[Ifeffit] Phase corrected Fourier transforms

[Scott Calvin]

Hi JA,

I'll put in my two cents on some of these 
questions, although in some cases it's a matter of personal taste:

At 03:16 PM 9/22/2006, you wrote:

>I have read Phase corrected Fourier transforms in Athena manual and now
>I have a big doubt, ¿phase correction or not in a publication?

I vote not. :)

Most of the time, phase corrections are desired 
for cosmetic reasons, to make the FT look more 
like a radial distribution function. But the FT 
is never a RDF, and so an attempt to make it look 
like one may mislead some readers without much 
experience in EXAFS. Also, adding a phase 
correction necessitates a discussion of what 
method for applying the phase correction was 
used, which complicates the publication.

On the other hand, I'm less opposed to using 
phase-correction for presentations and posters 
intended for a non-EXAFS audience, although I 
still don't do it. Like I say, this is a matter of personal tatse.


>I have high correlations between ss and SO2, and deltaR and deltaE. I
>tried different fits but I can not eliminate them, then...is the fit
>wrong?

No, this does not mean the fit is wrong! It is 
hard to eliminate the correlations you describe. 
And the algorithm used by Ifeffit to calculate 
the uncertainties works in such a way that you 
don't have to worry about correlations adding 
extra uncertainty beyond what's reported.

High correlations do help you, as the person 
doing the fits, diagnose why an uncertainty might 
be high. If you find the uncertainty for deltaR 
is too high to say anything useful about a bond 
length, for example, then check the correlation 
with deltaE0. If the correlation is high, there 
are a number of things you can try to reduce the 
correlation, and, hopefully, the uncertainties: 
fitting multiple k-weights, fitting more coordination shells, etc..

>Sorry for these easy questions but I am a novice in XAFS.

That's one of the things this list is for...

--Scott Calvin
Sarah Lawrence College