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 J

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: wojciech.gawelda@epfl.ch

WWW: http://lsu.epfl.ch

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