Hi Nicholas and all,


by coincidence, I experienced the same issue yesterday.

It looks like the  problem starts in the Athena project - if you import chi(k) data without background function, the program still somehow creates a false background function, which might become very high for higher k range. If you then import the Athena project data into Artemis, the fit doesn't seem to work properly for k ranges with a too high background value.

The simplest workaround is to directly import the chi(k) data into Artemis - or alternatively, save the data from Athena as chi(k), and use these chik data to import into Artemis. Don't import the Athena project data directly.

I hope I could help you!


Best,

Aaron

Von: Ifeffit <ifeffit-bounces@millenia.cars.aps.anl.gov> im Auftrag von Matt Newville <newville@cars.uchicago.edu>
Gesendet: Donnerstag, 8. Oktober 2020 05:32:05
An: XAFS Analysis using Ifeffit
Betreff: Re: [Ifeffit] Ifeffit Question
 
Hi Nicholas, 

On Wed, Oct 7, 2020 at 10:02 PM Nicholas Weingartz <nicholasweingartz2024@u.northwestern.edu> wrote:
Hello List,

I'm having a strange problem with my EXAFS fits in Artemis in which they seem to fly off at higher k.  I was having trouble with background fitting a particularly messy data set and I wanted more control over this so I used the program PySpline to fit the background after I did the rest of the preprocessing in Athena.  I then imported the data into Athena and loaded the project into Artemis.  Interestingly, when I only include a single path in the fit and plot both the single path and the overall fit, the single path does not fly off but the overall fit does.  I have attached an image of the issue.  When I use the spline fitting built into athena, I do not encounter this issue.  Has anyone ever experienced this issue before?


Sorry, but I'm not sure I understand what the issue is from that plot.  You said "the single path does not fly off but the overall fit does" but I sort of don't see that in the image you show.   Is something flying off there?

The k-range you show appears to be pretty limited to me, roughly k=3 to 9 Ang^-1.   For a single path, I can certainly believe that actually is the "best fit" even if it not "very good".  But it's hard to say from only one view of the fit.

If you suspect (or well, "made") the splines be different, I might suggest looking at the mu0(E) arrays themselves, or perhaps plotting the k^2*chi(k) for the two background subtraction.  


--Matt

Thanks for your help!

Nicholas Weingartz

Ph.D Student
Department of Chemistry
Northwestern University

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