![]() In this range multi-electron excitations and many-body final states in strongly correlated systems are relevant The X-ray absorption near-edge structure ( XANES), introduced in 1980 and later in 1983 and also called NEXAFS (near-edge X-ray absorption fine structure), which are dominated by core transitions to quasi bound states (multiple scattering resonances) for photoelectrons with kinetic energy in the range from 10 to 150 eV above the chemical potential, called "shape resonances" in molecular spectra since they are due to final states of short life-time degenerate with the continuum with the Fano line-shape.the bound core excitons in insulators with a Lorentzian line-shape (they occur in a pre-edge region at energies lower than the transitions to the lowest unoccupied level).the states at the Fermi level in metals giving a "rising edge" with an arc tangent shape.The absorption threshold determined by the transition to the lowest unoccupied states:.There are three main regions found on a spectrum generated by XAS data which are then thought of as separate spectroscopic techniques (Figure 2): For instance, excitation of a 1s electron occurs at the K-edge, while excitation of a 2s or 2p electron occurs at an L-edge (Figure 1). The edges are, in part, named by which core electron is excited: the principal quantum numbers n = 1, 2, and 3, correspond to the K-, L-, and M-edges, respectively. XAS data is obtained by tuning the photon energy, using a crystalline monochromator, to a range where core electrons can be excited (0.1-100 keV). Samples can be in the gas phase, solutions, or solids. The experiment is usually performed at synchrotron radiation facilities, which provide intense and tunable X-ray beams. X-ray absorption spectroscopy (XAS) is a widely used technique for determining the local geometric and/or electronic structure of matter. Figure 2: Three regions of XAS data for the K-edge
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