Extended X-Ray Absorption Fine Structure

Extended X-Ray Absorption Fine Structure

Extended X-Ray Absorption Fine Structure (EXAFS), or more simply X-ray Absorption Spectroscopy (XAS), is an experimental method in physics and chemistry of determining the bonding of solids by analyzing oscillations in x-ray absorption versus photon energy that are caused by interference. EXAFS spectra are displayed as graphs of the absorption coefficient of a given material versus energy, typically in a 500 – 1000 eV range beginning before an absorption edge of one of the elements in the solid. By analysing the oscillating spectrum at energies just above the absorption edge, it is possible to obtain information relating to the coordination environment of the central excited atom. EXAFS is distinguished from the closely related near-edge x-ray absorption (NEXAFS or XANES) method in that EXAFS considers the absorption spectrum out to much higher electron kinetic energies than that NEXAFS (also known as XANES) focuses on.

X-ray absorption spectra are produced over the range of 200 – 35,000 eV. The dominant physical process is one where the absorbed photon ejects a core photoelectron from the absorbing atom, leaving behind a core hole. The atom with the core hole is now excited. The ejected photoelectron’s energy will be equal to that of the absorbed photon minus the binding energy of the initial core state. The ejected photoelectron interacts with electrons in the surrounding non-excited atoms.

If the ejected photoelectron is taken to have a wave-like nature and the surrounding atoms are described as point scatterers, it is possible to imagine the backscattered electron waves interfering with the forward-propagating waves. The resulting interference pattern shows up as a modulation of the measured absorption coefficient, thereby causing the oscillation in the EXAFS spectra. A simplified plane-wave single-scattering theory has been used for interpretation of EXAFS spectra for many years, although modern methods (like FEFF, GNXAS) have shown that curved-wave corrections and multiple-scattering effects can not be neglected. The photelectron scattering amplitude in the low energy range (5-200 eV) of the phoelectron kinetic energy become much larger so that multiple scattering events become dominant in the NEXAFS (or XANES) spectra.

The wavelength of the photoelectron is dependent on the energy and phase of the backscattered wave which exists at the central atom. The wavelength changes as a function of the energy of the incoming photon. The phase and amplitude of the backscattered wave are dependent on the type of atom doing the backscattering and the distance of the backscattering atom from the central atom. The dependence of the scattering on atomic species makes it possible to obtain information pertaining to the chemical coordination environment of the original absorbing (centrally excited) atom by analyzing these EXAFS data.

Experimental considerations

Since EXAFS requires a tunable x-ray source, data are always collected at synchrotrons, often at beamlines which are especially optimized for the purpose. The utility of a particular synchrotron to study a particular solid depends on the brightness of the x-ray flux at the absorption edges of the relevant elements.


XAS is an interdisciplinary technique and its unique properties, as compared to x-ray diffraction, have been exploited forunderstanding the details of local structure in:

* glass, amorphous and liquid systems
* solid solutions
* Doping and ionic implantation materials for electronics
* local distortions of crystal lattices
* organometallic compounds
* metalloproteins
* metal clusters
* vibrational dynamics
* ions in solutions
* speciation of elements

Example of Significance

EXAFS is, like NEXAFS/XANES, a highly sensitive technique with elemental specificity. As such, EXAFS is an extremely useful way to determine the chemical state of practically important species which occur in very low abundance or concentration. Frequent use of EXAFS occurs in environmental chemistry, where scientists try to understand the propagation of pollutants through an ecosystem. EXAFS can be used along with accelerator mass spectrometry in forensic examinations, particularly in nuclear non-proliferation applications.

For an example of an EXAFS study of uranium chemistry in glass see [http://www.osti.gov/bridge/servlets/purl/459339-8dNh9T/webviewable/459339.pdf] , and for a general study of trivalent lanthanides and actinides in chloride containing aqueous media can be read at [http://www-ssrl.slac.stanford.edu/pubs/activity_rep/ar98/2525-edelstein.pdf]


A very detailed, balanced and informative account about the history of EXAFS (originally called Kossel's structures) is given in the paper "A History of the X-ray Absorption Fine Structure} by R. Stumm von Bordwehr", Ann. Phys. Fr. vol. 14, 377-466 (1989) (author's name is C. Brouder).


