On Wednesday 30 May 2007 17:13, Jacob Keller wrote: > ==============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?
Yes and no. The photon is not phase-shifted "from the protein scattering". It is phase-shifted compared to what it would have been if the f" term of the scattering atom was 0. There need not be any protein involved in this. And please note that "resonant scattering" is not a standard term. > If so, the question would be > whether this resonance event saps energy from the incident light It does not. There is no shift in photon energy. > --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. Nope. The photon does one of several things - passes straight through crystal (not diffracted, not absorbed) - is scattered (no change in energy, but possible phase shift) - is absorbed and re-emitted later as a fluorescent photon of lower energy - is absorbed and not re-emitted, leading to X-ray damage and heat > 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? I never said that. -- Ethan A Merritt Biomolecular Structure Center University of Washington, Seattle 98195-7742
