[Ifeffit] Linear combination fitting

Bruce Ravel bravel at bnl.gov
Tue Nov 6 16:09:47 CST 2007

On Tuesday 06 November 2007 11:33:16 Jens Kruse wrote:
> Hi,
> I have a general basic question about linear combination fitting (LCF)
> with Athena.
> I have XANES spectra from two different mixtures. Both were mixed from
> two reference compounds in a 1:3 ratio. In one mixture the ratio is
> based on the weight of both reference compounds and in the other the
> ratio is based on the concentration of the element of interest (the
> concentrations in the reference compounds are not equal).
> If I am doing LCF on these spectra, which spectra would yield to  the
> expected weighting parameters 0.25 and 0.75? 

Hi Jens,

Good question.  The answer is the latter -- the sample which is 1:3 in
terms of number of the absorber element will result in an LCF fit of
0.25:0.75.   The way to think about this is that each ray has a 1 in 4
chance of interacting with an absorber from the minority component and
a 3 in 4 chance of interacting with an absorber from the majority
component.  The rest of the stuff doesn't contribute to the XAS.

> What actually represent a normalized spectrum?  Is a normalized
> spectrum a spectrum what one atom of interest would produce ?

Normalizing is something we do so that we can compare samples measured
under different experimental conditions.  It is also something we do
so that we can compare data with theory.  If we set the gains on our
detectors differently between successive scans of the same sample, we
actually do measure the same thing, even though the numbers in the
data files are of different orders of magnitude.  Normalizing removes
the effect of gains and other purely empirical details from the
measurements.  Also, Feff is calculated such that its wiggles are the
same size as the wiggles in our normalized data.


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

 National Institute of Standards and Technology
 Building 535A, Room M7
 c/o Brookhaven National Laboratory
 Upton NY, 11973, USA

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

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