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

[SCHLEGEL Michel 177447]

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 
> <If/Io> 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