Like other absorption spectroscopies, XAS techniques follow Beer's law.
XAS spectra are most often collected at synchrotrons because the high intensity of synchrotron X-ray sources allows the concentration of the absorbing element to reach as low as a few parts per million.
Because X-rays are highly penetrating, XAS samples can be gases, solids or liquids.
The x-ray absorption coefficient is usually normalized to unit step height.
These spectra can be used to determine the average oxidation state of the element in the sample.
The XANES spectra are also sensitive to the coordination environment of the absorbing atom in the sample.
Finger printing methods have been used to match the XANES spectra of an unknown sample to those of known "standards".
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 photoelectron kinetic energy become much larger so that multiple scattering events become dominant in the XANES (or NEXAFS) spectra.
The wavelength of the photoelectron is dependent on the energy and phase of the backscattered wave which exists at 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.
The effect of the backscattered photoelectron on the absorption spectra is described by the EXAFS equation, first demonstrated by Sayers, Stern, and Lytle.
term which imposes the interference condition, leading to peaks in absorption when the wavelength of the photoelectron is equal to an integer fraction of
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.
Recent developments in the design and quality of crystal optics have allowed for some EXAFS measurements to take place in a lab setting,[3] where the tunable x-ray source is achieved via a Rowland circle geometry.
While experiments requiring high x-ray flux or specialized sample environments can still only be performed at synchrotron facilities, absorption edges in the 5 - 30 keV range are feasible for lab based EXAFS studies.
[4] 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: 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.
EXAFS is, like 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.
A very detailed, balanced and informative account about the history of EXAFS (originally called Kossel's structures) is given by R. Stumm von Bordwehr.