[Ifeffit] Question about shift in E0

Tue Jun 7 02:45:12 CDT 2011

Hi Brandon,
that was a reason why I wanted to clarify which parameters you use.
E0 (as a Fermi level) in Athena can be taken (but do not must) as a 
valence state of measured ion for K edges, whereas in Artemis delE0 can 
correspond to the potential of the one of nearest atoms to the measured 
one. If you talk about Artemis and calculations, would be also helpful 
to know the version of feff. It is known that for version up to 6. can 
be problem with fitting values of Enot. And of course Matt answered you 
more clearly...
The difference of 0.5eV between spectra of reference sample can be 
simply related with the energy resolution of the beam (beamline).

Maybe this will helps you more:
http://cars9.uchicago.edu/pipermail/ifeffit/2004-June/005607.html
http://cars9.uchicago.edu/pipermail/ifeffit/2006-February/006811.html

kicaj

W dniu 11-06-07 00:46, Brandon Reese pisze:
> Kicaj and Matt,
>
> Thanks for the replies. It is certainly reasonable to ask about the 
> experimental setup, that seems like a good first place to look for 
> oddities. Sorry about using E0 in two different contexts (Athena and 
> Artemis).  I'll switch to using E0 for the chosen parameter in 
> Athena's background subtraction and Enot for the guess parameter in 
> Artemis.
>
> I aligned the scans in Athena using the reference foil absorption 
> edge. I ended up shifting the scans in energy by ~1 eV or less based 
> on the reference foil. I then set the E0 parameter in Athena to the 
> peak of the first derivative. This value varied by about 0.5 eV (or 
> less) between the different samples. This is what I considered when I 
> made the statement about not seeing the oxidation change in the XANES. 
> There is a shift of about 4 eV compared to the reference foil, which I 
> would expect because I am looking at oxides.
>
> I then extracted the chi(k) into Artemis, and used the same Feff 
> calculations/paths for each sample. In the Artemis fits I am seeing 
> Enot shifts of 1-2 eV, which is a fair bit larger than the error bars. 
> If I chose E0 at peak of the first derivative the Enot came out to be 
> ~7 eV. Since this seemed a little on the big side, I also tried 
> setting E0 to be the top of the white line, in that case Enot came out 
> to be ~1 eV. The Enot shift between samples was about the same either 
> way. Could an argument be made that there is a small shift in the 
> overall oxidation state of the films, possibly contributing to the 
> changes in the conductivity changes in the films? Does anyone have any 
> references about using Enot shifts in this way?
>
> Scott - The amplitudes of the XANES features are very close. in the 
> normalized spectrum the difference practically 0, and in the 
> derivative the amplitudes are ~5% off.  There is bit larger of a 
> difference in amplitude between the samples (in fluorescence) and a 
> pure In2O3 powder (in transmission) of ~10% in the first derivative. 
> The samples are sputtered from oxide sources, so the presence of 
> metallic In should be pretty small. I am interested about your comment 
> on the relative sizes of the first derivative peaks. Comparing my 
> samples to the foil the oxide peak amplitude is bigger than the metal 
> peak by 10% or so. Could this be due to the differences between 
> fluorescence and transmission or not optimizing the experiment to 
> measure the foil?  Or could it be due to something else more heinous?
>
> I noticed on individual fits that the dR for the first shell came out 
> nearly the same (2.162 +/- .007 vs. 2.164) on the two samples, while 
> the Enot's had the 1-2 eV shift between the samples. I tried to fit 
> the two types of samples simultaneously while constraining the dR's 
> (and a few 2nd shell parameters) to be equal to each other and letting 
> the Enot's float. The relative Enot values came out close to what they 
> were before. If I let dR values fit independantly, there was no real 
> change present. In other words the multiple data set fit just made my 
> EB's a bit smaller, but the relative shifts stayed about the same.
>
> Would there be some reasonable way in a multi-data set fit to 
> constrain the Enot's?
>
> Brandon
>
> On Mon, Jun 6, 2011 at 9:04 AM, Matt Newville 
> <newville at cars.uchicago.edu <mailto:newville at cars.uchicago.edu>> wrote:
>
>     HI Brandon,
>
>     If I understand right (and to echo Darius's questions), it seems like
>     you measure samples with a metal reference, and aligned the spectra in
>     Athena so that the references matched.  That's a fine way to go.   I
>     would ask: how big were the needed energy shifts?
>     Are the experimental mu(E) spectra aligned well at this point?   If
>     you're studying metal oxides with different oxygen content, you might
>     very well see oxidation in the XANES.   If I understand correctly,
>     you're saying you don't see this.
>
>     Then, you extracted the chi(k) from the aligned spectra.  How much did
>     E0 vary for the shifted spectra in this background-subtraction step?
>
>     Then, you pulled these chi(k) into Artemis, and see different E0
>     shifts in the fits.  This E0 is a little different, in that it is the
>     E0 shift applied to the Feff calculation to match the experimental
>     spectra.  If you use different paths or different calculations, such
>     E0 shifts might happen, and wouldn't be highly meaningful.
>
>     I know that's not a complete answer, but hopefully that and Darius's
>     questions will help,
>
>     --Matt
>
>     PS on  Q2: 1/ (kR)^2 vs 1 / kR^2
>
>     This is probably either a typo, or a different convention based on the
>     definition of f(k) as the electron scattering amplitude.   In the Feff
>     world, it should be 1/kR^2, but using 1/(kR)^2 would just map f(k) to
>     k*f(k).
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