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

Thus for each position on the sample the

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energies will yield the distribution of oxidation state within the sample. Thus, for each position on the sample, the fluorescence yield at three energies has to be measured: one before the edge, one after, and one at the redox-sensitive maximum (see also Sutton et al. [545]). Applications As the position of the absorption edge is dependent on the oxidation state XANES spectra can be used for the determination of the valence state. This can be done by using the edge position, i.e., determination of position in com- parison to the variation observed for the end member valence states. However, the observed position of the edge may be affected by resonances occurring in the edge region that are due to multiple scattering or transitions of the photoelectrons to localized energy levels (these features are very sensitive to the local geometry around the absorber and therefore structure dependent). Another possibility is the use of the whole XANES spectra: if the end mem- ber spectra of each oxidation state are clearly known from available model compounds the oxidation state of the unknown sample can be determined by linear combinations of spectra of the model compounds (e.g., Garvie and Buseck [546]). As the structure of the XANES is also a “fingerprint” of the absorbing element in a given phase (e.g., examples in Fig. 7.143a), linear combinations of end member spectra can be used to determine the host phase of a given ele- ment and its relative abundance in a sample (e.g., Welter et al. [547]; Calmano et al. [548]; Vincze et al. [549], Salbu et al. [506]). The pre-edge feature that probes 1s- > 3d transitions (especially sensitive to oxidation state) is only affected by the first co-ordination shell around the absorber (usually oxygen). Thus, it will show much less dependence on changes in the medium-range environment or bulk structure than the structure of the spectra occurring at the main edge. The pre-edge position varies significantly with oxidation state and shows variation in intensity due to the geometry of oxygen-coordination polyhedron (octahedral, tetrahedral, etc. see Fig. 7.143b; Wilke et al. [550]). In the case of Fe, the difference between the pre-edge po- sition (measured as centroid) of ferric and ferrous Fe does not vary with the coordination geometry. Therefore, a linear variation of the position with redox ratio can be expected if the coordination does not change with oxidation state. Exploiting this effect, it was shown to give moderately precise estimates for the oxidation state (approx. 10 15% error) by the use of general cali- bration curves (Fig. 7.143c) (Bajt et al. [551] 1995; Wilke et al. [550]). These calibrations were applied to several geological problems where knowledge of
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684 D. Rammlmair et al. Integrated pre-edge intensity 7100 7120 7140 7160 7180 7200 4 Fe 2+ - Staurolite 6 Fe 2+ - Siderite 4 Fe 3+ - FePO 4 6 Fe 3+ - Andradite garnet Energy (eV) Energy (eV) 7100 7105 7110 7115 7120 7125 Normalized absorbance 4 Fe 2+ - Staurolite 6 Fe 2+ - Sideritet 4 Fe 3+ - FePO 4 6 Fe 3+ - Andradite garnet (b) (c) Normalized absorbance (a) Fe(II) Fe(II) Fe(III) Fe(III) Centroid position (ev) 0.35 0.40 Fig. 7.143. ( a ) XANES spectra collected at the Fe K-edge of several model com-
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  • Spring '14
  • MichaelDudley

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