On Wednesday 30 May 2007 16:24, Jacob Keller wrote: > I have been wondering recently whether the anomalous component of a > diffraction pattern is of a > different wavelength from the regular diffraction pattern.
The diffraction pattern satisfies Bragg's Law. If the input radiation is monochromatic, then the diffraction pattern shows a spot wherever that wavelength satisfies Bragg's Law for some set of planes in the crystal. In the presence of anomalous scattering, some of the incident radiation is absorbed rather than diffracted. The absorbed photon may then be re-emitted via X-ray fluorescence, as you mention. That emitted photon goes off in some random direction and does not contribute to the main Bragg diffraction pattern. In principle it could produce a diffraction pattern of its own as it travels through the rest of the crystal, but the diffraction pattern from a single photon will not be measurable in practice. Now you can, of course, have polychromatic input radiation. This is the basis of Laue diffraction experiments. In this case the diffraction pattern contains information from the Bragg scattering of photons with various energies. Because the scattering power changes as a function of energy, particularly near an absorption edge, this means that there is information about the anomalous scattering properties of your crystal in the recorded diffraction pattern. -- Ethan A Merritt Courier Deliveries: 1959 NE Pacific Dept of Biochemistry Health Sciences Building University of Washington - Seattle WA 98195-7742
