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 ~10^4 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.





Last change: 06-Jun-2006