[Ifeffit] errors in bond lengths

[Matt Newville]

Hi George,

I think this will not be a different answer from Matthew's or Anatoly's
answers, but just reiterate their points.  The Purans et al 2008 PRL from
2008 appears to use both non-linear fitting with Feff and EDA (which should
give basically the same results as Artemis/Ifeffit/Larch, though I do not
know in detail what error analysis is done), and the log-ratio method.  I
think they also fit the resulting sigma2 (derived from the non-linear fit)
to an Einstein model.

The log-ratio method can only determine relative changes in distance,
coordination number, and sigma2.  The main motivation for using this method
is that scattering factors in the EXAFS equation will cancel out (or mostly
cancel out) when comparing two similar experimental spectra.  In addition,
it is often argued that data extraction errors (energy scale, background
subtraction, etc) would tend to be the same for two experimental spectra
and so would also mostly cancel out.  There usually isn't much analysis of
what residual systematic errors happen with the log-ratio method.   The
working idea is that the ratio of the log of isolated single-shell EXAFS
chi(k) (or what we would call chi(q) in Artemis/Ifeffit/Larch)  amplitudes
vs k**2 should be linear (intercept = Delta N, slope=Delta sigma2) and the
phase difference vs k should also be linear (intercept=0 if E0 is truly
unchanged, and slope = Delta R).   For anyone who actually plots those
(even for spectra on the same sample), you will probably find that these
are "linear-ish", clearly showing both "yeah, that could work" and also
"maybe not perfectly".

But, if I'm reading this PRL correctly, it looks like they use the
log-ratio method to compare sigma2 and R of spectra at the same temperature
but with different isotopes. That does seem like a fine way to better
determine the subtle differences between those spectra.

--Matt


On Sat, Apr 11, 2020 at 12:41 PM George Sterbinsky <
GeorgeSterbinsky at u.northwestern.edu> wrote:

> Hello,
>
> As is well known, EXAFS is more accurate at determining relative changes
> in bond lengths than absolute changes in bond lengths due to cancelation of
> systematic errors in relative comparisons. When comparing the relative
> changes in bond lengths determined from EXAFS fits, as one might for a
> temperature series for example, is it appropriate to use the uncertainties
> returned by Artemis?
>
> My question arises in part from Phys Rev Lett vol. 100 pg. 055901 (2008),
> where the authors state that errors for changes in bond length in a
> temperature series were determined by empirical means rather than
> statistical means. This then raises the question as to if the authors
> believe that statistical means would overestimate the error. My inclination
> is to think that the uncertainties reported by Artemis would be appropriate
> because of the scaling by the square root of reduced chi squared. However,
> I want to see what others think about this before committing to it.
>
> Thank you,
> George
>
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