X-ray Absorption Spectroscopy
X-ray absorption spectroscopy is an element-specific probe of the local structure of elements in a material. The species of the material is determined based on its unique local structure. An important advantage to this technique is that it can be used to directly examine a wide variety of solid and liquid samples non-destructively. This method results from the absorption of a high energy X-ray by an atom in a sample. The absorption occurs at a defined energy corresponding to the binding energy of the electron in the material. The ejected electron interacts with the surrounding atoms to produce the spectrum. Occasionally, the electron can be excited into vacant bound electronic states near the valence band and distinct absorptions will result at these energies.
X-ray absorption spectroscopy is commonly divided into two spectral regions, the X-ray absorption near-edge structure (XANES) spectral region and the extended X-ray absorption fine structure (EXAFS) region.
The XANES spectra are unique to the oxidation state and species of the element of interest, as such are used to determine the oxidation state and coordination environment of materials. XANES spectra are commonly compared to standards in order to determine which species are present in an unknown sample. Once species are identified, their relative abundance is quantified using linear-combination fitting (or other curve-fitting algorithms) using XANES standards to reconstruct the experimental data.
The EXAFS spectra are described as a series of periodic sine waves that decay in intensity as the incident energy increases from the absorption edge. These sine waves are the result of the interaction between the elected photoelectron and the surrounding atomic environment. As such, their amplitude and phase depend on the local structure of the excited atom and can be determined by matching a theoretical spectrum to the experimental spectrum. This fitting yields diverse information, including the identity of neighboring atoms, their distance from the excited atom, the number of atoms in the shell and the degree of disorder in the particular atomic shell (as expressed by the Debye-Waller factor). These distances and coordination numbers are diagnostic of a specific mineral or adsorbate-mineral interaction. Consequently, the data is used to identify and quantify major mineral phases, adsorption complexes, and crystallinity. Linear combination of EXAFS spectra using standards is also commonly used in quantitation for samples containing numerous species, since it is difficult in practice to separate many species into their component shells.
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