X-ray absorption fine structure (XAFS) is a specific structure observed in X-ray absorption spectroscopy (XAS). By analyzing the XAFS, information can be acquired on the local structure and on the unoccupied electronic states.
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The X-ray absorption spectra show a steep rise at the core-level binding energy of X-ray-absorbing atoms and attenuates gradually with the X-ray energy. The XAS spectra are usually divided in three energy regions: 1) the edge region, 2) the X-ray Absorption Near Edge Structure (XANES); 3) the extended X-ray absorption fine structure (EXAFS). The absorption peaks at the absorption edges in a range of about 5 eV were first explained by Walther Kossel as due to electronic transitions to first unoccupied molecular levels above the chemical potential, and for many years was referred to as the “Kossel structure” now it is known as absorption edge region. The oscillatory structure extending for hundreds of electron volts past the edges was called the “Kronig structure” after the scientist, Ralph Kronig, who assigned this structure to the single scattering of the excited photoelectron by neighbouring atoms. This is now called EXAFS. The energy region of XANES (extending over a range of about 100 eV) between the edge region and the EXAFS region has been assigned to multiple scattering resonances. In the case of organic molecules this energy region has been later called near-edge X-ray absorption fine structure (NEXAFS), but NEXAFS is synonymous with XANES.
X-ray absorption edge spectroscopy corresponds to the transition from a core-level to an unoccupied orbital or band and mainly reflects the electronic unoccupied states. EXAFS, resulting from the interference in the single scattering process of the photoelectron scattered by surrounding atoms, provides information on the local structure. Information on the geometry of the local structure is provided by the analysis of the multiple scattering peaks in the XANES spectra. The XAFS acronym has been later introduced to indicate the sum of the XANES and EXAFS spectra.