[Ifeffit] XANES

[Bruce Ravel]

On Monday 03 March 2003 05:07 pm, Ahmed M Shahin wrote:
> I have some question. I have mixed oxidation states and I would like
> to fit XANES, to determine the ratio of the mixed oxidation states I
> have. I looked at the documentations in the Ifeffit website but I
> could not find it. I have a license of feff.  Would you please help
> me in how to fit XANES.

Ahmed,

I am not sure what you mean by "fit XANES".  That innocuous phrase can
have a variety of meanings.

My program, Athena, currently has the capability of fitting lineshapes
(i.e. an arctangent and a number of Gaussians or Lorentzians) to
normalized XANES data.  In the not-too-distant future, Athena will be
able to fit a linear combination of standards to a XANES spectrum and
be able to do principle component analysis (PCA).

Sam Webb's interface to Ifeffit, SixPack, already does the linear
combination thing and as well as PCA.
http://www-ssrl.slac.stanford.edu/~swebb/sixpack.htm 

Feff8 is often used to simulate XANES data.  That is, one or more
calculations are made and compared to the measured data.  Often the
Feff8 calculations are like the data in a qualitative sense more than
a quantitative sense, but very often it is possible to glean
information about your sample by observing hhow the calculations
change as you change the conditions under which the calculations are
made.  That is not the same as "fitting" XANES, but it often is a
useful approach.

If by "fitting XANES" you mean to somehow use the local partial
densities of state computed by Feff8 to compare to measured data and
use that comparison to iterate the conditions of the Feff calculation
until some criterion related to your data is met -- well I don't know
of anyone who has written using Feff.

Other researchers have done work fitting theory to XANES spectra,
albeit using code bases other than Feff.  Check out, for example,
  ** Benfatto and Della Longa, J. Synchrotron Radiat. 8 (2001)
     pp. 1087-1094
  ** Joly, Cabaret, Renevier, and Natoli, Phys. Rev. Lett. 82:11
     (1999) pp. 2398-2401

Hope that helps,
B

-- 
 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
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