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 () and fine structure (FS) terms,
Because the term is small and smooth, the contrast is roughly proportional to the ratio of the cross terms to .
High intensity Bragg reflections have larger absorption corrections than weak reflections. This is because the absorption contributions are weighted by the term, whereas the fine structure contributions from the and diffraction terms are weighted by and .
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.