[Ifeffit] viewing individual Fourier components with Athena

Richard Mayes mayes at ion.chem.utk.edu
Thu Mar 22 07:39:00 CDT 2007


There was a miscommunication dealing with what I was told to do.  Sorry 
for the confusion.


Bruce Ravel wrote:
> 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!).
> Richard,
> 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". 
> HTH,
> B

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