Matt, Outside of the question of fitting the data, can you collect absorption data on the additional 3+ metal you are adding the material?
Chris ******************************************* Dr. Christopher Patridge Assistant Professor Department of Math and Natural Sciences D'youville College Contact: 315-529-0501 On Dec 5, 2013, at 10:11 AM, Bruce Ravel <bra...@bnl.gov> wrote: > > Matt, > > At the risk of coming off sounding a bit mean, I don't think you are > asking a very well-posed question. > > Examining the history of this project, I see that you are fitting in q > space. Like Matt, this is not my favorite choice, but there is > nothing horribly wrong about it, so long as you understand what you > are doing. > > What is problematic is your expectation that, in doing so, you should > be better able to fit a particular feature in k space. If you examine > the data in k and q space, you will see that the act of Fourier > filtering the data (i.e. plotting in q space) has the effect of > suppressing the wiggle at 5 inv. Ang. that you are asking about. > Given that you are fitting in q-space, it is completely unreasonable > (from a numerical perspective) to expect that the fit could possibly > reproduce a feature that you have (intentionally or otherwise) > filtered out of the data. > > To say that another way, given how you constructed the fit, you got a > good fit. You made the fit in a way that it cannot possibly reproduce > the feature you are asking about, thus your question is ill-posed. > > I think the deeper problem is that you don't have a deep grasp of what > happens in Fourier analysis. So let's talk about that a bit. > > When you do the transform from k to R-space, you are representing the > frequency spectrum contained in the original data. Slow wiggling > features in the original data give rise to the low-R > (i.e. low-frequency) features in the chi(R) data. Fast wiggling > features in chi(k) give rise to high-R features in chi(R). Your > wiggle at 5 inv. Ang. looks to my eye like a pretty high frequency > feature. > > When you do the backwards transform from k to R with a restricted R > range (in your case, from 1 to 3.5), you are filtering frequencies out > of the data. The chi(q) data only contains those frequencies from the > original chi(k) spectrum that fall in your R range. > > What I am suggesting is that the wiggle in question is due to Fourier > components beyond 3.5 Ang in chi(R). > > So, how would you reproduce that feature in chi(k)? That's simple -- > fit features in the data beyond 3.5 in chi(R). That is, do an actual > good job of fitting the small signal from 3.5 to 5 Ang in R. > > Of course, that's going to be difficult to do in a statistically > robust manner because the signal is very small, there will be quite a > large number of paths contributing to that region, and the > parameterization of many paths for such a small signal is likely not > to be very robust. EXAFS is hard! > > Hope that helps, > B > > > On 12/04/2013 02:17 PM, Matt Frith wrote: >> Dear All, >> >> I need some help in fitting an amorphous iron oxyhydoxide sample. I am >> having difficulty producing a good fit, particularly in the k=4-6 range. >> Fitting this region well is very important for me, because if I add >> another metal(+3 oxidation state) into my system, this is where I >> observe the most quantifiable changes (The shoulder @ 5 A^-1 and the min >> @ 5.6 A^-1). Thus far I have been unable to fit the shoulder well enough >> to make meaningful comparisons. >> >> I have been fitting in kq with kmin=2.566 And kmax=10.877, and Rmin=1 >> and Rmax=3.5, and using the goethite O1.1, Fe.1, and Fe.3 paths. >> Attached is an Artemis file (P41_006_merge_norm_TRANS.fpj) for an >> amorphous Fe oxyhydroxide sample (Fe only, no other metals). The data >> was collected at the Fe K-edge. >> >> *Is there a way to fit just this region (k~4-6 range) in k? If so what >> is the best method for doing this? If not, does anyone have suggestions >> as to how I can improve my fitting? Should I fit the data in k since the >> shoulder is less evident in kq?* >> >> Thank you for your time. >> >> Sincerely, >> >> Matt Frith >> >> >> >> _______________________________________________ >> Ifeffit mailing list >> Ifeffit@millenia.cars.aps.anl.gov >> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit >> > > > -- > Bruce Ravel ------------------------------------ bra...@bnl.gov > > National Institute of Standards and Technology > Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2 > Building 535A > Upton NY, 11973 > > Homepage: http://xafs.org/BruceRavel > Software: https://github.com/bruceravel > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit