### X-ray Spectroscopic Methods for Diagnosing Atomic and Electronic Structure
#### 3.1.2 Vb Pes  
The main method for obtaining information about the energy distribution of occupied electron states in the valence band of a polyatomic system is valence-band photoemission spectroscopy (VB PES), which uses ultraviolet radiation (He(I) and He(II) resonance lines of gas-discharge sources with photon energies _hv_ of 21.2 and 40.8 eV, respectively) and synchrotron radiation in the soft X-ray range with photon energies _hv_ of 50-150 eV to excite photoemission spectra from the valence band (Figure 7a) [117]. The valence-band electronic structure of a polyatomic compound is characterized by the energy distribution of the density of states (DOS) for occupied electron states, which in turn are described by combinations of valence atomic orbitals (AOs) of all atoms of the compound. The different dependences of the photoionization cross sections on the energy of exciting photons for valence AOs of different atoms [122] cause noticeable differences in the shape of the experimental VB PE spectrum from the calculated energy dependence of DOS for occupied valence states. When the energy of the exciting photons changes, the intensity of the main PE bands usually changes significantly, which is clearly observed in Figure 8. Thus, by considering the behavior of the intensity of VB PE bands with changes in the energy of exciting photons, it is possible to obtain certain information about the origin of these bands in the valence band of a polyatomic system and their relationship to the partial DOS of individual atoms. In this regard, it is advisable to study VB PE spectra, exciting them with photons with energies in a wide energy range encompassing both ultraviolet and soft X-ray radiation. Additional information about the electronic structure of the valence band can also be obtained using the method of resonance photoemission (ResPES), which is especially useful in the study of compounds of transition and rare earth metals, in particular, 3\(d\) metals [123; 124; 125]. This method is implemented by measuring VB PE spectra at photon energies of localized core-excited states (absorption resonances) in the NEXAFS spectrum of metal atoms of the polyatomic system under study. When the VB PE spectra of [Ni(Salen)] are excited by photons with energies in the region of the Ni 2\(p_{3/2}\to 3d\) absorption resonance, distinct changes in the structure of the spectra are observed, namely, a strong increase in the intensity of some PE bands in the low \(E_{bin}\) region and the appearance of additional, very intense bands at high \(E_{bin}\) (Figure 9). The increase in intensity occurs due to the superposition of two processes. The first one is associated with the direct photoionization of occupied Ni 3\(d\)-derivated MOs in the valence band of [Ni(Salen)] (2\(p^{6}3d^{8}+h\
u\to 2p^{6}3d^{7}+\mathrm{e}\)), while the second process is due to the participator-Auger (\(pA\)) decay of the Ni 2\(p^{5}3d^{9}\) core (resonance) excitation (2\(p^{6}3d^{8}+h\
u\to 2p^{5}3d^{9}\to 2p^{6}3d^{7}+\mathrm{e}\)), the final state of which coincides with that of direct photoionization (Figure 10). In turn, the appearance of additional bands in the VB PE spectra is caused by the spectator-Auger (\(sA\)) decay of the Ni 2\(p^{5}3d^{9}\) resonance excitation. All of the above processes are shown schematically in Figure 10 and are clearly visible in Figure 9 when considering the VB PE spectra of the [Ni(Salen)] complex excited at a photon energy of 854.4 eV (corresponding to the value of maximum \(A\) of the most intense resonance in the Ni 2\(p_{3/2}\) NEXAFS spectrum). When considering Figure 9, it is important to emphasize that after the energy of the exciting photons passes through resonance \(A\), the intensities of the \(pA\), \(sA_{1}\), and \(sA_{2}\) bands gradually decrease, and at photon energies above 859 eV, the VB PE spectrum practically returns to its initial shape.