Paul, Are you sure that you are using a sensible crystal structure to run feff? 1.7 Angstroms is a suspiciously short Se-O distance. You will notice that your fit is also making DeltaR fairly large -- pushing that distance out to a much more sensible value. Given the very short Se-O distance in the feff calculation, that could account for odd value of E0 that you are getting in your fits. What distance do you expect for the Se-O bond? B On Tuesday 08 June 2010 03:34:55 pm Paul Bingham wrote:
Hi,
I have been struggling with a tricky problem and cannot resolve it - hopefully someone out there can help!
I have collected fluorescence XANES and EXAFS at the Se K-edge and I'm currently trying to fit the EXAFS (see attached files). The XANES agrees very well with a number of my standards and shows that the vast majority, if not all, of the Se is present in this material (an amorphous oxide-based system) as Se4+.
I have tried to fit the EXAFS using a single Se-O path from a range of cif files (the simplest being SeO2). Other ions than O present incorrect bond lengths and I am convinced the first shell is oxygen. I can obtain a decent fit in terms of most parameters, as long as I disregard the consistently high value of Enot. Guidelines indicate that Enot should be less than about 5(ish), but I consistently get values of 10-12 when I try to fit my spectra, of which one is present in the attached files.
I have checked my background subtraction and tried as many different options, changes and tweaks that I know or can find suggested (too many to list here) but I cannot obtain a fit with Enot below about 10 and so I ask my colleagues out there who are more experienced than I with EXAFS - can anyone shed any light on (a) the potential cause/s and (b) any potential solution/s?
Thanks in advance for your time.
Best Regards
Paul Bingham
-- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/