[Ifeffit] Using more than one E0

Fri May 28 09:51:00 CDT 2004

Hi Guys,

I wanted to add another comment about using multiple E0's.  One of the best formulas that I have found involves assigning different atom types different E0's.  This method seems to work well for M.S. paths.  Here is an example of how it goes.  I think that Daniel Haskel was the first to publish this type of E0 stuff. "Single and multiple scattering XAFS in BaZrO3: a comparison between theory and experiment" D. Haskel, B. Ravel, M. Newville and E. A. Stern  Physica B 208&209 (1995) 151 

Lets pretend that we have a copper oxide that we want to model.  First shell Oxygen called O1,  Second shell Cu called Cu1,  Third shell Oxygen called O2.  This method would use three E0's.  One for the first shell oxygen atoms, Eo1.  One for all the Cu atoms Ecu. and one for all other oxygen atoms Eo2.  The Cu-O1 path has E0=Eo1.  The Cu-Cu1 path has E0=Ecu.  The Cu-O2 path has E0=Eo2.  The Cu-O1-Cu-O1 path has E0=Eo1, (only scattering from O1 atom).  The Cu-O1-Cu1-Cu path has E0=0.5*Eo1+0.5*Ecu. (scattering once from O1 and once from Cu1) ... And so forth and so on.

Now if you set something like this up.  You need to look at the results carefully and see if it works.  Generally, scattering from an atom that has lost an electron due to an ionic bond will have a positive deltaE, and scattering from an atom that has gained an electron will have a negative deltaE.  Of course this negative/positive deltaE is added to the alignment deltaE.  If the values for Eo2 and Ecu are the same within the uncertainties of the measurement then you need to make them the same variable,  so then you will only have two energy shifts, but you can still use it.   This is most often the case as Matt mentioned but I always like to test it out.  In addition, one electron transfer is about a shift by 6 eV, so if these values differ by more than say 10 eV something is not right.  Since the shift is generally smallish, say 2 eV, your fit has to be really accurate to "see" these shifts.  In particular they seem to be most important when you have all the deltaR values constrained in some way so that the error in deltaR is really really small.  For example all deltaR values are related to unit cell parameters and atomic positions or and overall expansion term.

HTH

Shelly  
-----Original Message-----
From: Wojciech Gawelda [mailto:wojciech.gawelda at epfl.ch] 
Sent: Thursday, May 27, 2004 1:18 PM
To: ifeffit at millenia.cars.aps.anl.gov
Subject: [Ifeffit] Using more than one E0

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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