X-ray photoabsorption

Figure 1 Schematic representation of the three techniques (a) x-ray photoabsorption (NEXAFS/SEXAFS), (b) photoelectron spectroscopy (photoemission) and (c) photoelectron diffraction.

Solid state spectroscopy usually revolves around the determination of properties characteristic of a solid, such as the density of states in the conduction band, the position of the Fermi energy, etc. These can be probed by X-ray photoabsorption or more directly by photoemission (either angle-resolved [593] or inverse [594]) spectroscopies, in which case the initial quasiatomic state is reasonably well known since it is close to the wavefunction of a free atom, while the final state is the continuum characteristic of the solid, so that the dispersion of the wavevector k can be determined. 

Such models, applied by Gunnarsson and Schonhammer [616], or by Jo and Kotani [618] have provided a very successful framework for reconciling results from a wide variety of different spectroscopies (X-ray photoabsorption (XAS), photoemission (PES), bremstrahlung isochromat spectroscopy (BIS), etc.) by the use of only a few adjustable parameters. 

Barchewitz R, Cremonese-Visicato M, Onori G (1978) X-ray photoabsorption of solids by specular reflection. JPhys C 11 4439-4445... 

The real-photon method is essentially more direct and easier compared to the dipole-simulation method in obtaining absolute values of photoabsorption cross sections (o ), photoionization cross sections and photoionization quantum yields (t],). In the real-photon method, however, there is a practical need to use the big and dedicated facilities of synchrotron radiation where, in many cases, one should change the beam lines equipped with different types of monochromators depending on used photon-wavelengths—and to develop some specific new experimental techniques in the range from the vacuum ultraviolet radiation to soft X-ray. 

Ito et al. [20] studied the mechanism to produce strand breakage by phosphorus photoabsorption using a dinucleotide as a sample, and revealed that even with photoabsorption by phosphorus strand breakage occurs through the destruction of deoxyribose of the 5 end, leaving adenine and 5 -AMP as products. Yamada et al. [21] used penta-deoxythymidylic acid (d(pT)5) as a sample, and analyzed the products quantitatively with on-resonance (2153 eV) and off-resonance (2147 eV) x-rays. They found that the distribution of the products was independent of the x-ray energy, and that the yields of the products were proportional to the absorption cross section of the sample. Unexpectedly, they could not find any evidence of Auger-specific products, neither qualitatively nor quantitatively. 

These lines of results using repair-deficient strains suggest that inner-shell photoabsorption, followed by the Auger effect, more efficiently produces a nonrepairable type of damage than x-rays with other energies. However, studies to find or identify these types of damages at the molecular level have not yet been successful so far. 

In the soft x-ray region, an enhancement by the photoabsorption of phosphorus and of calcium was observed. The condensation of calcium near DNA in spores, as a form of dipicolinic acid, has already been reported [63]. The calcium atom has acted in the same manner as exogenous sensitizers. 

The dependence of photoabsorption cross-section on many-electron quantum numbers (sets of quantum numbers of a chain of electronic shells) is mainly determined by the submatrix element of the transition operator. Their non-relativistic and relativistic expressions for the most widely considered configurations are presented in Part 6. When exciting an atom by X-rays the main type of transitions are as follows ... 

Henke BL, Gullikson EM, Davis JC (1993) X-ray interactions photoabsorption, scattering, transmission, and reflection at E = 50-30 000 eV, Z = 1-92. Atomic Data and Nuclear Data Tables 54 181-342... 

The first experimental observations of the absence of core levels seem to have been made by Lukirskii et al.1 and Codling and Madden2 in soft X-ray absorption spectra for Xe. They found lines converging to what seemed to be the 4 3/2 threshold but no lines were found that could be associated with the 4p1/2 threshold. Later photoabsorption experiments on Te3), Xe4 and Cs5 confirmed these results for Xe and Cs and showed that in Te both 4p-thresholds were absent (see also Sect. 9). 

A single-particle effect that adds features in the X-ray absorption spectrum of molecules not present in that of atoms is the shape resonance (74, 75). (In the case of solids this effect, caused by a modification of the density of states due to the presence of the other atoms in the molecule, is automatically accounted for in band calculations.) Localization of the excited electron inside the molecule in states resulting from an effective potential barrier located near the electronegative atoms in the molecule causes strong absorption bands in free molecules and near the inner-shell ionization limits of positive ions in ionic crystals (74). Consequently, molecular inner-shell spectra depart markedly from the corresponding atomic spectra. The type of structure of an inner-shell photoabsorption spectrum depends on the geometry of the molecule, the nature of its ligands, etc., and can sometimes be used to determine the structure of the molecule. 

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