[Ifeffit] Phase corrected Fourier transforms

Matthew Marcus mamarcus at lbl.gov
Sat Sep 23 15:13:35 CDT 2006

OK, I have to put in my US$0.02/e0.02.  The phase correction can't mean much 
if the scattering atoms are different, which is usually
the case.  If the scattering atoms are very heavy, then the phase correction 
has a kink in it which could cause strange shapes if applied
to light-atom shells.  I've played with this phase correction and amplitude 
correction, while I was at it, and not been very impressed.
One place where it might be useful is in the aforementioned case of heavy 
and light scatterers.  If you use the correction for one of
these, then the corresponding shells sharpen and the other ones blur out, so 
you can get a rough idea of who's who.  I suspect
this works better if you do the amplitude as well.

A related technique is to use model compounds+FEFF to get 'semi-empirical' 
amps and phases which include all the artifacts
of the experiment.  Suppose, for instance, that you're looking at Cu in a 
matrix of Fe, and you have data for Cu metal but
not for any known Cu->Fe scattering pair.  You can synthesize a 
'semi-empirical' Cu->Fe phase and amp like this:

phi(Cu->Fe) = phi_exp(Cu->Cu)+(phi_theor(Cu->Fe)-phi_theor(Cu->Cu))
amp(Cu->Fe) = amp_exp(Cu->Cu)*amp_theor(Cu->Fe)/amp_theor(Cu->Cu)

If you don't want to do this for modeling, you can use this method for 
comparing two spectra which you think might
be alike except for the central atom, by "correcting" one spectrum with the 
difference to make it comparable to the other.

Another aspect to this whole thing is that people are very used to 
uncorrected FT's and are aware that you have to add 0.3-0.4A
to the distances.  If you show only corrected FT's, I wonder if that will be 
satisfying to the audience.

----- Original Message ----- 
From: "John J. Rehr" <jjr at leonardo.phys.washington.edu>
To: "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
Cc: "John J. Rehr" <jjr at phys.washington.edu>
Sent: Saturday, September 23, 2006 7:18 AM
Subject: Re: [Ifeffit] Phase corrected Fourier transforms

Dear Juan Antonio,

  I personally feel that adding phase correction to the XAFS FT is
highly desirable, and I encouraged its implementation in Athena.
The reasons are the following:

1) Peaks in  non-phase corrected FT are substantially in error.
2) The non-linearity of the phase shifts in high-Z materials leads
to multiple-peaks, thus blurring the FT.
3) Theoretical phase shifts are good enough that adding phase correction
tends to correct the peak positions and the problems due to non-linearities.
4) Adding phase correction does no-harm to the fits. That is, one gets
the same results whether or not phase correction is included.
5) Adding phase correction gives a FT which can be more easily
interpreted "by eye", that is the peaks have a more physical interpretation.
On the contrary, non-phase corrected FTs can be mis-interpreted.

Overall, my view is that the phase correction is like a prescription
lens which gives a sharper image. While the image may not be perfect,
at least it's generally much superior to the non-phase corrected FT.

  J. Rehr

On Fri, 22 Sep 2006, Juan Antonio [iso-8859-1] Maciá Agulló wrote:

> Hi all,
> 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 have read also that this correction is different (more complete) in
> Artemis and I am not sure if I should correct also in Artemis and which
> path should I use and why.
> I saw many papers dealing with EXAFS fits and they showed a "calculated"
> bond distance, I think it is: d = Reff + deltaR, right?
> I also ask for a paper where I can find that deltaE is ok (even for
> high-Z backscatterers) if deltaE < 10eV.
> 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?
> Sorry for these easy questions but I am a novice in XAFS.
> Thank you very much.
> Best regards,
> JA


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