand, dissociates between 100 and 450 K. Although the initial precursors to dissociative chemisorption were not isolated, the peroxide identifies an intermediate precursor state, and physically adsorbed oxygen identifies an extrinsic precursor state.