[Ifeffit] Contribution of multiple scattering paths - basic question

[Matt Newville]

Gerrit,

It is sometimes difficult to qualitatively compare |chi(R)|,
especially if you haven't applied sigma2 values to the Feff
paths.  Also, I couldn't tell what k-weighting you used (it
shouldn't matter too much, but some non-focussed triangle MS
paths decay quickly, so that k-weighting of 2 or 3 can focus on
the 'strong scattering').

But other than that, it does seem likely that the 'red paths'
are cancelling the others, and are probably a better description
of the data at least between 2.5 and 3.2 Ang (it might be worse
beyond that, but it's hard to tell).  And, yes, such destructive
interference can definitely happen!!

Of course, you'd probably want to do that more quantitavely, 
but it gives a reasonable place to start....

--Matt

On Mon, 9 May 2005, 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.
> 
> Regards,
> Gerrit
>