[Ifeffit] linear combination fitting

Bruce Ravel bravel at bnl.gov
Wed Aug 7 13:31:01 CDT 2013

On 08/07/2013 07:55 AM, Matt Newville wrote:
> How good are the data and standards used over these ranges?   As you
> might expect, a linear combination for chi(k) is generally intended
> for EXAFS, and so emphasizes distances  of the nearest neighbors while
> "norm" and "derivative" is generally for the XANES portion of the
> spectra, and so emphasizes chemical state.

I want to expand on this point.

Have you ever measured data on a metallic nanoparticle?  I'll use gold
as my example.  If you compare good XAS data on gold nanoparticles to
data on a gold foil, they are very similar, but not identical.  The
XANES for the nanoparticles is quite similar -- possibly
indistinguishable -- from the XANES for the foil.  The EXAFS, however,
is quite different due in part to the effect of undercoordination of
the scattering shells and in part to the additional structural
disorder introduced by the large surface to volume ratio.

Now imagine that you are trying to do linear combination fitting on a
system containing gold.  If you are doing LCF on the XANES, it
probably doesn't matter whether you use data on a foil or on
nanoparticles as a standard.  If you are doing LCF on the EXAFS, it
most certainly matters.

My point is that, like Matt, I think you asked the wrong question.
None of the ways of doing LCF are "better" in some general sense.  You
need to think hard about the problem you are trying to solve and do
the thing that is the most appropriate way to find your answer.  LCF
using chi(k) is sometimes a useful tool.  LCF using norm or deriv is
sometimes a useful tool.

As for the difference between norm(E) and deriv(E) -- well, I expect
that they would give the same answers, within numerical precision and
measurement uncertainty, for the same data.  Why does Athena allow
either one?  Well, it's not up to me to choose.  It's up to you!


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

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

  Homepage:    http://xafs.org/BruceRavel
  Software:    https://github.com/bruceravel

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