X-ray absorption fine structure for surface studies

Abstract

Over the past twenty years. extended X-ray absorption fine structure (EXAFS) has developed into a standard bulk technique for the determination of structure in condensed matter systems’ ‘. The strength of the technique is that it can be used to probe the local structure around a specific element within the material. Since it has no requirement for long range order as in diffraction techniques, it is ideal for studying amorphous. disordered and biological systems. The potential use of the technique for studying surfaces was soon realised”. but the technical difficulties met in transferring the technique to surfaces has meant that progress has been slower than in the bulk EXAFS. However, surface EXAFS is now regarded as a standard technique for determining the bond length and adsorption site of atoms and molecules on surfaces’.‘. The technique is particularly suited to molecular adsorption since these systems arc rarely amenable to study by the more traditional diffraction techniques. Surface EXAFS, unlike many other structural techniques, also has considerable promise for the study of technological surfdccs as well as the so called ‘ideal’ surfaces of single crystal substrates. The near edge region of an EXAFS spectrum also has important information for the study of molecular adsorbatcs. This region contains resonances which are much stronger than EXAFS structure and relate to the orientation of the molecule’-“. Since this information is collected with the same instrumentation as surface EXAFS, it is possible in one experiment to determine the molecular orientation, adsorption site and bond length (and occasionally, molecular conformation) making the techniques a very powerful combination. In this review. the basics 01‘ both techniques ~111 bc outlined and the sort of information it is possible to obtain will be discussed. The points will be illustrated by examples taken from the literature. but this is not intended in any way to be an exhaustive literature survey. In the last section. a variant of EXAFS will be discussed, namely electron energy-loss fine structure”. This has the considerable advantage of not requiring synchrotron radiation, using instead an electron beam as a probe. For use as a structural probe, it is fair to say that this is not as well developed as EXAFS but in the study of molecular near edge structure, it is arguable that it is far more advanced”‘. Whatever the pros and cons of Xray vs electron fine structure. the latter is something most surface scientists ought to bc aware of since it requires only standard cquipmcnt of the sort to be found in virtually every surface science laboratory.