[Ifeffit] Fit in R and k space
Bruce Ravel
ravel at phys.washington.edu
Tue Sep 20 08:05:35 CDT 2005
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 at anl.gov -or-
ravel at 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/
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