[Ifeffit] determining reasonable fitting parameters

[Matthew Marcus]

Did you do  any correction for the multi-electron bumps?  Here's a note 
I made for myself when doing CeO2 EXAFS:

Problem:  Ce L3-edge spectra are polluted by multi-electron peaks (MEP) 
which appear as
bumps right in the important range and can corrupt 1st-shell analysis,
Gomilšek, et, al (Acta Chim. Slov. 51,23(2004)) give an estimate for the 
MEP feature
for 3+ and 4+ states:

For 3+, a single Lorentzian peak, height 2.5% of edge jump, FWHM 17eV, 
position 5852eV.
For 4+, two peaks:
Pos(eV)	Height	FWHM(eV)
5857	2.7%	12
5877	1.1%	 7

Their energy calibration is with respect to Cr (5989eV),  but it seems 
from their Figure 1 that it's
the same as mine,  The first peak of CeO2 appears at 5730eV, as I assume.

Thus, the procedure might be:
Reduce the file to .e (pre-edge subtracted, post-edge normalized).
Fit this to 3+ and 4+ references to get the 3+ fraction x3.
Subtract from the .e a contribution x3*(3+ MEP)+(1-x3)*(4+ MEP).
Convert to k-space and proceed as normal.

Here, ".e" files are pre-edge subtracted, post-edge normalized XANES.  I 
wrote  a program that fits an unknown .e to a sum of CeO2 and Ce3+ refs 
to get the Ce4+ fraction, then subtracts the peaks.  This results in 
something unpolluted by MEP.
	mam

On 7/20/2016 3:59 PM, Neil M Schweitzer wrote:
> Being somewhere between a novice and an advanced user of XAS, I have two
> “philosophical” questions about EXAFS fitting (I hope this is the
> correct forum to ask them!). I also have a bug to report. I have
> included a project file of a CeO2 powder for those interested.
>
>
>
> 1)      Generally, I have developed my own methodology for EXAFS fitting
> which I think generates ok fits, at least from the standpoint that the
> red line seems to follow the blue line pretty closely. My question
> relates to interpreting the statistics of the fit. Generally, I am
> studying catalyst (i.e. very small metal or oxide nanoclusters supported
> on a different oxide phase). Due to the size of the clusters of
> interest, I wouldn’t expect them to resemble a bulk crystal, and
> therefore it is difficult for me to reduce the number of independent
> variables using know crystal structures. For example in this reference
> material, I am giving every single scattering path its own delR and ss,
> and using a single delE and SO2 for all the paths since they come from
> the same feff calculation. Once I am through with my own methodology, I
> can usually get a close fit even while making every variable a guess
> (refer to fit 17 in the project file). Admittedly, the reduced chi is
> rather large (5262) and the error of each of the DWF’s is the same
> magnitude as the value itself, which is troubling. However, if I use the
> best fit values of fit 17 and set all of the values except for the 3
> DWF’s and generate a new fit, all of the statistics significantly
> improve even though the values of the DWF’s did not change at all (Fit
> 19). That is, merely setting the other variables improved the fit (the
> reduced chi square is now 2951) and improved the error of the guess
> variables (the errors of the DWF’s are cut close to half). Why is the
> fit better since none of the values actually changed, and why do the
> errors improve? How should I be reporting the statistics (i.e. the
> errors) in a publication? Should I report the fit 17 errors or the fit
> 19 errors for the DWF’s?
>
> 2)      In terms of the DWF’s in general, what value is considered too
> high? I know the DWF’s have a component that relates to temperature
> induced disorder in the scattering shell and a component that relates to
> physical disorder in the scattering shell, but what value would be
> considered too big for a sample measured at room temperature. I have
> seen values as high as 0.03 and 0.04 in presentations (sorry, no
> references) but these seems too large to me. At some point, if the
> sample is disordered enough, it seems like EXAFS is no longer an
> appropriate characterization tool to use. What value of DWF would that
> represent (for a sample measured at room temperature)?
>
> 3)      I found a bug in the log files in the history window. I am
> running Demeter 0.9.21 on Windows 7. When I generate a new fit, if I
> change the k-range of the fit, the k-range of previous fits in the
> history window will also change to the current range (e.g. fit 19 was
> run from 2.1-9.3, as shown in the data window. Fits 14-16 were run in
> smaller ranges). What’s even stranger, is if I go to a very old fit
> (i.e. fit 1 or 2 in the project file) and then go back to the new fit,
> the log file will report the k-range from the earlier fit, not the
> current fit. Obviously, this is making it very difficult to keep track
> of fits generated with different k-ranges. I have not tested to see if
> the R-range behaves similarly.
>
>
>
> Neil
>
>
>
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