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Conclusion
The information
content of an EXAFS signal is estimated by the product of the lengths
of intervals it occupies in the wavevector space and its Fourier transform
space. A good spectrum may contain 25 "independent points"
i.e. up to 25 parameters can be meaningfully determined in a variational
refinement of a model of the structure of the sample. This may not seem
much in comparison to the single-crystal XRD method where the intensities
of ~104 diffraction points may be determined in a single measurement.
On the other side, the information content of EXAFS is entirely comparable
to the powder XRD where similar methods of refinement are employed.
The main advantages of X-ray absorption spectrometry methods, as already
mentioned, are the insensitivity to the long-range order and the sensitivity
to the chemical species. EXAFS probes the immediate neighborhood of
the atomic species chosen by the energy of its absorption edge: the
depth of the probing may be somewhat greater in ordered materials but
useful results are obtained also from amorphous samples, even from liquids
and molecular gases where diffraction methods cannot be applied. The
strong sensitivity to first neighbors makes EXAFS, and especially XANES,
the tool of choice for the coordination chemistry and chemistry of catalysts
and other nanostructures.
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| E-mail:iztok.arcon@p-ng.si Last change: 23/2/2004 |
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