[Ifeffit] 3rd and 4th parameter in Artemis path fitting

[Bruce Ravel]

On 02/26/2014 08:23 AM, RAFAYAHER at alum.us.es wrote:
> Hello,
>
> I have been analyzing some EXAFS data with Artemis, and I was wondering
> what are the 3rd and 4th parameters in the path parameters window.
>

Hi Rafael,

Those are the third and fourth cumulants of the absorber-scatterer
distribution.  They are best thought about in relation to the deltaR
and sigma^2 parameters.

Remember that we always measure EXAFS at finite temperature.  That
means that, even in a very well ordered material, there is thermal
motion.  In other materials there may be static sources of disorder as
well.

DeltaR is the centroid of the distribution of distances at which the
scatterer is observed.  sigma^2 is the mean square deviation in that
distance.  In the case of thermal motion, sigma^2 measures how much
the scatterer is "jiggling" about it's average position due to thermal
energy.

The third cumulants is next higher moment of that distribution and the
fourth is simply related to the fourth moment.

Some reading material on this topic:

   http://dx.doi.org/10.1016/0167-5087(83)90655-5

   http://dx.doi.org/10.1103/PhysRevB.46.687

   http://dx.doi.org/10.1103/PhysRevB.48.585

Those will ground you in the theory and math of the cumulant expansion
in EXAFS.  There are literally thousands of examples of papers in the
literature where the 3rd and 4th cumulants are included in the
analysis.

B


-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
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