==============Original message text=============== On Wed, 30 May 2007 6:51:09 pm CDT Ethan Merritt wrote:
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. Although I am no authority on this matter, the way I think of resonant scattering is scattering which is phase-shifted from the protein scattering by a certain absolute amount, due to a resonance event which takes a certain amount of time. Is this a correct model? If so, the question would be whether this resonance event saps energy from the incident light--then the emitted light from the heavy atoms would be of lesser energy. If this were true, then the heavy atoms would indeed be sending out a diffraction pattern of their own, which would be, I believe, at just slightly higher scattering angles and somewhat different Bragg conditions due to their lesser energy. Were you saying that there was some resonant scattering from the heavy atoms which was the same wavelength as the incident light, and some which was different? Thanks for your response, Jacob Keller *********************************** Jacob Keller Northwestern University 6541 N. Francisco #3 Chicago IL 60645 (847)467-4049 [EMAIL PROTECTED] ***********************************
