Pieter,

Please use the ifeffit mailing list for questions about XAFS Analysis.  I am CCing this reply to that list.

On Thu, Jun 2, 2016 at 2:35 AM, Pieter Tack <Pieter.Tack@ugent.be> wrote:

Dear Dr. Newville,


During the analyses of some metallic Au EXAFS I noticed the Au-Au single scatter first shell is characterised by a split peak in the chi(R) spectrum. I was wondering what caused this, as I would expect a single scatter path resulting in only one scatter distance and thus one peak in the chi(R). However, I could not find much on the matter except for a quote of you stating it is "due to the resonance in the scattering for heavy scatterers" ( http://permalink.gmane.org/gmane.science.physics.ifeffit/2352 )


In short:  XAFS chi(R) is *NOT* a pair distribution function, due mostly to the complex electron scattering that alter the amplitude and shift the phase of potoelectron in addition to the simplistic "backscatter from a point source atom".   The most obvious effect of this is a ~0.5 Ang shift to lower R from the value you might expect if chi(R) was a pair distribution function.   Other effects include broader and asymmetric peaks,  and the resonance effect such as you notice for heavy elements (easily noticeable for Z>40 or so, but unmistakable for Z > 60  -- that Z is for the *scattering atom*).  

At certain energies (or for a narrow range of energies), the photo-electron has very low scattering probability from a heavy element, and the phase jumps dramatically, as if the electron was able to tunnel through or hop over the potential well of the scattering atom.
This is usually (and I think properly) called a Ramsauer-Townsend resonance.

There are many papers, review articles, and slides from talks available that describe this in more detail.  Google "ramsauer townsend effect exafs" for detailed articles.

I don't really understand what this means, which physical process causes this. Could you perhaps provide me with additional info on this matter or direct me to some papers that explain this?


Thanks in advance!

Best regards

Pieter Tack


---
X-Ray Micro-Spectroscopy and Imaging Group
Department of Analytical Chemistry
Ghent University
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--Matt