EXAFS

Abstract

X-ray Absorption Fine Structure (XAFS) spectroscopy is an important technique for determining the local structure around an absorbing element in a sample. It is normally divided into two techniques: Extended XAFS (EXAFS) describing the fine structure more than about 50 eV above an absorption edge, and near edge structure or XANES. In an X-ray absorption spectrum, a series of oscillations in the measured absorption coefficient can be observed on the high-energy side of an absorption edge. These oscillations are due to the fact that the final state wavefunction of the emitted photoelectron consists of an outgoing part and a part that is scattered from neighbouring atoms. The EXAFS oscillations, as they are known, correspond to an interferogram of the spatial distribution of nearby atoms and can be analysed to provide structural information about the local environment of the absorbing atom such as bond length, the number and type of neighbouring atoms, bond angles and a measure of order/disorder and/or chemical lability. EXAFS typically gives very accurate interatomic distances, in particular for nearest neighbours, of the order of ±0.02 Å or better. Achieving precision in the determination of coordination number is more difficult and errors as large as 20% in the fitted result are common. Careful choice of suitable standard compounds with known structure followed by EXAFS analysis, however, can greatly improve this determination.