[Ifeffit] Help: some operation using Athena and Artemis

Bruce Ravel bravel at bnl.gov
Mon Nov 26 10:01:36 CST 2012


Again, sorry for the very long wait to get a response to this question.

On Monday, October 22, 2012 07:44:05 AM Bingjie Ouyang wrote:
> Hi, I just joined the mailing list recommended by a friend, I hear this is
> a really good place to communicate and learn. So I'm here, actually I need
> help for some basic questions on Iffeffit software:
> 
> 1, In Athena, how to make a fourier transform draft?

I don't understand this question.  I don't know what a FT "draft" is.

> 2, In Artemis, since I was told that each shell in R-space can be fitted
> separatly, is it OK to fit each shell in the R-range respectively, and then
> join up the fitted curve and just ignore the not match part in other
> R-ranges? And can a shell be comprised of several different paths (like
> Fe-O and Fe-S)?

There is, currently, no way to exclude an interior region from a fit
in Artemis (i.e. there is no way to fit from 1 to 2 and from 2.5 to
3.5, but to exclude the region from 2 to 2.5).  I actually think
that's a bad idea and doubt that I would ever implement such a thing.

The reason I don't like this idea is that, in general, shells in the
EXAFS signal are not completely isolated.  Because of the limited data
range and other reasons, the signals form the various shells have long
tails and overlap considerably.  That overlap is a very important part
of the problem.  Getting the small parts of the chi(R) spectrum right
is just as important as getting the large parts right.

Excluding an interior region could artificially remove this overlap
from the evaluation of the fit.  That would have a very serious impact
on the statistical quality of the analysis.  I also worry that an
interior exclusion would be used in an attempt to minimize the impact
of multiple scattering paths on the analysis.  That, too, would be a
statistics disaster.

In short, you should embrace the correlations between the parts of
the problem.

HTH,
B


-- 

 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 Homepage:    http://xafs.org/BruceRavel
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