[Ifeffit] self absorption correction

[Bruce Ravel]

On Wednesday, January 09, 2013 10:28:17 AM sumitk at barc.gov.in wrote:
> Thanks Bruce. Till the time i look into the suggested works, here is the
> missing informations.

That's kind of backwards.  You should do your research *before* asking
a question.  One tends to ask better questions that way!

> Question: how to check the appropriateness of the self absorption
> correction? I don't have corresponding transmission data.
> The question came to my mind when i compared the uncorrected (of courese
> corrected for dead time) data and the corrected (with SiO2Eu0.002 formula;
> the formula corresponds to the mole (roughly atom)percentage. The edge
> jump enhanced after correction to ~1 from ~0.04.
> Material: Eu(III) sorbed on silica.

Again, you are not providing enough information so that your question
can be answered well.  Where did that stoichiometry come from?  It
would be useful to know how thick the Eu film is on the silica
substrate and then compare that to the information depth (aorund 50
microns, if I understand correctly what the sample is) of Eu around
its L3 edge.

Judging from amount the data changes when applying the correction, it
would seem as if your film is quite thin.  The XANES changes very
little and the EXAFS not at all when the correction is applied.

Are you fretting about the change in edge jump?  If so, why?   

Edge jump is, in most cases, a pretty arbitrary number for a
fluorescence measurement [1].  If you change the gain on I0 by one
order of magnitude, the edge jump changes by a factor of 10.  If you
change the gasses in IO or change the gains on your energy
discriminating detectors, the edge jump again changes.  This is in
contrast to a transmission experiment where, by virtue of the natural
log, the edge jump tells you something about the physical thickness of
the absorbing part of the sample.

By applying the self-absorption correction, you did some kind of
linear transform to the data, changing the absolute scale of the
numbers in some way.  Comparing the normalized data (remember, the
normalization algorithm is used explicitly to remove all of these
arbitrary scling factors from the data) shows that the correction did
very little to your data.

Perhaps I don't quite understand what your concern is with these data,
but I doubt that self-absorption should be among them.

Cheers,
B

[1] Here's a counter example http://dx.doi.org/10.1021/ac060476m, but
note that great care was taken in the work to make the ensemble of
measurements in a consistent fashion.


-- 

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

 National Institute of Standards and Technology
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