[Ifeffit] Another stab at the data

Dave Baker dabaker at unity.ncsu.edu
Wed Oct 29 12:15:26 CST 2003


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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North Carolina State University
Dept. of Physics
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email: dabaker at unity.ncsu.edu
 
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On Tuesday 28 October 2003 11:00 pm, Dave Baker wrote:
> 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
 
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