[Ifeffit] Question about shift in E0
Brandon Reese
bjreese at gmail.com
Wed Jun 8 02:03:46 CDT 2011
Chris - I have done it both ways, but I generally leaned to constraining
them all to some reasonable value.
Thanks
Brandon
On Mon, Jun 6, 2011 at 7:55 PM, Christopher Patridge
<patridge at buffalo.edu>wrote:
> Brandon,
>
> I have a question about your method. You mention that you pick E0 based on
> the 1st peak of the derivative for each sample then extract chi(k). Have
> you tried constraining E0 across the set? I have thought that in order to
> make a good comparison between samples in a set, a common E0 should be
> chosen in order to replicate closely the background function across the
> whole set. Constraining may also provide a trend in the fitting.
>
> Buena salud,
>
> Chris
>
> On Jun 6, 2011, at 6:46 PM, Brandon Reese <bjreese at gmail.com> wrote:
>
> 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>
> 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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