Extended X-Ray Absorption Fine Structure

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Extended X-Ray Absorption Fine Structure (EXAFS) 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) focusses 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 and as such is capable of undergoing subsequent transitions (although these are usually ignored in data analysis). 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 due to single scattering events shows up as a modulation of the measured absorption coefficient, thereby causing the oscillation in the EXAFS spectra. 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.

Contents

1 Significance
2 Experimental considerations
3 History
4 External links

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 [link], and for a general study of trivalent lanthanides and actinides in chloride containing aqueous media can be read at [link]

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.

History

The first paper about EXAFS was published by Edward A. Stern and his students at the University of Washington's physics department in 1971.

External links

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).

[EXAFS theory Introduction]
[FEFF], the University of Washington's EXAFS data analysis program.

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