Bruce,
Thanks for the advice. I’ve run through the list, redone
processing and started the analysis with said list in mind, and the most
plausible solution is that I shouldn’t be using a crystalline material
for the FEFF calculation. One interesting thing (at least to my untrained eye) to
note is that the shapes of the first path and the peak that I’m trying to
fit (this is before any attempt at fitting) are nearly identical, but the FEFF
path is shifted about 2~3 angstroms higher than the peak. The only improvement that I could muster
is that the e0 variable has gone down from 80 to 50.
-Dave Baker
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Dept. of Physics
Ph: 919-515-5017
Fax:919-515-7331
email: dabaker@unity.ncsu.edu
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On
>
I am a novice student in the EXAFS world, and I am starting to work my
>
way through Athena and Artemis. Currently, I'm working with amorphous
>
GeTe and the Atoms/FEFF runs are based on the
crystalline data. I have
>
performed a fit in Artemis of the Ge K edge, and my
e0 for three
>
contributing paths is exceedingly large, (-70 ~ -80 volts). Two
>
questions: 1.) What symptom is this an indicator
of, and 2.) What are
>
some possible solutions?
Hi
Dave,
Glad
to see you were able to get started using the software after that
very
short conversation we had at X11A a couple weeks ago.
I
have seen this effect on occassion when, for some
silly reason, I
have
made the Feff calculation using the wrong edge (i.e.
K instead of
L3 or vice versa).
Another
possible indication of an absurdly large e0 shift might be if
the
distances between atoms in the feff calculation are
very different
than
the distances between atoms in the real material. That is, if
the
material used in the feff calculation is a very poor
approximation
of
the real material.
Another
possibility that occurs to me from the way you phrased your
question is
that you are allowing e0 to float independently for each
path. That's almost certainly not physically
supportable. At least
as a
first stab at theproblem, you should use the same e0
for each
path.
One
thing you might try is to not allow everything to vary in the
fit. Try fixing e0 to be something sensible --
like 0.0. Vary all
the
rest of the parameters to get a sense of how the parameters behave
and
how well they describe the data.
This sort of manual examination
of
the parameter space might help give you a clue of why you are
getting
such an odd result.
HTH,
B