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 inves tigated by high-resolution electron energy loss spectroscopy. Dissocia tive chemisorption occurs up to the saturation atomic coverage [nu(per pendicular-to) (Ru-O) = 595 cm-1], after which a small amount of a mol ecularly 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 ox ygen was found to exhibit a resonance in the electron scattering cross section at an electron beam energy of about 5.5 eV. Heating the surfa ce to 55 K leads to desorption of the physically adsorbed overlayer. T he peroxide species, on the other hand, dissociates between 100 and 45 0 K. Although the initial precursors to dissociative chemisorption wer e not isolated, the peroxide identifies an intermediate precursor stat e, and physically adsorbed oxygen identifies an extrinsic precursor st ate.