From what you have said, it would seem that you are on the right
Hi Bridget, Welcome to the list! track. Without knowing what CuWO4 looks like or how it behaves, I have a few suggestions that you might want to think about
I started off with CuWO4 as the model compound and constrained the fitting parameters: - take all 1st oxygen shells (x6 at slightly different distances) and give them all amp1, dr1*reff, enot and ss1 - take all 1st copper shells (x2) and give them both amp2, dr2*reff, enot and ss2 - take all 2nd oxygen shells (x6), amp3, dr3*reff, enot, ss3 - take all 1st tungsten shells (x6), amp4, dr4*reff, enot, ss4
This gave me 13 parameters with 14.8 independent points.
I think you have not quite explained what your constraints are. 4 shells times 4 parameters is 16 variables, not 13. It is hard to justify independent amplitude parameters for the different shells of a crystal. If you have any confidence at all in the crystallography, then it is hard to justify floating the amplitude for each shell. More appropriate would be to constrain each shell to have the coordination indicated by the crystallography and float a single S02 parameter. Similarly, it is hard to justify a priori the use of an independent enot for each shell. If we have faith in the theory, then you should only need one enot parameter to line up the energy grid of the data with the energy grid of the theory. Those two suggestions reduce the number of parameters from 16 to 10. You probably need to float sigma^2 independently for each shell, but you might be able to constrain the deltaR parameters in some way. I don't know what the structure of CuWO4 is, but there might be some clever constraint you could use to reduce the number of deltaR's. That might give you enough freedom to try to determine more about the first oxygen shell than just an overall deltaR. Who knows? It's a shame that you data range only goes out to 9 inverse Angstroms.
(Other factors: fitted in kw=2&3, k-range 2-9, used Athena with default background subtraction parameters as the chi(k) from that seemed sensible. The data were collected at SSRL, beamline 11-2, in transmission.)
It is good that you are using the multiple-k-weight trick. You might consider including k-weight of 1 as well. Using 1 and 3 emphasizes the high and low bits quite differently. Here's some reading on the topic: http://www.google.com/search?q=site:millenia.cars.aps.anl.gov+correlation As you say, getting both edges would be a boon, particularly given that your data range is so limited. Fortunately, it's *only* a 14 hour plane ride from New Zealand to California, so measuring more data will be no trouble at all ;-) HTH, B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/