[Ifeffit] Using the amplitude reduction factor as a linear combination fitting parameter

Bruce Ravel bravel at bnl.gov
Tue Apr 29 08:11:13 CDT 2008


On Tuesday 29 April 2008 03:06:28 Bruggeman Christophe wrote:
> I have recently collected EXAFS spectra of uranium on a FeS2 surface.
> Using principal component analysis of the XANES and k3-weighted EXAFS
> spectra, I have found that there are two uranium species which compose
> the spectra. As a first tentative guess, I believe these two uranium
> species are uraninite (UO2(c)) and a uranyl species. I would like now to
> fit the fourier transform functions (real parts and magnitudes) using
> the theoretical paths and path degeneracies created by feff, and use the
> amplitude reduction factor S02 as a fitting parameter to derive the
> relative amounts of the two uranium species in my samples.
>
> Normally, this S02 is taken as a constant (between 0.7 and 1.0), and the
> path degeneracies are fitted. So normally, S02 is not really a fitting
> parameter (some papers derive it even with theoretical functions).
> However, given the fact that S02 and N are completely correlated, I
> think it is justified to use this approach.

Hi Christophe,

The use of S02 in EXAFS analysis and its correlations with N and other
parameters are discussed at considerable length in the archives of
this mailing list.  Here is a good place to start:

  http://cars9.uchicago.edu/iffwiki/FAQ/FeffitModeling

Also seaching google for "site:millenia.cars.aps.anl.gov s02" turns up
several more useful posts.

In short, you seem to be on the right track.  S02 is completely
correlated with N in a first-shell fit.  If you are fitting multiple
corodination shells, you might be able to disentangle S02 from the
various N's.  And, of course, if you know N (for instance, when you
measure EXAFS on a well-described crystal), then you might be able to
determine S02 from your data.

A word of caution, though.  There are lots of other things that can
effect the amplitude of your EXAFS, including error in normalizing
data, error in Feff's self-energy model, detector non-linearity,
sample inhomogeneity -- pretty much every part of the experiment from
sample prep through theory can introduce systematic uncertainty and
inaccuracy in your determination of EXAFS amplitude.  Consequently,
one often throws an amplitude parameter at a fit to account for all
those various systematic issues.  Given that, once the experiment is
finished, those systematic issues are probably unknowable and
unfixable, a variable amplitude parameter may be your best bet for
extracting meaningful results from your data.

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

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/




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