QUANTITATIVE STRUCTURAL STUDY OF AN NA-O COADSORPTION PHASE ON AL(111) USING X-RAY STANDING WAVES

Using normal-incidence X-ray standing wavefield absorption (NIXSW) inc luding site triangulation via measurements at (111) and (($) over bar 111) reflections of both the O and Na X-ray absorption, we have invest igated the local adsorption sites produced by O chemisorption onto a r oom-temperature-prepared Al(111)(root 3 x root 3)R30 degrees-Na surfac e. The structure of the pure alkali-metal phase has been shown previou sly, by SEXAFS and NIXSW, to involve Na substitution of top-layer Al a toms, and these sites are unchanged in the early stages of O chemisorp tion. The new NIXSW results indicate that in this stage the O atoms ad sorb atop the Na atoms with an Na nu O spacing of ca. 2.05 Angstrom. T his site provides a clear rationalisation for the results of Na and Al 2p core-fever spectroscopy which indicate that the dominant O bonding is to the Na atoms and not to the Al, as in O chemisorption on the cl ean Al(111) surface. An alternative interpretation of the NIXSW data b ased on adsorption to near-hcp hollow sites is difficult to reconcile with the very short Na nu O bond length associated with this site, and with the implied inequivalence of hcp and fee sites despite the local NavO bonding found in photoemission.