"Can ss be negative," as in, can a fit with a negative best-fit value for ss be considered a valid fit? That's really what you're asking, I think, and the answer is that it could be, depending on what you are trying to claim. Since the uncertainty in your case is quite large, it's certainly possible that your fit is consistent with believable values. But it also means that your fit gives you very little idea of what ss should actually be. The twin facts that the uncertainty is large and that the best-fit value is very close to 0 make the fit less convincing.

The simplest explanation in your case is that Ifeffit is finding a fit with an S02 and a ss that are both a bit low. Since they tend to correlate highly, that's not uncommon. Have you tried fitting using different k-weights, or, better yet, several k-weights simultaneously?

On Jun 28, 2010, at 4:10 PM, Chris Patridge wrote:

Hello all,

I am working on W L3 edge data. W is acting as a substitution dopant in vanadium dioxide at rather low concentration. In a past mailing conversation discussing Feff6 overestimation of E0 for heavier elements it was mentioned that the E0 could be past the rising edge due to the white line from W data. Well using this comment I aligned data using the theory method well explained by Shelly Kelly SnO2 example. Literature suggests W approximates WO2 cubic structure locally instead of the VO2 unit structure. Then fitting the first oxygen coordination shell paths which are well isolated from the other paths, it gives reasonable values for amplitude and enot of 0.77 (0.17) and -1.14 (3.06) respectively. delr is -0.067 (0.028) and then ss comes out to -0.00036 (0.00414). Can ss be negative if the uncertainty brings it above 0?

Thank you all,

Chris Patridge
PhD Candidate
Department of Chemistry NSC 405
SUNY Buffalo
315-529-0501
_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit