Am 23.04.2009 um 02:31 schrieb James Holton:
Fluorescent x-rays have a VERY different wavelength from the incident beam and therefore cannot interact coherently with Bragg- scattered photons, so they contribute to nothing but background. Fluorescence is also a true absorption-reemission process, and must occur from one atom at a time. The "core hole lifetime" before emission occurs is small, but there is still a "random delay" before the fluorescent photon is emitted. This means there is essentially no interference between fluorescence events from different atoms. Scattering, on the other hand, occurs from every atom in the crystal simultaneously for each incident photon, and this is why we see interference.
yes, this is certainly true for real fluorescence effects. But the anomalous scattering can be best thought of as a resonance phenomenon without any frequency change, and as such, it has a distinct phase relationship to the elastically scattered photon and does have an effect on the intensities (which, I think, was the background of the original question?). But for the lighter atoms in biological macromolecules, where in a typical experiment the measurement frequency is far away from any resonance frequency, this effect can be neglected.
This leads me to my follow-up question to the experts: why is the resonance effect "anomalous scattering" measured by a fluorescence scan that should have all the effects mentioned by James? Don't we get as a result a mixture of signals from resonance (i.e. anomalous) and from absorption-emission (i.e. fluorescence) effects?
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