[Ifeffit] New Method for Normalization?

Matt Newville newville at cars.uchicago.edu
Thu Sep 29 17:00:12 CDT 2005

Hi Dave,

Thanks for bringing this up.  I've talked with Tsu-Chien about
the MBACK procedure discussed in the JSR article, and about
whether he thought it would be worth adding to Ifeffit.  I
interpreted our conversation to be that it wasn't a very high
priority for either of us two to do the implementation soon.
(I don't recall if Tsu-Chien is on this list, but I'd be happy 
to be corrected).

I do not mean to say that I think it should not be done.  I do
think the Cromer-Libermann background (bkg_cl) command in
ifeffit is sort of close, but it does ignore any drifts in
instrumentation response.  And Ifeffit always does a linear
pre-edge.  I've come to understand (especially from people with
very dilute samples and solid state detectors) that this can be
a noticeable problem.

I think including the MBACK approach is a good idea, and would
not want to discourage anyone from making that happen.  But
having an option for a Victoreen pre-edge, say, is probably even
more important.  I think that coupling a better pre-edge
function with the Cromer-Libermann background might get 3/4 of
the way to the MBACK approach.

I'd love to hear other opinions on this....


On Thu, 29 Sep 2005, Barton, David (DG) wrote:

> XANES Aficionados,
> I know most users on this list are hard-core EXAFS analysts,
> but some of us use XANES analysis to decipher mixed phases or
> use it to get structural clues when data quality isn't
> sufficient for EXAFS analysis.  Anyway, I have a question for
> those interested in quantitative analysis of XANES or those
> that are willing to offer an opinion on methods to normalize
> raw spectra to get reliable, repeatable comparisons of
> near-edge features.
> Has anyone considered using the methods of Penner-Hahn to
> normalize their data to get reliable intensities of near-edge
> features?  The reference to their method of normalization is
> below:
> J. Synchrotron Rad. (2005). 12, 506-510    [doi:10.1107/S0909049504034193]
> "A method for normalization of X-ray absorption spectra"
> T.-C. Weng, G. S. Waldo and J. E. Penner-Hahn
> It is my understanding the Ifeffit uses a linear function to
> remove the pre-edge and a quadratic for the post-edge and that
> the Athena's flatten is a subtraction of these functions.  
> This is an excellent method under most scenarios since the
> background is usually fairly smooth and can be well
> approximated by a quadratic equation.  Personally, I have had
> only a few rare cases where the quadratic function on the
> post-edge was not sufficient to reliably normalize the data
> and in those cases a third-order polynomial was sufficient.  
> Does anyone else have an opinion on using alternative
> normalization routines?
> Dave  
> _________________________________ 
>   David Barton
>   The Dow Chemical Company 
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> Ifeffit at millenia.cars.aps.anl.gov
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