[Ifeffit] How to distinguish whether the coordination element is heavy or light

[Bruce Ravel]

On 07/02/2014 01:06 PM, ZHAN Fei wrote:
> Dear all,
>
> when I try to solve my question{*[Ifeffit] path contribution to fit in
> low R-space position, but the fit bond length is much longer than
> that*},I find a method from Prof.Calvin to distinguish whether
> coordination shell is composed by light or heavy element:
>
> http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2007-December/007983.html
>
> A clue can perhaps be obtained by noting the relative height of the
> peak near 2.3 angstroms compared with the large peak you've fit. As k-
> weight is raised from  0 to 1 to 2 to 3, the peak at 2.3 angstroms
> does not grow relative to the first peak. That suggests the scattering
> may be from another low-Z element like oxygen.
>
> But I still don't know ,why it works.

Hi Zhanfei,

First off, I am very pleased to see that you are using the archive of 
the mailing list as a learning resource.  Well done!  Lots of questions 
have been answered here over the years!

On this page, http://xafs.org/Tutorials, you will find a link to a 
document by Matt called "The Fundamentals of XAFS".  Look at figure 3.3 
on page 15.  It's a plot of the back-scattering amplitudes of three 
different elements.

As you can see, they have very different behavior as a function of k. 
Light things don't scatter very strongly at high k whereas heavy element do.

When you change the k-weight, you change how strongly the different 
regions contribute to the Fourier transform.  If you have different 
elements contributing to the spectrum, then changing the k-weight may 
give you a way of emphasizing one contribution over another.

HTH,
B



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

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2
  Building 535A
  Upton NY, 11973

  Homepage:    http://xafs.org/BruceRavel
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