[Ifeffit] Help: some operation using Athena and Artemis

[Scott Calvin]

A comment on this thread, started by Bingjie:

On Nov 26, 2012, at 11:01 AM, Bruce Ravel wrote:

2, In Artemis, since I was told that each shell in R-space can be fitted
separatly, is it OK to fit each shell in the R-range respectively, and then
join up the fitted curve and just ignore the not match part in other
R-ranges? And can a shell be comprised of several different paths (like
Fe-O and Fe-S)?

There is, currently, no way to exclude an interior region from a fit
in Artemis (i.e. there is no way to fit from 1 to 2 and from 2.5 to
3.5, but to exclude the region from 2 to 2.5).  I actually think
that's a bad idea and doubt that I would ever implement such a thing.


Sure there is. Just make a copy of the data set and treat it as a multiple dataset fit, with the two epsilons forced to the same value.

I've considered using that trick in the past, but never done so in earnest, in part because of the very important considerations Bruce describes.

So why did I consider it? To get around a really messy forest of glitches in data. While a single glitch can be removed, replacing a forest of glitches with zeroes is creating data that isn't there. Better to somehow exclude the data without fitting it, working only with the non-glitchy data above and below.

As I've said, I never got quite desperate enough to do that on a real fit, though--I always found some better way out of the problem.

But I agree strongly with Bruce that it's problematic to use this trick to avoid data, as opposed to glitches.

The reason I don't like this idea is that, in general, shells in the
EXAFS signal are not completely isolated.  Because of the limited data
range and other reasons, the signals form the various shells have long
tails and overlap considerably.  That overlap is a very important part
of the problem.  Getting the small parts of the chi(R) spectrum right
is just as important as getting the large parts right.

Excluding an interior region could artificially remove this overlap
from the evaluation of the fit.  That would have a very serious impact
on the statistical quality of the analysis.  I also worry that an
interior exclusion would be used in an attempt to minimize the impact
of multiple scattering paths on the analysis.  That, too, would be a
statistics disaster.

In short, you should embrace the correlations between the parts of
the problem.


--Scott Calvin
Sarah Lawrence College