X-ray K absorption edges

Herman Pines and Luke A. Schaap The Use of X-Ray K-Absorption Edges in the Study of Catalytically Active Solids Robert A. Van Nordstrand The Electron Theory of Catalysis on Semiconductors Th. Wolkenstein... 

The Use of X-Ray K-Absorption Edges in the Study of Catalytically Active Solids... 

Z is the atomic number, is the wavelength at the K-absorption edge. On passing the X-ray K-absorption edge from lower to higher frequencies the absorption coefficient p will increase by the factor 63.868/Z° . 

Figure 1. X-ray absorption spectrum of a silica supported ruthenium-copper catalyst at 100 K In the vicinity of the K absorption edge of ruthenium. Reproduced with permission from Ref. 8. Copyright 1980, American Institute of Physics.

Electronic Structure Determination, Cox and Beaumont have studied the polarized x-ray absorption edge of a single crystal of Zn 2 (23), in which the Zn has tetragonal (D ) site symmetry. The observed anisotropic K-absorption edges were explained in terms of a ls+4p and a ls-K4p, 4p ) transition. 

Figure 5.4 X-ray tube output spectrum, showing continuous emission and line spectra of the target material (in this case gold). The K absorption edges for major elements in silicate glasses are shown below the diagram, indicating that the gold M lines are particularly effective for the analysis of the light elements Na to P.

K EDGE ABSORPTION SPEGTRA. The absorption of x-rays in the vicinity of the K absorption edge of transition metals gives information on both oxidation state and coordination geometry of the central absorbing atom. The energy positions of various absorption features have been demonstrated to be correlated to the formal valence of V in an extensive series of oxides, vanadates and intermetallics (13"). 

Takakura, K. Auger effects on bromo-deoxyuridine-monophosphate irradiated with monochromatic X-rays around bromine K-absorption edge. Radiat. Environ. Biophys. 1989, 28 (3), 177-184. 

Maezawa, H. Hieda, K. Kobayashi, K. Furusawa, Y. Mori, T. Suzuki, K. Ito, T. Effects of monoenergetic X-rays with resonance energy of bromine K-absorption edge on bro-mouracil-labelled E. coli cells. Int. J. Radiat. Biol. Relat. Stud. Phys. Chem. Med. 1988, 53 (2), 301-308. 

Traditionally, X-ray absorption edge measurements have been used to determine oxidation states of metals in complex materials. The extended X-ray absorption fine structure (EXAFS), on the other hand, provides structural information such as bond distances and coordination numbers even with powdered samples, crystalline or amorphous, the fine structure essentially resulting from short-range order around the absorbing atom. The technique is also useful for studying solid surfaces (SEXAFS). The observation of fine structure beyond the K-absorption edges of materials dates back to... 

Fig. 221. Anomalous scattering of X-rays of wavelength near that of an absorption edge. Iron atom,. K absorption edge, a. Definitions4 6, o and d. A/, A/, and 4 in relation to A/Ajp (the ratio of incident wavelength to that. of the K absorption edge of the iron atom).

Figure 2.73 (a) The X-ray absorption spectrum of copper showing the X- and L-absorption edges, (b) The K-edge in more detail. From A.R. West, Solid Stale Chemistry and its Applications, Fohn Wiley and Sons, Chichester (1984). Reprinted by permission of John Wiley and Sons, Ltd. 

The importance of X-ray diffraction (discussed in Chapter 3) to biochemistry is obvious, but techniques related to absorption of X-rays and y-rays have also come into widespread use.117 Abbreviations such as XANES and EXAFS are common in the metallopro-tein literature. The names arise from the sharp increase in the absorption coefficient for X-rays as their energy is increased to what is called the K absorption edge. At slightly lower energies absorption of an X-ray by an atom leads to expulsion of an electron or the raising of an electron to an excited state. Absorption of X-rays will expel all except the inner Is electrons. 

Fig. 18. Average environment of calcium ions in amorphous calcium phosphates as determined by X-ray absorption spectroscopy near the K absorption edge of calcium. The diagram shows two bidentate and two monodentate phosphate tetrahedra and two water molecules contributing to the eight oxygens of the first coordination sphere of the calcium ions. A third shell, possibly comprising the two phosphorus atoms of the monodentate phosphate ions, can be seen in some of the preparations. The positions of the protons are not established (Holt et al., 1988, 1989b Holt and Hukins, 1991).

Waychunas, G. A., Apted, M. J. Brown Jr, G. E. (1983) X-ray K-edge absorption spectra of Fe minerals and model compounds I. Near-edge structure. Phys. 

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