Alkali metal and simple gas atom adsorption and coadsorption on transition metal surfaces

Alkali metal coadsorption systems represent a step along the pathway from simple model adsorbate overlayers to more technologically relevant real systems. Such is their complexity, however, that very few systems have been solved structurally. Presented here are SXRD and STM investigations of two such systems. The first study involves potassium adsorption on the Ni(100)(2x2)p4g-N surface, where a clock reconstruction is present with the nickel substrate atoms rotated in alternate clockwise and anti-clockwise directions about the nitrogen atoms which lie in alternate four-fold hollow sites. Our work indicates the removal of this reconstruction upon coadsorption of an alkali metal. The resulting Ni(100)c(2x2)-(N+K) structure involves a lifting of the nitrogen atoms allowing the nickel substrate atoms to return to their bulk in-plane positions. STM results, obtained at lower potassium coverages, indicate initial step site adsorption and provide an explanation for previous SPA-LEED results. An SXRD investigation of a simple gas atom adsorption system, Rh(100)-O, is also presented. This system is also a member of the 'clock' reconstruction family, although it has been proposed that the adsorbate site differs from that of other simple p4g systems, SXRD has been used to confirm this alternative adsorption site and to determine the reconstruction accurately. A greater challenge is presented by the structure determination of the Ni(100)-(3x3)-(O+K,Cs) coadsorption system. This system is formed by adsorption of potassium or cesium on the Ni(100)c(2x2)-O overlayer. The difficulty of the structural fit is compounded' by the size of the unit cell. In this study, Anomalous Scattering was used to investigate whether there is a contribution from the nickel substrate to the reconstruction. Measurements of the fractional order rods at 10 eV and 200 eV below the nickel K edge (8333 eV) showed no discernible differences and involvement of the nickel substrate in the reconstruction can be eliminated. (author)