On 07/20/2016 06:59 PM, Neil M Schweitzer wrote:
In terms of the DWF’s in general, what value is considered too high? I know the DWF’s have a component that relates to temperature induced disorder in the scattering shell and a component that relates to physical disorder in the scattering shell, but what value would be considered too big for a sample measured at room temperature. I have seen values as high as 0.03 and 0.04 in presentations (sorry, no references) but these seems too large to me. At some point, if the sample is disordered enough, it seems like EXAFS is no longer an appropriate characterization tool to use. What value of DWF would that represent (for a sample measured at room temperature)?
This one is a bit easier than your first question, so I'll take a stab as I am drinking my morning coffee.
As you have certainly noticed, defensible sigma^2 values tend to be 0.00something. Values for sigma^2 that are 0.0something tend to be very unrobust parameters in the sense that they cause so much attenuation that they more-or-less serve to remove the scattering path from the fit. It is likely that the uncertainty will be similarly large in a fit like that.
One point I often try to get across when answering questions here is that often things that are presented as problems are actually useful information. That is, a sigma^2 of 0.0something is trying to tell you that the Fourier components represented by the path are not represented (or represented very weakly) in the actual data. A big sigma^2 is Artemis' way of suggesting one of your assumptions about the structural model might not be quite right.
In that case, it is likely that the fit will not change -- and might improve by virtue of reducing the count of guess parameters by one -- if you remove the path and remove the parameter.
To look at it another way: a result of "it's too disordered to measure" *is* a result. It may not be quantitative. It may not be what the boss is looking for. But it's honest and it is a result.
HTH, B -- Bruce Ravel ------------------------------------ bra...@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 743, Room 114 Upton NY, 11973 Homepage: http://bruceravel.github.io/home/ Software: https://github.com/bruceravel Demeter: http://bruceravel.github.io/demeter/ _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit
