On Tuesday 26 September 2006 15:45, Scott Calvin wrote:
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.
I'd like to expand on this a bit. At the level of Artemis, users are often concerned that the background corefinement is somehow a cheat. It isn't. (Well, not any more than anything else Artemis does -- but that's a separate story ;-) The way to think about it is in terms of Fourier components. Artemis uses Feff calculations to fit the Fourier components between Rmin and Rmax and, if the background corefinement is turned on, it uses a spline to fit the components between 0 and Rmin. Considered in that light, background corefinement is very handy indeed. If you set Rmin to be the same as Rbkg, this gives you a chance to evaluate the correlations between your background removal and the parameters you are actually interested in. As Scott says, this can give you a certain confidence in your results, or, I suppose, encourage you to go back to Athena and do the background removal differently. So, the background corefinement is the use of splines to optimize low-frequency Fourier components. In the Autobk algorithm, we use splines to optimize low frequency Fourier components. It is, therefore, reasonable to consider background corefinement as as additional round of playing with Autobk while simultaneously trying to figure out something about your chi(k) data. 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/