[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.


 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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