[Ifeffit] Question about shift in E0

Tue Jun 7 13:09:29 CDT 2011

Sorry Brandon, I see now everything I was asking for was I'm the first few
emails. Enot is a local atomic shift in E0 as explained before. (sorry I'm
not referencing, I only have my phone available this week) What I understand
is you have set E0 to 27944 and 27950 then get Enot of -7 and -1
respectively. This leads to a local edge in the EXAFS equation of 27951 in
both cases which is what I was getting at previously. But you say ~ so that
means you have an real difference between the two, and the opposite of what
I was thinking. Sorry if I caused any confusion.

Andy Korinda
On Jun 7, 2011 6:27 AM, "Brandon Reese" <bjreese at gmail.com> wrote:
> I was using feff6. Does your comment mean that when fitting Enot with
Feff6,
> the results should be considered unreliable? I assumed the 0.5 eV shift in
> the mu spectra was small enough that the spectrum could be considered
> essentially at the same energy. That is why I was surprised with the
> (somewhat) larger shift in Artemis.
>
> Andy, are you asking what the difference is between E0-Enot for the two
> samples? If I picked E0 to be 27944 for all the samples( a round number
near
> the 1st derv peak), Artemis would fit Enot to be ~5.5 for one group of
> samples and ~7 for the other group. Approximately the same offset was
> present if I let the parameters get chosen automatically, regardless of
the
> absolute E0 (within reason). Is this what you were asking, or did I
> misunderstand?
>
> Brandon
>
> Andy, E0-Enot came out to be 2794
> 2011/6/7 "Dr. Dariusz A. Zając" <kicaj at ifj.edu.pl>
>
>> **
>> 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
>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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>>
>>
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