On Tuesday 20 September 2005 02:59, Michel Schlegel wrote:
So maybe (or even surely, I daresay), the contribution from non-modelled distant shells would affect the structural parameters from the modelled shells.
I daresay "surely" as well ;-) Although I prefer to think about the EXAFS problem in R space, I don't think this is the reason to do the fit in R space. Regardless of which fitting space you use, you are relying on Feff to supply some fraction of all the Fourier components in the data. The first-shell feff path only has Fourier components (or frequencies, if you prefer) corresponding to the first-shell portion of chi(R). Thus that feff path can only fit those frequencies. That's true in k-space as well as in R-space What's more, I think the problem of non-modelled shells affecting the modelled portion of the data exists in R-space just as much as in k-space. Because of sigma^2 and the finite data range, peaks have width. In R-space, a given path is centered at a particular R-value (or, in other words, its contribution to the spectrum is dominated by a particular frequency). However, the peak has width (it contains frequencies below and above the dominant frequency). Thus longer paths always interfere with shorter paths to some extent. B -- Bruce Ravel ----------------------------------- bravel@anl.gov -or- ravel@phys.washington.edu Environmental Research Division, Bldg 203, Room E165 Argonne National Laboratory phone: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/