Relevant Websites

* [http://www.i-x-s.org/ International XAFS Society]
* [http://leonardo.phys.washington.edu/feff/ FEFF Project, University of Washington, Seattle]
* [http://gnxas.unicam.it GNXAS project and XAS laboratory, Università di Camerino]
* [http://srs.dl.ac.uk/xrs/Theory/theory.html EXAFS theory Introduction]
* [http://xafs.org/XAFS Community web site for XAFS]


* B.-K. Teo, "EXAFS: basic principles and data analysis", Springer 1986
* "X-ray Absorption: principles, applications and techniques of EXAFS, SEXAFS and XANES", a cura di D.C. Koeningsberger, R. Prins, Wiley 1988


* J.J. Rehr e R.C. Albers, "Theoretical approaches to X-ray absorption fine structure", "Reviews of Modern Physics" 72 (2000), 621-654
* A. Filipponi, A. Di Cicco e C.R. Natoli, "X-ray absorption spectroscopy and n-body distribution functions in condensed matter", "Physical Review" B 52/21 (1995) 15122-15148
* F. de Groot, "High-resolution X-ray emission and X-ray absorption spectroscopy", "Chemical Reviews" 101 (2001) 1779-1808
* F.W. Lytle, "The EXAFS family tree: a personal history of the development of extended X-ray absorption fine structure", "Journal of Synchrotron Radiation" 6 (1999), 123-134
* Dale E. Sayers and Edward A. Stern, Farrel W. Lytle, [http://dx.doi.org/10.1103/PhysRevLett.27.1204 New Technique for Investigating Noncrystalline Structures: Fourier Analysis of the Extended X-Ray—Absorption Fine Structure,] Phys. Rev. Lett. 27, 1204–1207 (1971).
* A. Kodre, I. Arčon, Proceedings of 36th International Conference on Microelectronics, Devices and Materials, MIDEM, Postojna, Slovenia, October 28-20, (2000), p. 191-196

Wikimedia Foundation. 2010.

Игры ⚽ Нужно решить контрольную?

Look at other dictionaries:

  • Extended X-Ray Absorption Fine Structure — EXAFS La spectroscopie EXAFS (Extended X Ray Absorption Fine Structure) est une technique d analyse de spectrométrie d absorption des rayons X utilisant principalement le rayonnement synchrotron. Elle apporte des informations sur l environnement… …   Wikipédia en Français

  • Extended X-ray absorption fine structure — EXAFS La spectroscopie EXAFS (Extended X Ray Absorption Fine Structure) est une technique d analyse de spectrométrie d absorption des rayons X utilisant principalement le rayonnement synchrotron. Elle apporte des informations sur l environnement… …   Wikipédia en Français

  • extended x-ray absorption fine structure spectroscopy — abbr. EXAFS Extended x ray absorption fine structure spectroscopy, a technique used to characterize detailed structures on the surfaces of catalysts and other solids …   Petroleum refining glossary

  • X-ray absorption fine structure — (XAFS) is a specific structure observed in X ray absorption spectroscopy (XAS). By analyzing the XAFS, information can be acquired on the local structure and on the unoccupied electronic states.The X ray absorption spectra show a steep rise at… …   Wikipedia

  • Near edge X-ray absorption fine structure — NEXAFS (Near Edge X ray Absorption Fine Structure) is an element specific electron spectroscopic technique which is highly sensitive to bond angles, bond lengths and the presence of adsorbates. It is widely used in surface science and has also… …   Wikipedia

  • X-ray Absorption Near Edge Structure — XANES XANES, qui signifie X ray Absorption Near Edge Structure (soit en français Spectroscopie de structure près du front d absorption de rayons X) est une méthode d analyse spectroscopique par absorption de rayons X. Sommaire 1 Principes 2 Notes …   Wikipédia en Français

  • X-ray absorption spectroscopy — The X ray absorption spectroscopy is currently a widely used technique giving information on the local structure and on the electronic states in gas phase, molecular and condensed matter.X ray absorption spectra are obtained by tuning the photon… …   Wikipedia

  • X-ray Raman scattering — (XRS) is non resonant inelastic scattering of x rays from core electrons. It is analogous to Raman scattering, which is a largely used tool in optical spectroscopy, with the difference being that the wavelengths of theexciting photons fall in the …   Wikipedia

  • x-ray — /eks ray /, n. Also, x ray, X ray. 1. Physics. a. Often, x rays. a form of electromagnetic radiation, similar to light but of shorter wavelength and capable of penetrating solids and of ionizing gases. b. such radiation having wavelengths in the… …   Universalium

  • Spectre d'absorption — Spectrométrie d absorption La spectrométrie d absorption est une méthode physique d analyse chimique. Elle s utilise principalement sur les liquides. La couleur d un corps en transmission (transparence) représente sa capacité à absorber certaines …   Wikipédia en Français

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”