> On Friday 24 April 2009 11:53:27 Jacob Keller wrote: > > Aha, so I have re-invented the wheel! But I never made sense of why > f' is > > negative--this is beautiful! Just to make sure: you are saying that > the real > > part of the anomalous scattering goes negative because those photons > are > > sneaking out of the diffraction pattern through absorption-- > >fluorescence? > > I am not sure about that "because". Let's not confuse correlation with > causality. The negative f' is adequately explained by the Kramers- > Kronig > equation as being a result of the resonance interaction. > http://www.rp-photonics.com/kramers_kronig_relations.html > The maximum resonance is at the absorption energy, which is also the > maximum for the fluorescence. Both effects are "because of" the match > between incident photon energy and the energy required to kick an > electron > out of its current orbital state. I am uneasy saying that one effect > causes the other effect. > > Ethan
There is a very good technical description in Jens Als-Nielsen's "Elements of Modern X-ray Physics" in the chapter Resonant Scattering", pg 235 ff. In fact, there is also a good description of the breakdown of Friedel's Law and the MAD experiment in that chapter. I would like to iterate Ethan's comment about resonance. The effects are not anomalous at all, we know very well what is happening: the changes in f', f" and mu as a function of energy are all effects of the resonance of the photon energy with transition energy of the electron. So, we really should call it resonance scattering, not anomalous scattering. I have to admit MRD and SRD aren't as euphonic at MAD and SAD and the change will probably never happen. Joe Joseph D. Ferrara, Ph.D. Rigaku Americas Corporation > > > Jacob > > > > ******************************************* > > Jacob Pearson Keller > > Northwestern University > > Medical Scientist Training Program > > Dallos Laboratory > > F. Searle 1-240 > > 2240 Campus Drive > > Evanston IL 60208 > > lab: 847.491.2438 > > cel: 773.608.9185 > > email: j-kell...@northwestern.edu > > ******************************************* > > > > ----- Original Message ----- > > From: "Ethan Merritt" <merr...@u.washington.edu> > > To: "Jacob Keller" <j-kell...@md.northwestern.edu>; > <CCP4BB@jiscmail.ac.uk> > > Sent: Friday, April 24, 2009 1:40 PM > > Subject: Re: [ccp4bb] Reason for Neglected X-ray Fluorescence > > > > > > > On Friday 24 April 2009 11:28:16 Jacob Keller wrote: > > >> Dear Dr. Holton and CCP4BBers, > > >> > > >> Are you saying that a resonant event is always accompanied by a > > >> fluorescence > > >> event? 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? > > > > > > Yes. > > > That is why the real component of the scattering factor, f', is > negative. > > > > > > > > > -- > > > Ethan A Merritt > > > Biomolecular Structure Center > > > University of Washington, Seattle 98195-7742 > > > > > > > > > > > > > -- > Ethan A Merritt > Biomolecular Structure Center > University of Washington, Seattle 98195-7742
