Since you know the spectra are well calibrated relative to one another, I would use a single E0 for all background subtractions. In such a situation, I tend to play with the background parameters for one of the spectra and then apply these background parameters to all other spectra. Thanks, Matt! Yes, that's pretty much what I have been doing, just had some

Vadim, The ss2 of multiple-scattering (ms) paths and single-scattering (ss) paths are not simply related unless the legs in the ms paths are collinear. In that case, as published in Frenkel, Stern, Qian, Newville, Phys. Rev. B, 48, 12449 (1993), if the intervening atom is a first nearest neighbor of the absorber, this atom, to a good approximation, does not affect the ss2 of the double scattering and triple-scattering path connecting the absorber, the 1NN and the 1NN to the intervening atom in the forward scattering direction. It also describes other relationships between the ss2 of the 1NN path and the ms paths when the intervening atom is the absorber. The complete set of these relationships can be found in the Appendix of an article by D. Pease, A. Frenkel et al., - I will send it to you as an attachement in a separate email. Anatoly Frenkel Yeshiva University -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov on behalf of Vadim G Palshin Sent: Sun 7/2/2006 5:25 PM To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] Right way of choosing E0 in Athena trouble getting reasonable fit values for E0's in my last set of samples. Aligning chi(k) of the standard to theory - great guide, Shelly! - and then applying the same parameters to the other spectra helped solve this. Now, more questions: 1. Many experts advise to do multiple k-weight fitting to deal with correlated variables. Should one always use multiple k-weights, or is it better to switch to one kw value once the correlations are taken care of - to refine the remaining variables? Does it make any difference? 2. When modeling the Debye-Waller factors for multiple-scattering paths, is it possible to express them in terms of the sigma^2's of single-scattering paths that correspond to the atoms involved in the multiple scattering events; i.e. for a core-atomA-atomB-core path, can sigma^2 be obtained by some combination of core-atomA and core-atomB sigmas? It seems intuitively that they should be related, and also that the amplitudes of multiple scattering paths should be more sensitive to disorder. Does this make any sense? Thanks again for your replies! Vadim. _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

Hi Vadim,

1. Many experts advise to do multiple k-weight fitting to deal with correlated variables. Should one always use multiple k-weights, or is it better to switch to one kw value once the correlations are taken care of – to refine the remaining variables? Does it make any difference?

I wouldn't say that multiple k-weights "take care of correlations", as the correlations are generally unavoidable, and don't always drop by very much when using multiple k-weights either. As an example, I just tried kweight=2 v. kweight = 1,2,3 on a test case, and saw Correl(S02, sigma2) drop from 0.90 to 0.88, and Correl(DelR,E0) drop from 0.87 to 0.80. The correlations are smaller, but hardly gone. But I still recommend using multiple k-weights most of the time because it seems to improve the stability and reliability of the fit results (that is, it's harder for a small change in fit parameters to give a different result). That is, I use it unless the performance hit is unacceptable.

2. When modeling the Debye-Waller factors for multiple-scattering paths, is it possible to express them in terms of the sigma^2's of single-scattering paths that correspond to the atoms involved in the multiple scattering events; i.e. for a core—atomA—atomB—core path, can sigma^2 be obtained by some combination of core—atomA and core—atomB sigmas? It seems intuitively that they should be related, and also that the amplitudes of multiple scattering paths should be more sensitive to disorder. Does this make any sense?

In general, there is not a simple relation between the sigma2 for single- and multiple-scattering paths. As Anatoly points out, for some collinear paths there are some fairly simple approximations that seem to work. Other than that, it is often safe to assume that a sigma2 for a MS path will be larger than that of a SS path that is one of its legs. ;). There are other approaches to _calculating_ sigma2 for single- and multiple-scattering paths. From what I can tell, these tend to be computationally intensive, highly system-dependent, and difficult to reduce to a set of parameters that can be refined to match data. --Matt