The image is actually from
http://www.ruppweb.org/Xray/tutorial/vectordiag.htm
but the first image is missing.
There you see that the 'vectors' are actually (general acentric)
structure factors for a single anomalous atom, and each of those
of course has a real and imaginary component.
The real part of the anomalous structure factor f'
has to be collinear with the fo part (in contrast to what the
images insinuate, f' - being negative -
points opposite to fo, but that makes those drawings even more
confusing)
The f" structure factor part then of course also has a real and
imaginary part, but the f" contribution to the scattering is
retarded by 90 degrees (i.e. -270 = +90, offset by pi).
The key is to remember that these arrows are structure factors
of a single atom, not the scattering factors themselves.
BR
-----Original Message-----
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of
William Scott
Sent: Wednesday, October 13, 2010 10:48 AM
To: [log in to unmask]
Subject: [ccp4bb] embarrassingly simple MAD phasing question
Hi Citizens:
Try not to laugh.
I have an embarrassingly simple MAD phasing question:
Why is it that F" in this picture isn't required to be vertical (purely
imaginary)?
http://www.doe-mbi.ucla.edu/~sawaya/tutorials/Phasing/phase.gif
(Similarly in the Harker diagram of the intersection of phase circles, one
sees this.)
I had a student ask me and I realized that there is this fundamental gap in
my understanding.
Many thanks in advance.
-- Bill
William G. Scott
Professor
Department of Chemistry and Biochemistry
and The Center for the Molecular Biology of RNA
228 Sinsheimer Laboratories
University of California at Santa Cruz
Santa Cruz, California 95064
USA
phone: +1-831-459-5367 (office)
+1-831-459-5292 (lab)
fax: +1-831-4593139 (fax)
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