ARE: [Ifeffit] Re: Ifeffit Digest, Vol 5, Issue 1

Hello everyone,

> > I would second Matt's reservations about averaging I0 for
> > different XAFS scans. The main reason we record I0 is to
> > compensate for fluctuations in I0, which are mirrored in
> > If (statistically they are highly correlated) so the ratio
> > If/I0 eliminates the variation (as long as the
> detector chain is linear).

> I totally agree.  This can lead to big problems in the data.  We
> accidentally tried it once and then made sure that the
>  average was performed instead.

I think the best way to do it statistically is to weight each
individual XAS spectrum with 1/s^2, where s is an estimate of
the noise in the spectrum. This makes sure that a noisy
spectrum does not alter the overall quality of the fit . Of
course, a simpler way to do it is to retain only the spectra
of overall similar -and hopefully good - quality, and discard
the noisy ones.

I think an evaluation of the spectrum noise may be obtained
from the FT contributions peaking at high 'distances' (e.g.,
> 10Å). Hence, with a correct script, we should be able to:
- extract chi(k) functions from individual XAS spectra
- perform FTs for these spectra,
- perform inverse FT on the spectra over a given limit, and
  calculate s from that,
- usecalculated s values to sum weight-average individual
  XAS spectra.

As I'm a very lousy IFEFFIT user (means that I've stopped
short from hitting buttons on Athena and Artemis), I would
humbly ask Bruce if there's any chance to see such a methodology
implemented on Athena one of these days (not in a hurry, though)

Best regards,

Michel Schlegel