[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Because xafs works as a local probe the

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Because XAFS works as a local probe, the aforementioned information is also accessible in systems lacking long-range order as silicate glasses and melts or aqueous solutions and hydrothermal or even supercritical fluids. As XAFS is element specific it also allows investigation of elements that are only near or at the surface of a mineral or phase, thus providing a helpful tool for the investigation of processes at the mineral–water interface. XAFS spectrometers usually rely on the continuous spectrum at high pho- ton flux available at synchrotron radiation facilities. In order to detect the fine structure at the absorption edge a monochromatic beam with a sufficiently small bandwidth is needed. Typically used in the energy regime suitable for
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Methodological Developments and Applications 681 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 7080 7120 7160 7200 7240 7280 7320 7360 7400 Normalized absorbance Energy (eV) EXAFS XANES Pre-edge Hercynite Fig. 7.142. XAFS-spectrum at the Fe K-edge of hercynite (Fe–Al-spinel) showing the respective regions within the spectrum as explained in the text. Note that the EXAFS region extends much further than shown here investigations of the 3d-transition elements are Si (111) based double-crystal monochromators providing a resolving power E/∆E of approx. 4000 at 7 keV. While such a resolution is adequate enough for investigations of the EXAFS region it may be insufficient to resolve all spectral features present in the XANES region so that monochromator crystals with higher intrinsic resolu- tion have to be used, e.g., Si (220) or Si (311). The actual resolution needed is determined by the energy width of the transitions that are probed. The width is a function of the core-hole lifetime and increases with increasing atomic number. Use of a small bandwidth monochromator, however, may be limited due the accompanied loss in photon flux that potentially makes high-resolution measurements unfeasible due to low counting rates at the detector. XAFS and Fluorescence Yield Originally X-ray absorption spectroscopy is performed by measuring the in- tensity of the X-ray beam before and after the sample (I 0 and I tr , respec- tively), so that the absorption coefficient µ = ln I 0 /I tr . A further possibility
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682 D. Rammlmair et al. is the indirect measurement of µ through detection of the fluorescence yield as µ I f /I 0 . This relation holds as long as the concentration of the absorbing element is low or the sample is very thin. For thick and concentrated samples the oscillations and resonances above the edge are significantly damped, and peaks in the pre-edge region artificially are enlarged due to self-absorption of the incoming beam. These effects are dependent on the detection angle and are minimized by detection in grazing exit geometry (see Tr¨ oger et al. [538]; Manceau et al. [539]).
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  • Spring '14
  • MichaelDudley

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