[Ifeffit] Fit in R and k space

[Scott Calvin]

At 03:07 PM 9/20/2005 +0200, you wrote:

>I think the effect of interfering contributions in the Konigsberger and 
>Prins. What I noticed, though, is that the fit of closer shells  can affect 
>that of the furthest shell because of the "spillof" from the first peak. 
>The recciprocal I was not aware of. But I do agree with you on the whole.
>However, I think that in the FT case, these interences are limited to the 
>nearest shells, whereas in k-space, nothing protects you from shells at, 
>say, 2R and 3R (so to speak.
>
As part of a larger unpublished study that keeps sitting on my back-burner
(one of these days maybe I'll finish up the paper and send it somewhere!),
I looked at the seriousness of the "leakage" (or spill-over or interference
or whatever you want to call it) from higher-R shells in a first-shell fit
of fcc metals. Although I don't have the results at my fingertips right
now, my recollection is that it was sufficient to completely screw up the
dependence of lattice parameter and sigma2 on temperature for temperatures
below room temperature. The fact that I've heard occasional assertions that
EXAFS is not sufficiently sensitive to accurately yield the thermal
expansion of fcc metals below room temperature suggests to me that some
people may be underestimating the effect of this leakage. When the outer
shells are accounted for (even crudely--no new free parameters are
necessary), I found it relatively easy to get the right expansion
characteristics.

Having said that, of course the problem of leakage is LESS for fits in
R-space than k-space, right? At least we're filtering out PART of the
signal from outer paths when we do a fit on a portion of R-space, but
that's not possible when we do a k-space fit.

--Scott Calvin
Sarah Lawrence College
>