Hi Matti，
Thanks for your attention and patience.
The back fourier transform of the specific peak (use window)of Chi(R) gives
the amplitude,donated by amp_bft。As in previous mails,I ask whether the compare
the amp_bft and the amplitude of specific Z number element can determine the
Z.And thanks for telling me the useful trick using the total phase shift of the
specific element.
The discrepancy between R and Rphcor is below,the Zn and Br is close to Se(the
best fit)'s 0.013,should the enot also be important criterion in this trick?and
dose the plus or minus of enot indicate the lighter or higher element relative
to
the specific coordination shell?
Zn -0.014
GE -0.004
Se 0.013
Br 0.018
Rb 0.042
"a bond valence sum can be an independent check on the consistency of N, R,
and valence",can your give the ref. paper of this method? Thanks
Sinceerly,
Zhanfei
-----原始邮件-----
发件人: "Matt Newville" <newvi...@cars.uchicago.edu>
发送时间: 2014年7月7日 星期一
收件人: "XAFS Analysis using Ifeffit" <ifeffit@millenia.cars.aps.anl.gov>
抄送:
主题: Re: [Ifeffit] How to distinguish whether the coordination element is heavy
or light
Hi Zhanfei, Bruce, Scott,
Sorry for jumping in to this conversation a bit late. Like Bruce said, there
is no magic trick for determining whether a part of the R-space spectrum comes
from one or more scatterers. And while XAFS is sensitive to Z, the sensitivity
is weak.
Because it's XAFS School week here in Chicago, I thought I'd go through a test
of the Z dependence, and also try out a "trick" (I believe I first heard this
from Mali Balasubramanian, but I suspect others may know this trick too) that
relies on phase-corrected Fourier transforms, and is somewhat related to
Scott's description of Joe Woicik's comments.
The phase-correction "trick": If you correct for the phase-shift, the peak
in |chi(R)| should be at the interatomic distance (ignoring subtleties in the
XAFS equation). Turning this around, the peak in the phase-corrected |chi(R)|
will be at the interatomic distance if and only if the phase-shift applied was
correct... which means that Z is correct (to within some uncertainty). How
well does this actually work on real data?
To work through these two related ideas, I used ZnSe as a test case -- a very
simple structure with a well-isolated first shell, and I have some decent data
on it lying around. This also seemed like a useful enough category of
analysis, that I thought it would be useful to better document. Scripts and
results are at
http://xraypy.github.io/xraylarch/xafs/feffit.html#example-6-testing-exafs-sensitivity-to
(turning this into an Artemis project is left as an exercise for the interested
reader). The results of just changing scatterer in the fits are pretty clear,
and suggest that Z +/- 2 might be a reasonable rule-of-thumb even when
refining, R and S02, at least in this case of a well-isolated first shell.
The results might be different for lighter backscatterers, but there are
claims, especially in the bio-XAFS literature, that one can distinguish N and
O ligands at least in some cases. Still, given that the ZnSe case is so
clear, it seems reasonable to stick with the more pessimistic "Z +/- 5"
rule-of-thumb, as long as the possibility that one can do better in certain
cases (and may do much worse in others!).
The phase-correction approach is interesting in that it asks "is this
particular fit self-consistent?" instead of "which of these fits is best?".
This independent of the fit quality could make it a useful secondary check of Z
and R (much like a bond valence sum can be an independent check on the
consistency of N, R, and valence). It does not seem highly accurate on its
own -- also suggesting Z +/- 2 or 3 is about as well as one can do without
further knowledge of the scatterers. That might be partially related to how
well one can actually determine the peak position for chi(R) on a grid of 0.03
Ang, and partly related to the fact that other terms in the EXAFS equation
alter the phase. In principle, those could be accounted for -- another
exercise for the interested reader.
I don't think the phase-correction "trick" would help Zhanfei -- it will NOT
work on a mixed coordination shell. But the approaches described might be
useful and/or inspiring to others.
--Matt
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