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 -- Bruce Ravel ----------------------------------- ravel@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/