[Ifeffit] Using more than one E0

[Wojciech Gawelda]

Hi everybody,
 
I would like to address a couple of questions which are partially
related to my recent struggles in fitting some EXAFS data. I’m trying to
fit my data using several shells of different neighbors including a few
single scattering paths and also so multiple scattering contributions
(mainly collinear multiple scattering paths) all calculated with the
help of FEFF 8.20. Now, I found once in the FEFFIT manual the following
suggestion:  one might consider using several different E0’s for
different paths in order to improve the fit. Ok, the explanation was
based on some approximations coming from FEFF code which include
incomplete core-hole shielding, lack of angular variations of the
valence charge distribution and charge transfer between atoms in polar
materials. 
My question is the following: does anyone of you have some experience
with such procedure? And if yes, shall than distinguish between the
first shell of nearest neighbors and the rest of the atoms in terms of
their E0 corrections (using 2 parameters)? Or perhaps one can use
separate E0’s for each path?
That’s one thing

The second question concerns the background correction in Artemis: I was
trying to find an easy explanation of what this procedure does during
the fit but I guess I’m still somewhat confused
how one can judge
whether the spline co-refinement does the right job? Some people on this
mailing list underline that one should avoid high correlations between
fitting parameters and background parameters in order to make sure that
the fit is correct. But is it the only criterion? In some of my fits
background line looks kind of constant over the entire R-range which was
taken into the fit. Sometimes it becomes like a modulation peaking
underneath some scattering paths
even if the background parameters don’t
correlate significantly with fitting parameters I have my doubts whether
the result is trustful. Any hints about this procedure?
Well, I’ll greatly appreciate any comments and feedback from all you
guys. I hope I’ll be able to share some of my own soon :-)
Cheers,
 
Wojciech
 
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Wojciech Gawelda
 
Laboratoire de Spectroscopie Ultrarapide (LSU)
Institut des Sciences et Ingénierie Chimiques (ISIC),
Faculté des Sciences de Base (SB-BSP)
Ecole Polytechnique Fédérale de Lausanne (EPFL)
 
CH-1015 Lausanne-Dorigny, Switzerland
Tel.:  +41 (21) 693 0452
Fax.: +41 (21) 693 0422 
E-mail:  <mailto:wojciech.gawelda at epfl.ch> wojciech.gawelda at epfl.ch
WWW:  <http://lsu.epfl.ch> http://lsu.epfl.ch
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