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X-ray Absorption Near-Edge Spectroscopy
The X-ray absorption near-edge structure (XANES) can be used to determine the oxidation state of the target (X-ray absorbing) element in solution or in the solid state. The energy at which an absorption edge appears depends on the ionization potential of the ion. This ionization potential increases with the ion’s valence state, so in general the absorption shifts to higher energy with increasing oxidation state. This known effect has been observed in plutonium and can therefore be used to determine the oxidation of plutonium in soils and concrete samples from RFETS.
An example of the differences in XANES spectra of plutonium is shown in the inset to (b), where XANES of Pu(III) and Pu(VI) are compared. There are distinct differences in the energy of the rising absorption edge, the intensity of the peak (sometimes referred to as the “white line”), and the structure in the absorption features at the higher energies beyond the absorption peak. All of these differences in the XANES spectra are used to identify the oxidation state of plutonium in RFETS samples.
For plutonium studies, researchers prefer to use the LIII X ray absorption edge (which appears near 18,060 eV) because it has the highest absorption intensity (c). However, in RFETS environmental samples there were other elements present in the samples that exhibited X-ray absorption in this same region, and therefore interfered with the plutonium measurements. As a result, the plutonium X-ray absorption studies of RFETS samples were performed instead at the LII X-ray absorption edge, which appears near 22,270 eV. This absorption edge has a lower absorption intensity than the LIII edge, making the RFETS samples very hard to study using X-ray absorption spectroscopy.
Next Article ... "Extended X-ray Absorption Fine Structure"
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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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