[Ifeffit] Contribution of multiple scattering paths - basic question

[Bruce Ravel]

On Monday 09 May 2005 11:24, Gerrit Schmithals wrote:
> Hello,
>
> I want to fit a cobalt-porphyrin for which I have an atom.inp-file and
> EXAFS experimental data at Co K-edge . I started with a simple sum of
> paths for all paths with a distance below 5 A and an amplitude above 10
> (see attachment). If one compares experiment (black curve) with this
> first fit (red curve) the first shell peaks overlap quite nicely but not
> the second and third (fourier transforms of the single paths are also
> drawn as blue and red curves). Then I excluded four paths (the red ones)
> and did a second sum of path. Now the other shell peaks overlap as well
> (green curve). The four exluded paths are multiple scattering paths via
> one nitrogen and one carbon atom. I conclude that the extinction of the
> second shell peak in the first simulation is due to the fact that these
> paths have a different phase shift than the other paths with a similar
> frequency.
>
> Do I make a simple error (adjustments of the fitting parameters
> necessary?) or does that tell me something about the system? I
> appreciate any help.


I'll take a stab at this.  I should mention that the image you sent
did not reproduce very well on my computer.  It was very hard to read,
so my answer might be tainted somewhat by that.

I think the simplest way to understand what you observed is to
remember that when you do a sum of paths, you are doing so without any
reference to sigma^2.  It is quite possible, particularly in an
organometallic thingie, that the MS paths will be quite strong in the
sigma^2=0 Feff calculation, but quite weak in the real analysis due to
their large sigma^2's.  This seems reasonable.  One might expect a MS
path to be quite floppy in an organometallic.  At finite temperature,
that floppiness translates into a large sigma^2 which suppresses that
contribution.

I must say, though, that your data look to be a bit more complicated
that that.  Perhaps you have some interesting collinear MS paths which
don't sigma^2 away while other, non-collinear MS paths do.  The reason
I say that is because the green line looks more like the data between
2 and 3.5, but the red looks more like the data at 4.  Of course, I
haven't seen the real or imaginary plots, so that might not really be
the case...

In any case, you are wise to proceed the way you have.  It is a very
good idea to begin any new analysis project by exploring the various
contributions from Feff.  You can learn a lot by doing so and that
knowledge is very helpful as you start to actually fit the data.  It
is certainly a poor idea to jump right into the analysis without
exploring the contributions from Feff in this manner.  I think that
being able to visualize the individual contributions in an easy manner
is what makes Artemis competitive with other data analysis solutions
out there.

Good luck.  I'll be interested to hear what other have to say.
B


-- 
 Bruce Ravel  ----------------------------------- bravel at anl.gov  -or-
						  ravel at phys.washington.edu
 Environmental Research Division, Building 203, Room E-165
 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

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