[Ifeffit] viewing individual Fourier components with Athena

Bruce Ravel bravel at anl.gov
Tue Mar 20 17:42:45 CDT 2007

On Tuesday 20 March 2007 16:36, Richard Mayes wrote:
> Basically, I have been asked to plot what Artemis gives for the theory
> when an individual path is plotted, except I'm supposed to do it for the
> raw data...that is, plot the individual paths that sum to the observed
> chi(k).
> However, looking at the picture from some old rhodium dimer EXAFS given
> to me by the good guy that signs my checks ;-) (a pic I'm supposed to
> use as a guide), I'm "chasing a rabbit."  Problem solved (it's a "use
> the theory, Homer" moment, i.e. DO'OH!).


I'm confused in the same way that Scott is.  There is no way to
deconstruct your chi(k) data into path components a priori.  The only
way that we have understanding data in a path-by-path manner is to
compute some theory and plot each of the paths from the theory.  Even
better is to do a fit and plot each of the paths after the fitted
parameters have been evaluated.

It occurs to me that you may be thinking of Fourier filtering the
peaks.  That is done in Athena by placing an R window around some
region of your chi(R) spectrum then pressing the "kq" button.  A word
of caution -- plotting a Fourier filtered spectrum is NOT the same
thing as plotting a path.  In certain situations, a path from Feff and
the Fourier filter of a region of chi(R) might be very similar.
Indeed, they might be sufficiently similar that the filtered spectrum
can be interpretted as the contribution from a path. 

More often, however, that is a very flawed assumption.  In any
situation where two or more paths conbtribute to a peak in chi(R) (and
excellent example would be the first peak in iron metal, which
contains contributions from the first two coordination shells and
which cannot be separated by Fourier filtering) it would be quite
incorrect to call the filtered specutm a "path contribution". 


 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel 
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/

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