Extended X-ray absorption fine structure
This article may be too technical for most readers to understand.(June 2019) |
When the incident x-ray energy matches the binding energy of an electron of an atom within the sample, the number of x-rays absorbed by the sample increases dramatically, causing a drop in the transmitted x-ray intensity. This results in an absorption edge. Every element has a set of unique absorption edges corresponding to different binding energies of its electrons, giving XAS element selectivity. XAS spectra are most often collected at synchrotrons because of the high intensity of synchrotron X-ray sources allow the concentration of the absorbing element to reach as low as a few parts per million. Absorption would be undetectable if the source is too weak. Because X-rays are highly penetrating, XAS samples can be gases, solids or liquids.
Background
EXAFS
The normalized absorption spectra are often called
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
If the ejected photoelectron is taken to have a
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 frequently 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:
- amorphous and liquidsystems
- solid solutions
- doping and ionic implantation of materials for electronics
- local distortions of crystal lattices
- organometallic compounds
- metalloproteins
- metal clusters
- vibrational dynamics[citation needed]
- ions in solutions
- chemical speciation analysis
XAS provides complementary to diffraction information on peculiarities of local structural and thermal disorder in crystalline and multi-component materials.
The use of atomistic simulations such as molecular dynamics or the reverse Monte Carlo method can help in extracting more reliable and richer structural information.
Examples
EXAFS is, like
History
A very detailed, balanced and informative account about the history of EXAFS (originally called Kossel's structures) is given by R. Stumm von Bordwehr.[1] A more modern and accurate account of the history of XAFS (EXAFS and XANES) is given by the leader of the group that developed the modern version of EXAFS in an award lecture by Edward A. Stern.[2]
See also
- X-ray absorption spectroscopy
- X-ray absorption near edge structure
- Surface-extended X-ray absorption fine structure
References
Bibliography
Books
- Calvin, Scott. (2013-05-20). XAFS for everyone. Furst, Kirin Emlet. Boca Raton. )
- Bunker, Grant, 1954- (2010). Introduction to XAFS : a practical guide to X-ray absorption fine structure spectroscopy. Cambridge: Cambridge University Press. )
- Teo, Boon K. (1986). EXAFS: Basic Principles and Data Analysis. Berlin, Heidelberg: Springer Berlin Heidelberg. OCLC 851822691.
- X-ray absorption : principles, applications, techniques of EXAFS, SEXAFS, and XANES. Koningsberger, D. C., Prins, Roelof. New York: Wiley. 1988. )
Book chapters
- Kelly, S. D.; Hesterberg, D.; Ravel, B.; Ulery, April L.; Richard Drees, L. (2008). "Analysis of Soils and Minerals Using X-ray Absorption Spectroscopy". Methods of Soil Analysis Part 5. Soil Science Society of America. ISBN 9780891188575. Archived from the original (PDF) on 2019-07-16. Retrieved 2019-07-16.)
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Papers
- Stern, Edward A. (1 February 2001). "Musings about the development of XAFS" (PDF). PMID 11512825.
- Rehr, J. J.; Albers, R. C. (1 June 2000). "Theoretical approaches to x-ray absorption fine structure". ISSN 0034-6861.
- Filipponi, Adriano; Di Cicco, Andrea; Natoli, Calogero Renzo (1 November 1995). "X-ray-absorption spectroscopy and n-body distribution functions in condensed matter. I. Theory". PMID 9980866.
- de Groot, Frank (2001). "High-Resolution X-ray Emission and X-ray Absorption Spectroscopy". S2CID 44020569.
- F.W. Lytle, "The EXAFS family tree: a personal history of the development of extended X-ray absorption fine structure",
- Sayers, Dale E.; Stern, Edward A.; Lytle, Farrel W. (1 October 1971). "New Technique for Investigating Noncrystalline Structures: Fourier Analysis of the Extended X-Ray—Absorption Fine Structure". Physical Review Letters. 27 (18). American Physical Society (APS): 1204–1207. ISSN 0031-9007.
- A. Kodre, I. Arčon, Proceedings of 36th International Conference on Microelectronics, Devices and Materials, MIDEM, Postojna, Slovenia, October 28–20, (2000), p. 191-196