From: Ravel, Bruce via Ifeffit <ifeffit@millenia.cars.aps.anl.gov>
Date: Friday, July 25, 2025 at 12:49 PM
To: ifeffit@millenia.cars.aps.anl.gov <ifeffit@millenia.cars.aps.anl.gov>
Cc: Ravel, Bruce <bravel@bnl.gov>
Subject: [Ifeffit] Re: Problems with linear combination fitting S-XANES data in Athena
I should say also that Larch and Larix probably have smarter ways of
choosing initial parameters for the data reduction. Athena's initial
guesses are pretty good for EXAFS data, but often do funny things (like
you saw here) for data measured over a shorter energy range.
B
On 7/25/25 13:07, Ravel, Bruce wrote:
> Investigating further, it seems that the problem might instead be that
> you are putting too much faith in the default values for the various
> parameters used to process the data. I mean this in the sense of the
> third paragraph of this page:
>
> https://urldefense.us/v3/__https://bruceravel.github.io/demeter/documents/Athena/bkg/index.html__;!!G_uCfscf7eWS!cTLg9RWK1TAZRd267t_3ddjg2A3zeSjd5FNxTsRIYhs1g4IzWpbezllxLHh3QPr6XT9wd06Z_BFEk-k5CSK2PXgjs08iPw$
>
> In short, you need to be more mindful of the values of the various
> processing parameters. I am skeptical of the value of E0. The values
> of the normalization parameters are certainly incorrect for this
> measurement, as is clear from the green and purple lines. And when I
> click the orange normalization button, sure enough ... it plots the data
> inverted.
>
> Playing around just a bit with the parameters explained here:
>
> https://urldefense.us/v3/__https://bruceravel.github.io/demeter/documents/Athena/bkg/norm.html__;!!G_uCfscf7eWS!cTLg9RWK1TAZRd267t_3ddjg2A3zeSjd5FNxTsRIYhs1g4IzWpbezllxLHh3QPr6XT9wd06Z_BFEk-k5CSK2PXif0Epc3w$
>
> and setting them to sensible values for your data completely fixes the
> problem.
>
> Professorial advice: don't just trust things, make sure you understand
> what is happening with your data.
>
> B
>
> On 7/24/25 16:28, Muretta, Julie via Ifeffit wrote:
>> Hello, First of all, thank you all so very much for offering this
>> platform to address XAS data processing!! It is wonderful to know it is
>> available. I am having a problem using Athena "Linear Combination
>> Fitting" with x-ray fluorescence
>> ZjQcmQRYFpfptBannerStart
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>> Hello,
>>
>> First of all, thank you all so very much for offering this platform to
>> address XAS data processing!! It is wonderful to know it is available.
>>
>> I am having a problem using Athena "Linear Combination Fitting" with x-
>> ray fluorescence data from SSRL. Last year, we collected sulfur K-edge
>> XANES data at SSRL, beamline 4-3 and processed them using SixPack
>> (averaging 5 sweeps and normalization) and then used Athena to conduct
>> linear combination fitting of the XANES region. It worked great!
>>
>> This year's data are presenting a problem when I get to the LCF step,
>> however. When I import the sample data .mu file into Athena and plot it
>> by pressing the orange "E" button, the normalized spectrum looks like it
>> should (albeit noisy and sometimes not the best data, but that is not
>> Athena's fault). When I attempt to apply the Linear Combination Fitting
>> algorithm, fitting a number of spectra from reference samples also
>> collected _last year and this year_, Athena inverts the sample.mu
>> spectrum (between -1 and 0 rather than 0 and 1). It does not invert old
>> or new reference sample spectra. When I try this with old data, using
>> old and new reference sample spectra, I do not have this problem. With
>> the inverted sample spectrum, it often does not find a fit, but
>> sometimes it does.
>>
>> Attached is an example Athena project file, the .mu file, and a plot of
>> the output of a test fit, for your reference.
>>
>> I am hoping someone can please help me figure out why this is happening.
>> I cannot see anything wrong, based on my limited experience, but
>> hopefully it is something I have done wrong that can be easily corrected.
>>
>> Thank you in advance,
>> Julie
>>
>> *Julie Muretta, Ph.D.*
>>
>> /Materials Testing Laboratory Manager/
>>
>> /Center for Advanced Materials Processing (CAMP)/
>>
>> _(406) 496-4808_ | jmuretta@mtech.edu
>>
>> _Montana Technological University <https://urldefense.com/v3/__https://
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>> cA8TPa7rlyGu5Vmwp--j7WUFcufVPj1XStOTpvP1kNOf8HO7A1eqt_eFg-
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>>
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--
Bruce Ravel ------------------------------------ bravel@bnl.gov
National Institute of Standards and Technology
Synchrotron Science Group at NSLS-II
Lead Beamline Scientist, 6BM (BMM)
Building 743, Room 114
Upton NY, 11973
Homepage: https://urldefense.us/v3/__http://bruceravel.github.io/home/__;!!G_uCfscf7eWS!cTLg9RWK1TAZRd267t_3ddjg2A3zeSjd5FNxTsRIYhs1g4IzWpbezllxLHh3QPr6XT9wd06Z_BFEk-k5CSK2PXguwFhVWg$
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