[Ifeffit] sigma^2 values for multiple scattering paths

[Shelly Kelly]

Hi Han Sen:

I agree that the small sigma2 values for the MS paths is suspect. The
data is saying that it wants more amplitude from these paths than is
physically possible. One way to increase the amplitude of the paths is
to make the scattering angle closer to 180deg for a linear path.  It
is a bit tedious to edit the feffit input file to get that to run, but
can be done with a bit of geometry.

This will not work for a triangle MS path. In that case I would look
for additional contribution from other MS or SS paths at the same
distance.

Another way to force sigma2 to be larger would be to use a Debye or
Einstein Model to model both the SS and MS path.  That will make
sigma2 for both of them at least reasonable. Although the fit will not
be as perfect.

HTH,
Shelly

On Wed, Oct 6, 2010 at 12:26 AM, Han Sen Soo <hssoo at lbl.gov> wrote:
> Hello,
> I briefly read through the FAQ about this but I'm not sure if it answers my question. Are there situations where the sigma^2 for a multiple scattering path can be smaller than the direct paths? So small that they're on the order of 0.001-0.003 for a degeneracy of 12 such paths?
> I'm working on a fitting model that does not work well with additional shells but it looks almost perfect with a multiple scattering path included. I'm skeptical however, because of the small sigma^2 values. I am also not discounting the fact that the data quality may be poor. But I would appreciate any physical reasons for small sigma^2 values. Thanks!
> han sen
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