Oxygen adsorption on an alkali metal-covered Ni(100) surface

A87 moment of & = 0.25 D. On a stepped-kinked Ru(0001) surface, the terrace-width, the stepheight and step-orientation of which are well characterized with LEED, however, two coexisting xenon adsorption states are distinguishable by an unprecedented separation in Xe Sp3/2,1,2 electron binding energies of 800 meV, by their different UPS intensities and line shapes, by their difference In adsorption energy of tiad = 3 kcal/mole and finally by their strongly deviating dipole moments of pS = 1 .O D and PT = 0.34 D. The two xenon states (which are also observed on a slightly sputtered surface) are identified as corresponding to xenon atoms being adsorbed at step and terrace sites, respectively. Their relative concentrations as deduced from the UPS intensities quantitatively correlate with the abundance of step and terrace sites of the ideal TLK surface structure model as derived from LEED. Furthermore, ledge-sites and kinksites are distinguishable via Ead. The Ead heterogeneity on the stepped-kinked Ru(0001) surface is interpreted in terms of different coordination and/or different charge-transfer-bonding at the various surface sites. The enormous increase in Xe 5p electron binding energy of 0.8 eV for Xe atoms at step sites is interpreted as a pure surface dipole potential shift. - The observed effects suggest selective xenon adsorption as a tool for local surface structure determination. Surface Science 104 (1981) 240-252 North-Holland Publishing Company OXYGEN ADSORPTION ON AN ALKALI METAL-COVERED

Ni(100) SURFACE

M.KISKINOVA,L.SURNEVandG.BLIZNAKOV Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria

Received 8 April 1980; accepted for publication 22 September 1980 The oxygen chemisorption on an alkali (Na, K, Cs) covered Ni(100) surface and its initial oxidation were studied by Auger and electron energy loss spectroscopy (ELS). It was found that in the presence of an alkali metal, the sticking coefficient S remains unity up to a given oxygen coverage of Sb whose value depends on the alkali overlayer concentration and the ionicity of the Ni-alkali metal bond. At a given oxygen coverage, the line shapes of Auger and loss spectra are almost the same for alkali-covered and clean Ni(lOO), which suggests that alkali metals cause no change in the character of the Ni-0 bond. The effect of alkali metals is associated with increasing electron charge in the surface region, which facilitates oxygen chemisorption. The enhanced surface oxygen concentration in the presence of an alkali metal results in the formation of an oxide phase at lower oxygen exposures than is the case of clean Ni surfaces.

Surface Science 104 (1981) 253-269 North-Holland Publishing Company THE SURFACE CATION DENSITIES OF IRON OXIDE-CHROMIUM OXIDE SOLID SOLUTIONS

M.C.KUNG and H.H.KUNG Department of Chemical Engineering, Materials Research Center and Ipatieff Laboratory, Northwestern University, Evanston, Illinois 60201) USA

Received 28 July 1980; accepted for publication 6 October 1980 The surface cation densities of Fe,Crz_,Os solid solutions were determined for x = 0.5, 1, and 1.5 using Auger Electron Spectroscopy on samples that had been equilibrated in air at 798 or 973 K. Cr(V1) was found to be present on the surface by iodometric titration. The results indicated that the Cr(VI)/Cr(III) ratio was constant, independent of composition. Compared to the bulk densities, Fe was found to be enriched on the surface for x = 0.5 and 1, but depleted forx=l.S.