[Ifeffit] Debye factor and S02 correlation
Dr. Paul Fons
paul-fons at aist.go.jp
Sat Nov 2 08:27:17 CST 2002
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?
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