[Ifeffit] Fitting using Experimental standard

Matthew mamarcus at lbl.gov
Mon Jan 4 21:34:56 CST 2010


You're describing the way many of us, including me, used to analyze data before FEFF became as reliable and easy to use (courtesy of 
Artemis and the like) as it is now.  As shown in the paper,
you extract amplitude and phase from model compounds, by back-transforming filtered shells, then use those to fit data.  Multiple 
data sets aren't new, either; I used to do that with data taken on
dilute solid solutions at different temperatures.  Another trick I used to use a lot was if I didn't have a standard for the right 
pair of atoms, I'd use FEFF to get the difference in scattering factors (phase+amp)
between the pair I wanted and a nearby (in Z) pair I had data for.  For instance, you can get CuAl2 as  a compound and extract the 
phase+amp for Cu as the central atom and Al as the scatterer.  You can't do
that for Cu and Si because there's no Si-rich intermetallic in the CuSi system.  Therefore, you can do:

Amp(Cu->Si) = Amp_exp(Cu->Al)*Amp_Feff(Cu->Si)/Amp_Feff(Cu->Al)
Phi(Cu->Si) = Phi_exp(Cu->Al)+Phi_Feff(Cu->Si)-Phi_Feff(Cu->Al)

and get a "semi-empirical" Cu->Si "standard".  Why the +- for phase and the */ for amp?  You can think of it as extrapolating the 
log of a complex signal chi~(k) = Amp(k)*exp(i*Phi(k)), of which the measured chi is the Im().
This expresses everything in terms of relatively slowly-varying quantities and treats amp and phi as parts of the same quantity, 
which they really are.

One of the programs available from the ALS beamline 10.3.2 website http://xraysweb.lbl.gov/uxas/Beamline/Software/Software.htm is 
called EXAFSfit and does this sort of fitting to amp and phase.  You feed it amp and phases which
come from the FT program, also available at the website.  The amp and phase files are simple 2-column ASCII, thus readable anywhere.
    mam
----- Original Message ----- 
From: "Frenkel, Anatoly" <frenkel at bnl.gov>
To: "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
Sent: Monday, January 04, 2010 6:21 PM
Subject: Re: [Ifeffit] Fitting using Experimental standard


I know of one very obscure conference proceeding describing what you are looking for. It was written during pre-Artemis times but 
the method can be used with Artemis, of course.

A. I. Frenkel, M. Vairavamurthy, and M. Newville,
A study of the coordination environment in aqueous cadmium-thiol complexes by EXAFS spectroscopy: experimental vs. theoretical 
standards
J. Synchrotron Rad., 8 , 669-771 (2001).

The link to PDF is here:
https://pubweb.bnl.gov/~frenkel/EXP-FEFF/thiols.pdf

Anatoly

________________________________

From: ifeffit-bounces at millenia.cars.aps.anl.gov on behalf of abhijeet gaur
Sent: Mon 1/4/2010 9:15 AM
To: ifeffit
Subject: [Ifeffit] Fitting using Experimental standard


A very happy new year to all

Generally the fitting is done using theoretical standards. For that in Artemis, we give input as crystallographic data. But If we 
want to use an experimental standard instead of
theoretical standard, how that can be done. Is it possible to use experimental standard in Artemis? or we have to use some other 
method.

with thanks
Abhijeet Gaur
School of Studies in Physics
Vikram University, Ujjain (India)





--------------------------------------------------------------------------------


> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
> 




More information about the Ifeffit mailing list