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The NEXAFS stands for near-edge X-ray absorption fine structure. Technically, the NEXAFS is a synonym for the XANES. In practice, the term NEXAFS is generally used only for low-energy edges, typically those below 1000 eV.

As discussed in the previous section, the formal theories of the XANES and EXAFS are essentially the same and both are given by the Fermi’s golden rule. When an effective single-particle description of the spectrum is reasonable, this leads to

(2.12)

Several of the approximations appropriate for the EXAFS regime (beyond about 20–30 eV above the edge) are not valid in the near-edge regime, with some of these related to the reduction of the many-body formulation to an effective single-particle description. For example, in the highly correlated systems such as transition metal oxides and f-electron systems, many-body effects can change the qualitative behavior of the near-edge spectrum while the main effect on the EXAFS region is simply an overall reduction in the amplitude of the fine structure, which is taken into account by the factor in the EXAFS equation. Vibrational effects damp the EXAFS via the Debye–Waller factor exp(−2k²σ²). In the XANES, the effects of vibrations and disorder are sometimes more related to symmetry breaking, which allows transitions to states that are previously dipole forbidden, resulting in additional peaks in the near-edge spectrum.

Many-body effects can be traced down to the different energy of photoelectron. The photoelectron with a larger kinetic energy is less affected by the neighboring coordinating atom. Under normal circumstances, it is only scattered by the neighboring coordinating atom. However, if the kinetic energy of the photoelectron is very small, it will be scattered many times by an unknown neighboring coordinating atom scattering. This is the biggest difference between the simplified models of the EXAFS and XANES. Based on single scattering, the EXAFS can generally only give average structural information. The multiple scattering signal that occurs on the high-energy side of the XANES region records the superposition of the scattered waves when scattered by more than one neighbor atom. Therefore, it can reflect the three-dimensional coordination environment of the absorbing atom, combined with the relevant information of the transition, and provide strong evidence to judge the absorption atomic coordination geometry.

The EXAFS has limitations. At high temperature, taking in situ reaction conditions as an example, it is difficult to analyze the EXAFS under such conditions [108]. The XANES is highly sensitive to the local symmetry of the short-range order of absorbing atoms, and the short-range order of matter still exists at high temperatures. Therefore, the XANES is widely applicable. In principle, the XANES can distinguish mixed systems. The reason is that the characteristic of the XANES spectrum is fingerprint authentication, and a mixture of multiple systems can be distinguished.

Although the central atoms are completely different, the lines and shapes of oxides and fluorides with the same short-range order structure in the multiple scattering zone are the same. This has been confirmed by a large number of experimental spectra. This is to identify the coordination geometry of the central atom. At present, the identification of this part of the spectrum is mainly based on experience and comparison with the standard samples.

The EXAFS is also less sensitive to the nonspherical details of the potentials, and a simple overlapped atomic muffin tin potential is adequate for most practical calculations. On the other hand, near-edge spectra can be quite sensitive to the details of charge transfer and changes in Fermi level due to the solid-state effects. Thus, the use of self-consistent potentials and often nonspherical symmetry are essential for accurate calculations of the XANES. Finally, calculations of the single-particle Fermi golden rule must be treated differently in the near-edge region because the path expansion detailed in the equation often fails to converge (or converges very slowly) for low-energy photoelectrons. This slow convergence is caused by two factors. First, the inelastic mean free path becomes large for low energy electrons so that very long paths must be included in the expansion. Second, large angle scattering amplitudes are not small at low energies, so that the XANES signal is not dominated by the nearly linear scattering paths, and all multiple scattering paths must be considered.