X-ray Absorption spectroscopy (XAS)
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.
XAFS is a spectroscopic technique that uses x-rays to probe the physical and chemical structure of matter at an atomic scale. XAFS is element-specific, in that x-rays are chosen to be at and above the binding energy of a particular core electronic level of a particular atomic species. An energy-tunable x-ray source is needed to measure XAFS, which typically means a synchrotron is used.
Extended X-ray Absorption Fine Structure (EXAFS) refers to the oscillations of the total X-Ray absorption coefficient observed over an energy range of several hundred eV starting ~50-100eV above the absorption edge of the element of interest. X-Ray Absorption Near Edge Structure (XANES) analysis on the other hand refers to the fine structures around the absorption edge (up to ~50-100eV above the edge) including as well the pre-edge region (figure). XAFS is a general terminology for both EXAFS and XANES techniques.
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.