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Optimization of DAFS measurements

The optimization of a DAFS experiment requires choosing Bragg reflections that balance three factors: intensity, contrast and size of the absorption correction. High intensity reflections are preferred over low intensity reflections for several reasons. Low intensity reflections have poor counting statistics, require long measurement times, and have large background signals due to fluorescence and diffuse scattering. High intensity reflections, however, have poor contrast. The DAFS contrast is a measure of the size of the fine-structure signal relative to the Bragg reflection intensity. Equation 10 shows the intensity dependence on the smooth (tex2html_wrap_inline1514) and fine structure (FS) terms,
eqnarray291
Because the tex2html_wrap_inline1578 term is small and smooth, the contrast is roughly proportional to the ratio of the cross terms to tex2html_wrap_inline1512.

High intensity Bragg reflections have larger absorption corrections than weak reflections. This is because the absorption contributions are weighted by the tex2html_wrap_inline1512 term, whereas the fine structure contributions from the tex2html_wrap_inline1584 and tex2html_wrap_inline1586 diffraction terms are weighted by tex2html_wrap_inline1588 and tex2html_wrap_inline1590.

All of the samples used for the work described in this chapter were thin epitaxial films grown on substrates. Thin films have several advantages over other samples: they provide more intense diffracted beams than powder samples of the same volume; they typically have broad smooth mosaic distributions which allow easy tracking of the Bragg peaks versus photon energy; their diffracted intensities can be analyzed using the kinematic formalism; and their absorption corrections are relatively small so that optimization consists primarily of finding a compromise between high intensity and high contrast.

There are additional optimization considerations for experiments that use the wavevector and site selectivities of DAFS. For wavevector selective DAFS experiments, Bragg reflections from the desired components should be well separated from the other Bragg reflections. For site selective experiments, Bragg reflections should be chosen that produce easily separable linear combinations of signals from the different sites. Generally, even after applying these restrictions, many suitable Bragg reflections will be available.


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Next: DAFS ANALYSIS METHODS Up: EXPERIMENTAL METHODS Previous: EXPERIMENTAL METHODS