LOCAL GEOMETRICAL STRUCTURE OF A NA O COADSORPTION PHASE ON AL(111) -ATOP BONDING DUE TO CHEMICAL HETEROGENEITY/

Normal incidence X-ray standing wavefield absorption (NIXSW) including site triangulation, has been used to study the local geometry of O an d Na when O chemisorbs onto the Al(111)(root 3 X root 3)R30 degrees-Na surface alloy phase. The favoured geometry places O atop the surface Na atoms, a result which has no parallel on elemental surfaces and whi ch must be attributed to the chemical and electronic heterogeneity of the surface alloy. This structure rationalises earlier implications of local Na-O bonding from core level photoelectron spectroscopy.