X-ray absorption spectroscopy (EXAFS and XANES) at surfaces

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Abstract
X-ray absorption spectroscopy is now being applied to the study of a wide range of surface structures. The modulation of the absorption coefficient as a function of photon energy, caused by photo-electron scattering, contains information on the local geometric structure around the absorbing atom. Far above an absorption edge, in the EXAFS (extended X-ray absorption fine structure) regime, single scattering usually applies and near-neighbour distances (R) and coordination numbers (N) can readily be obtained. Some of the factors influencing the scattering and thus an accurate determination of R and N are critically discussed. The usefulness of surface EXAFS (SEXAFS) is illustrated by reviewing measurements on the oxidation of aluminium, the adsorption of Te and I on silicon and germanium, the clean surface structure of ion-bombarded silicon, and the ionic-covalent transition in submonolayer coverages of caesium adsorbed on silver. Near to the absorption edge, the XANES (X-ray absorption near edge structure), also called NEXAFS (near-edge X-ray absorption fine structure), is often complicated by multiple-scattering effects and may in consequence produce a spectrum rich in structure but awkward to interpret. The atomic adsorption of oxygen on nickel is given as an example. However, for molecular adsorbates, the XANES is often dominated by intra-molecular scattering, and may then quite simply yield the molecular orientation and intra-molecular distances, and hence details of the bonding to the surface. These ideas are illustrated by the adsorption of various carbon- and oxygen-containing small molecules.