### X-ray Spectroscopic Methods for Diagnosing Atomic and Electronic Structure
#### 3.1.3. NEXAFS and EXAFS  
X-ray absorption spectroscopy is currently one of the most powerful experimental methods for obtaining information about the local atomic and electronic structure of polyatomic systems. Within the framework of this technique, two spectroscopic methods are usually distinguished, namely, near-edge X-ray absorption fine structure (NEXAFS) and extended X-ray absorption fine structure (EXAFS) spectroscopy, differing in their origin, methods of obtaining spectra, and analysis. NEXAFS spectroscopy is used to  
Figure 10: Schematic representation for resonant core electron excitation process (**a**) and Auger electron decay processes of a resonant excitation: (**b**) participator-Auger (\(pA\)) electron decay and (**c**) spectator-Auger (\(sA\)) electron decay [53]. Figure 9: (**a**) Ni \(2p_{3/2}\) NEXAFS spectrum of [Ni(Salen)]; the photon energies used to excite the VB PE spectra are marked with numbered dots. (**b**) A series of resonant VB PE spectra of [Ni(Salen)]. \(pA\)—electron band due to participator-Auger decay; \(sA_{1}\) and \(sA_{2}\)—low- and high-energy electron bands due to spectator-Auger decay; \(nA_{1}\) and \(nA_{2}\)—electron bands due to normal Auger decay of Ni \(2p_{3/2}\)\({}^{-1}3q^{9}\) excitations. The lowercase letters \(a\), \(b\), \(c\), \(d\), \(e\), \(f\), \(g\), \(h\) and \(i\) denote the photoemission signals of individual sub-bands of the valence band [53]. obtain information about low-lying unoccupied electronic states in a polyatomic system and is based on the analysis of absorption bands in the immediate vicinity of the X-ray absorption edge--about 10 eV below and 30 eV above the absorption edge (Figure 11a). It is conventional to describe these spectra using multiple (resonant) scattering of photoelectrons ejected from the core of an absorbing atom on the nearest neighbor atoms [126,127]. At certain photoelectron energies, a quasi-molecule formed by neighborhood atoms can temporarily trap a photoelectron, resulting in the formation of metastable states (shape resonances). These resonances, depending on their lifetime, are observed in the spectrum as narrow lines or broad absorption bands. Resonance localization in the quasi-molecule field allows us to consider them as a result of dipole-allowed transitions of core electrons to the unoccupied MOs of this polyatomic system. Figure 11: (**a**) X-ray absorption spectrum with regions corresponding to NEXAFS and EXAFS, and (**b**) schematic representation of NEXAFS mechanism due to multiple scattering. (**c**) Ni \(2p_{3/2,1/2}\) and N \(1\)s NEXAFS spectra of [Ni(Salen)] compared to the corresponding spectra of nickel porphyrin NiP [45]. It should be emphasized that the dominant role of the processes of multiple scattering of photoelectrons in the NEXAFS region is due to their low kinetic energy, which leads to large photoelectron wavelengths, which turn out to be comparable to the interatomic distances between the absorbing atom and its neighboring atoms (Figure 11b). As the kinetic energy of photoelectrons increases (>50 eV), their wavelength quickly decreases and the processes of multiple scattering of photoelectrons become unlikely. The promise of using the above quasi-molecular approach in the analysis of NEXAFS spectra is demonstrated by a direct comparison of the corresponding spectra of the nickel salen [Ni(Salen)] and nickel porphyrin NiP complexes in Figure 11c. Both complexes have similar square-planar coordination centers (quasi-molecules) [NiN\({}_{2}\)O\({}_{2}\)] and [NiN\({}_{4}\)] with close interatomic distances. Despite the general similarity of the spectral behavior of the corresponding Ni \(2p\) and N 1s NEXAFS spectra of the complexes, the compared spectra demonstrate noticeable differences in the fine structure (energy shifts of absorption bands, changes in their intensities, the appearance of additional bands, etc. ), which are logically associated with a change in the coordination center when moving from salen to the porphyrin complex.