Hi Juan, Excellent questions! At 03:52 PM 9/26/2006, Juan Antonio Maciá Agulló wrote:
I read in Artemis manual that a good fit must be consistent for different k-weights. When two fits (one with k1 and other with k3) are consistent? I mean, which is maximum allowed difference, 1%, 5%...between these two fits?
Ideally, the fitted parameters should have uncertainty ranges that overlap. For example, one bond length might be 2.03 +/- 0.04 angstroms, and the equivalent bond length using a different k-weight might be 2.09 +/- 0.03 angstroms.
I have done different fits changing fit k-weights and k-weights in the plotting options. Usually, for k-weights = 1 or 2, I have obtained very strange spectra (why?) and then I have decided to change only k-weight in the plotting options to 3. After that, spectra looked more like before. I am not sure if this gimmick is correct. I mean, can I use different k-weights for fit and for plotting? I have obtained "consistent" fits with fit k-weights of 1 and 3, and both plotted in k-weight 3, that's ok?
What do you mean, "strange" spectra? They will look different for different k-weights, of course. That's not a problem. In general, it's OK to fit in one k-weight and plot in another, and there are a variety of good reasons for doing that: for example, you're comparing to published data with a certain k-weight.
Sometimes when I have changed fit k-weight from 3 to 1, I have observed that two high correlations (between ss and S02, and delR and delE0) have dissapeared (<0.85). What does it mean? Parameters obtained with fit k-weight = 1 are more reliable?
Umm...not more reliable. Just less correlated. :) You can look at the uncertainties on the parameters to see if that also was reduced when you changed k-weights. Sometimes one k-weight manages to break correlations more effectively than another, but there isn't a simple pattern to it. Different parameters are weighted by different powers of k in the EXAFS equations, and different scattering elements have large amplitudes at different values of k, so the overall effect is sometimes difficult to predict: it's best to just try it and see what happens. Ideally, the different k-weights will yield similar values for the parameters, although the graphs may look quite different.
And another question, I have seen an option to fit background in Artemis, is it obligatory to fit the background to publish EXAFS fits?
No. This is another one of these issues where people can reasonably have different preferences. Personally, I prefer not to use the Artemis background refinement in my published fits, partly because it's one more processing step to explain and justify. But I do use the ability of Artemis to do background refinement to test if my Athena background is OK: if fitted parameters correlated highly with Artemis background parameters, I know there's a problem, and I go back and look at the background subtraction process in Athena again. If they don't correlate highly, then the Artemis background wasn't doing anything except improving the visual aesthetics of the fit anyway, and I turn it back off. --Scott Calvin Sarah Lawrence College