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X-ray Absorption Spectroscopy
X-ray absorption spectroscopy (XAS) is one of the most powerful tools available for mapping local atomic structure. In this technique, a sample is probed with X-rays that are tuned to the energy of a core electron shell (1s, 2s, 2p, etc.) in the element of study. The number of absorbed X-rays is monitored as a function of energy.
A plot of the X-ray absorption as a function of energy shows a decrease in absorption with increasing energy; the presence of a sharp rise at certain energies referred to as absorption edges; and if taken with sufficient accuracy, small oscillations that are the result of the local environment’s influence on the target element’s basic absorption probability (a). From the spectrum, one can extract the oxidation state of the absorbing element, the distances between the absorber and its near-neighbor atoms, and the number and type of nearneighbor atoms—all parameters that determine local structure. By selecting different X-ray energies, one can obtain this information for any element in the sample.
In the example of the absorption spectrum in the X-ray region (b), the step-like rises occur where the X-ray energy has come into resonance with a core electron shell of one of the elements in the sample (plutonium in this example), exciting the electron into the continuum (a). Because of the shape of the spectral feature (b), the data are referred to as an absorption edge. For the most part the edges are widely separated, and the target element is selected simply by scanning over an appropriate energy range. Following the edge, the absorbance decreases monotonically with increasing X-ray energy as the X-ray penetration depth becomes larger.
Fine structure is observed when the spectrum is expanded past a specific edge (b). The X-ray absorption near-edge structure (XANES) region occurs as peaks and shoulders over a 20-to-30-eV-wide region immediately past the edge onset. The fine structure on the high-energy side of the edge that damps out over several hundred eV is termed extended X-ray absorption fine structure (EXAFS) (c). This fine structure in both the XANES and the EXAFS regions is well understood and enables XAS to be applied to the determination of chemical speciation and local structure.
Next Article ... "X-ray Absorption Near-Edge Spectroscopy"
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A schematic energy-level diagram of an absorbing atom shows the atom’s first few core-electron orbitals: 1S, 2S, 2P1/2 , 2P3/2 . The ionization potential energy is denoted by E0 . With the absorption of a photon of energy E, the electron undergoes a transition to an unbound state in the continuum and, by conservation of energy, acquires a kinetic energy (E-E0 ).
The abrupt jumps in this simplified absorption spectrum are the absorption edges, which correspond to the excitation of an electron from a specifi c orbital. The absorption edge due to excitation of the 1S electron is called the K edge, while excitations from the less strongly bound 2S, 2P1/2, and 2P3/2 electrons are called the LI, LII, and LIII edges, respectively. The inset shows how variations in the near-edge structure within the first 20 to 30 eV of the edge change with oxidation state of the absorbing atom.
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