[Ifeffit] Right way of choosing E0 in Athena

[Frenkel, Anatoly]

Vadim,

The ss2 of multiple-scattering (ms) paths and single-scattering (ss) paths are not simply related 
unless the legs in the ms paths are collinear. 

In that case, as published in Frenkel, Stern, Qian, Newville, Phys. Rev. B, 48, 12449 (1993), 
if the intervening atom is a first nearest neighbor of the absorber, this atom, to a good approximation, 
does not affect the ss2 of the double scattering and triple-scattering path connecting the absorber, 
the 1NN and the 1NN to the intervening atom in the forward scattering direction. 

It also describes other relationships between the ss2 of the 1NN path and the 
ms paths when the intervening atom is the absorber.

The complete set of these relationships can be found  in the Appendix of an article by 
D. Pease, A. Frenkel et al., - I will send it to you as an attachement in a separate email.

Anatoly Frenkel
Yeshiva University

-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov on behalf of Vadim G Palshin
Sent: Sun 7/2/2006 5:25 PM
To: ifeffit at millenia.cars.aps.anl.gov
Subject: [Ifeffit] Right way of choosing E0 in Athena
 




>Since you know the spectra are well calibrated relative
>to one another, I would use a single E0 for all
>background subtractions.  In such a situation, I tend to
>play with the background parameters for one of the
>spectra and then apply these background
>parameters to all other spectra.
Thanks, Matt! Yes, that's pretty much what I have been doing, just had some
trouble getting reasonable fit values for E0's in my last set of samples.
Aligning chi(k) of the standard to theory - great guide, Shelly! - and then
applying  the same parameters to the other spectra helped solve this. Now,
more questions:
1. Many experts advise to do multiple k-weight fitting to deal with
correlated variables. Should one always use multiple k-weights, or is it
better to switch to one kw value once the correlations are taken care of -
to refine the remaining variables? Does it make any difference?
2. When modeling the Debye-Waller factors for multiple-scattering paths, is
it possible to express them in terms of the sigma^2's of single-scattering
paths that correspond to the atoms involved in the multiple scattering
events; i.e. for a core-atomA-atomB-core path, can sigma^2 be obtained by
some combination of core-atomA and core-atomB sigmas? It seems intuitively
that they should be related, and also that the amplitudes of multiple
scattering paths should be more sensitive to disorder. Does this make any
sense?
Thanks again for your replies!
Vadim.

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