Quantitative structural study of an Na–O coadsorption phase on Al(111) using X-ray standing waves

Abstract

Using normal-incidence X-ray standing wavefield absorption (NIXSW) including site triangulation via measurements at (111) and (text-decoration:overline111) reflections of both the O and Na X-ray absorption, we have investigated the local adsorption sites produced by O chemisorption onto a room-temperature-prepared Al(111)(√3 ×√3)R 30°-Na surface. The structure of the pure alkali-metal phase has been shown previously, by SEXAFS and NIXSW, to involve Na substitution of top-layer Al atoms, and these sites are unchanged in the early stages of O chemisorption. The new NIXSW results indicate that in this stage the O atoms adsorb atop the Na atoms with an NaνO spacing of ca. 2.05 A. This site provides a clear rationalisation for the results of Na and Al 2p core-level spectroscopy which indicate that the dominant O bonding is to the Na atoms and not to the Al, as in O chemisorption on the clean Al(111) surface. An alternative interpretation of the NIXSW data based on adsorption to near-hcp hollow sites is difficult to reconcile with the very short NaνO bond length associated with this site, and with the implied inequivalence of hcp and fcc sites despite the local NaνO bonding found in photoemission.