[Ifeffit] Novice seeks advice

Bruce Ravel ravel at phys.washington.edu
Wed Oct 29 07:03:03 CST 2003

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

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.


 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
 Code 6134, Building 3, Room 222
 Naval Research Laboratory                          phone: (1) 202 767 5947
 Washington DC 20375, USA                             fax: (1) 202 767 1697

 NRL Synchrotron Radiation Consortium (NRL-SRC)
 Beamlines X11a, X11b, X23b
 National Synchrotron Light Source
 Brookhaven National Laboratory, Upton, NY 11973

 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/

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