X-ray absorption fine structure abbr., XAFS (rus. тонкая структура спектров поглощения рентгеновских лучей) — Oscillations observed in x-ray absorption spectra of. Often XAFS implies XAFS-spectroscopy.

Description

In the modern sense, the term XAFS combines the notations of several X-ray absorption ranges close to absorption discontinuity caused by the photoelectric effect: the XANES region, sometimes referred to as the NEXAFS (near edge X-ray absorption fine structure) with the energy range limited to ± (30-50) eV in relation to the excitation threshold energy (absorption edge) and the EXAFS region (extended X-ray absorption fine structure), located above the absorption edge and lying in the range from about 30 eV to about 1,500-2,000 eV in relation to the excitation threshold. The conventional division of the spectrum into such regions is due to the need to use different physical approximations to describe them.

X-ray absorption by the matter is related to the x-ray photons' interaction with the electrons in the inner shells of an atom. As a result of such interaction, electrons are knocked out from atoms, which leads to a sharp increase in X-ray absorption (edge) when the photon energy exceeds the energy of the electron-nucleus bond (the excitation threshold). The excitation threshold is a characteristic value for each chemical element that can uniquely identify a chemical element by the excitation threshold position.

XAFS, or XAFS-spectroscopy, provides information about the nature, number and location of the neighbouring atoms with respect to the atom in question, both in the first and in more distant coordination spheres. In this regard, XAFS-spectroscopy is used for structural analysis, along with the X-ray diffraction analysis. At the same time, it has some extra advantages that enable the analysis of matter in any aggregate state, to study chemically complex substances, including cases where the concentration of the atoms is very low (for example, impurities in alloys, catalysts, the active sites in enzymes, make environmental pollution analysis) as well as study the transformations during chemical reactions. The evolution of XAFS-spectroscopy was due to the advent of synchrotron radiation sources, without which XAFS-spectroscopy experimental application would be a problem since XAFS spectra are measured in the X-ray energy range of 1-100 keV.

Illustrations

а — Example of abrupt enhancement of absorption in the XAFS spectrum. Excitation threshold is a
а — Example of abrupt enhancement of absorption in the XAFS spectrum. Excitation threshold is around 9000 eV. 
b — Example of splitting of XAFS spectrum into XANES and EXAFS. Horizontal axis represents the x-ray radiation energy relative to the excitation threshold [1].

Author

  • Tolkachev Nikolay N.

Source

  1. Fetisov G. V. Synchrotron radiation. Methods of studying the structure of matter (in Russian). — Мoscow: Fizmatlit, 2007. — 672 pp.

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