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?
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).
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?
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).