[Ifeffit] Debye factor and S02 correlation

[Dr. Paul Fons]

I meant to send this off to the list (instead I sent it to Scott Calvin 
only).  Ahh the wonders of email.

I thought I would parrot back something I read in one of Bruce's 
tutorials (that seemed to work well for me), namely that the Debye 
factor, sigma has a k dependence while S02 does not.  This means that 
if you can Fourier isolate the first shell (and you can assume a 
coordination number), you can determine a value for S02 by a fit using 
different k weights.  You simply fit sigma for various fixed values of 
S02 (e.g. 0.6 - 1.0) and repeat the process for different k weights 
(e.g. k, k^2, k^3).  The resulting plot of sigma vs. S02 will result in 
a straight line for a given k weighting.  The three different k weights 
will intersect at a single point (or almost so) and the value of S02 at 
this point will be the value of S02 for the sample.

While we are discussing S02, I thought I would ask a little more about 
it myself.

1.
S02, the passive electron reduction factor is a term that encompasses 
many body effects, in particular the effect of the core hole.  It is 
also as such said to have a "weak energy and path dependence" [J.J. 
Rehr and R.C. Albers in Reviews of Modern Physics, v. 72, no. 3 
(2000)].  The question is how fair is it to assume that S02 is a 
constant independent of shell, or in other words should S02 be allowed 
to vary with shell (e.g. with significant multiple scattering 
contributions?)

2.
I don't remember the paper offhand, but as I recall, in a paper by 
Bruce, the value of S02 was ascribed a value greater than unity.  I 
didn't grok the explanation at the time, and was wondering if Bruce (or 
anyone else) in his ever-expanding knowledge of exafs has an 
explanation for when/if S02 can be greater than unity.  Certainly if 
you consider Rehr's definition of the overlap integral of the N-1 
electron wavefunction with and without a core hole, the value of S02 
cannot be greater than unity[same reference, p. 636].  Any comments?