[Ifeffit] Athena: problems with LCF

[Bruce Ravel]

Nina,

I agree with Stefan.  Your data are too short for adequate
normalization and your criterion for choosing the post-edge line is
ill-conceived.  Normally, we choose a region well beyond the edge as
the basis for the post-edge line.  This is because the differences
between the bare atom and the atom in a solid are small well above the
edge.  Thus regressing a line to that region well above the edge is
likely to provide a consistent normalization which is insensitive to
chemistry.

Your criterion, on the other hand, is highly sensitive to chemistry.
You chose a post-edge line that follows the slope of half of one
particular oscillation in the region near the edge where the
sample-to-sample variation due to chemistry are as large as they get.
In my opinion, your method is guaranteed not to work.

The only solution that I can recommend is that you abandon this data
set and return to the synchrotron to remeasure your data.  Take the
time to measure at least 100 or 150 volts before the edge and many
hundreds of volts after the edge.  If you need to save time, you can
measure sparsely in the pre- and post-edge regions -- but you have to
measure something out there if you want to normalize your data in a
way that is defensible.

As for your question about the algorithm, I answered that the first
time you asked.  If you have a specific question, please ask and I
will happily answer it.  But please do not repeatedly ask the same
vague, open-ended question.  You won't get a better answer by simply
re-asking the same question.

B


On Monday, August 15, 2011 05:56:18 am Stefan Mangold wrote:
> Dear Nina,
> 
> you used for this linear combinations very, very short scans. In my humble
> opinion, the scans are much to short to get a reasonable background
> correction and normalization.
> 
> That is not a problem of athena or other solutions, there are just not
> enough data points for a reasonable background fit.
> 
> Best regards
> 
> Stefan Mangold
> 
> Am 15.08.2011 um 11:35 schrieb Nina Siebers:
> > Dear All,
> > 
> > I acquired Cd L3-edge spectra of some binary and ternary mixtures in
> > varying proportions and for the individual components. The mixtures were
> > created on Cd-mass basis. Then, I tried to fit the reference spectra to
> > the spectra of the mixtures using linear combination fitting of Athena
> > to get their abundance. However, the results were disappointing despite
> > all spectra were carefully energy calibrated and normalized, so I
> > decided to create simple mathematical binary and ternary mixtures by
> > summing up the spectra of the individual reference spectra. After that I
> > did an edge-step normalization in excel and imported the normalized
> > calculated mixtures into Athena. Then, I tried the fitting again to
> > exclude mixing-failures and check sensitivity of LCF with the idealized
> > spectra. Even though the results of the LCF of the mathematical mixtures
> > were better compared to the real mixtures, LCF was also not able to
> > reliable deconvolute these spectra into the individual reference
> > spectra.
> > 
> > Does anybody have an explanation for that? It would be nice if somebody
> > could give me information about the mathematical fitting algorithm
> > implemented in Athena.
> > 
> > Attached is a data file of three mixtures (two ternary and one binary
> > mixture) including the mathematical mixture created in excel (named
> > calculated at the end). Mixing ratios are named 1to1to1 (meaning 1:1:1
> > of the components in the same order). For the 1:1:1 ternary mathematical
> > mixture the deconvolution was very good, but the others need
> > improvement.
> > 
> > I hope I made my problem clear this time.
> > 
> > Thanks a lot!
> > Wishes,
> > Nina
> > 
> > 
> > <Mixtures.prj>_______________________________________________
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> > Ifeffit at millenia.cars.aps.anl.gov
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> 
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-- 

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

 National Institute of Standards and Technology
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