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.

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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.

Applications

XAS is an interdisciplinary technique and its unique properties, as compared to x-ray diffraction, have been exploited for understanding 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 a EXAFS study of uranium chemistry in glass see [1], and for a general study of trivalent lanthanides and actinides in chloride containing aqueous media can be read at [2]

History

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). The first paper using a Fourier Transform method for analyzing EXAFS was published by Edward A. Stern and his students at the University of Washington's physics department in 1971.

References

Relevant Websites

• International XAFS Society
• FEFF Project, University of Washington, Seattle
• GNXAS project and XAS laboratory, Università di Camerino
• EXAFS theory Introduction
• Community web site for XAFS

Books

• 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

Papers

• 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, 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