[Ifeffit] Aligning EXAFS E0 to Theory

Edmund Welter edmund.welter at desy.de
Thu Jun 5 02:30:51 CDT 2014

Dear Carter,

the question is: Why? Why would you want to align your experimental
spectra to a theoretical standard? For a number of reasons (the somewhat
arbitrary definition of E0, the calibration of your monochromator, the
way FEFF calculates the theoretical spectra...) delta E0 is almost never
0. Some(!) eV are normal and should just be repoerted in your
publication. If delta E0 is larger than ~10 eV you should start to find
a possible explanation for this behaviour. If you are confident about
the calibration of your DCM it is usually indicating a problem with your
fitting model. If you align your exp. spectra to the (so far...) best
fit you are just obscuring these problems...

Best regards

On 04.06.2014 22:18, Carter Abney wrote:
> Hello, Everyone --
> This is my first time trying to work through EXAFS analysis of
> experimental data, and I've got what (I think) should be a fairly
> straightforward question.  I'm trying to minimize my delta E0 by
> aligning the EXAFS data with my best fit (i.e. - align the k-grid of
> the data with the k-grid of the theory to obtain a small delta E0). 
> However, I have been unsuccessful in "reading the theoretical chi(k)
> spectrum into Athena
> <http://www.mail-archive.com/ifeffit%40millenia.cars.aps.anl.gov/msg02494.html>"
> to accomplish this.    
> I assume the crux of the problem is the format in which I am exporting
> my theoretical chi(k) spectrum from Artemis.  The only way I have been
> able to export the data with Artemis so Athena can import it is to
> click the "Save next plot to a file" button, which then allows me to
> save the data as a text file.  I know I need chi(k) data that is not
> k-weighted
> <http://www.mail-archive.com/ifeffit%40millenia.cars.aps.anl.gov/msg01623.html>,
> so I plot the best fit with k-weight of 0 in k space with deltaE0
> equal to 0.  Considering the problems I've had, I'm assuming somewhere
> in this process is where I'm going wrong. 
> When I import this data, I have four columns: k, experimental data,
> fit, window.  Since k is the wavenumber of the photoelectron, I choose
> that as "Energy."  Then I select column 3, the best fit, as
> "Numerator," and change the data type to chi(k).  I then import the
> data.  I've attached two pngs to this e-mail which, assuming they come
> through, should display the Athena data import window and the
> corresponding plot of chi(k).  When I plot this data in k-space or
> R-space using Artemis, the resulting plots look appropriate.
> When I try to align the experimental EXAFS data in Athena, I know it
> aligns the first derivative of E for the selected data to the first
> derivative of E for the standard.  I want my best fit to be the
> standard to which the data is aligned.  However, I cannot plot chi(k)
> data in E, which causes me to believe I shouldn't be able to align
> data to the first derivative of E.  True enough, when I try to auto
> align, I get impossible values for E0 adjustment.
> Clearly I'm doing something wrong -- possibly even several somethings
> wrong.  Any guidance would be greatly appreciated.
> Thanks,
> - Carter
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