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Jacob Keller wrote: > Dear Dr. Holton and CCP4BBers, > > Are you saying that a resonant event is always accompanied by a > fluorescence event? no. For example, with selenium only ~59% of the core holes decay by emitting a fluorescent x-ray. The rest by emitting an Auger electron. The latter seldom escape the sample. On the other hand, there are generally a lot more absorbed photons than scattered ones. For Se again at 12680 eV (just above the edge) the ratio is about 120 absorption events for every elastically scattered photon. And since 59% of the absorptions make a fluorescent x-ray, there are about 70 "fluorescence events" for every "resonant event". Anyway, the ratio is definitely not 1:1. > If that were true, wouldn't the resonant event end up manifesting as > *negative* scattering component from the resonant atom, due to the > elimination of an otherwise-scattered photon, this making the resonant > atom "darker" than would be expected? Sort of. I personally like to think of the core electrons as "disappearing" from the "normal scattering" as they start to scatter out of phase. However, ALL of the electrons scatter any given photon. Even the "anomalously scattering" electrons don't really disappear any more than a beat-deaf member of a marching band disappears when they get out of step with the rest of the rank and file, but they do stop contributing to the total effect. > > Also, in your selenium crystal example, I think there would still be > an anomalous signal, because there would always be regular scattering > as well as the anomalous effect. Isn't that true? No. There is no anomalous scattering from crystals with only one atom type. That is, Friedel's law holds because the phase lag from every atom is the same. Friedel's law also holds for centrosymmetric crystals, despite any anomalous effects. I suppose you might be able to see the atomic form factor change as the core electrons go out of phase as you approach the absorption edge, and well, okay, technically that is an anomalous scattering effect. But Friedel's law is not broken for elemental crystals nor for centrosymmetric crystals. > > By the way, while we're on the topic of comparing uv-vis fluorescence > to x-ray fluorescence, does anybody know of an example of the use of > FRET in x-ray fluorescence? I cannot think, off hand, of an > application for such, but theoretically it could be done easily with > two types of heavy atoms, such as a Se-met and some appropriate acceptor. Rick Donahue (or health physicist here at ALS) told me a story once where they found a sample of what I think was some metal carbonate that was emitting fluorescent x-rays from the metal, but it was the carbon in the carbonate that was radioactive. One could consider this an example of a transfer of excitation in the x-ray regime, but I'll have to check with Rick to be sure. -James Holton MAD Scientist