[Ifeffit] Re: A very quick question

[Barton, David (DG)]

On Thursday 17 June 2004 03:17 pm, Ahmed M Shahin wrote:
AM> Hello, I would like to ask a very quick question and I would
AM> appreciate if someone can help me.  While running XANES fitting
AM> for mixed valence state metals such as cerium (3/4), should I use
AM> two arctangent functions with selected group of Lorentzian
AM> functions or I have to use only one arctangent function with a
AM> group of Gaussian functions?  I would appreciate if someone give
AM> some advice.  Yours, Ahmed

I have used the XANES peak fitting option some in Athena and would concur with Bruce's suggestions that peak fitting needs to be handled in a *consistent* manner to be useful.  With that said, I appreciated Bruce adding the feature to be able to fit multiple arctangents because it can help tease out the relative ratios of mixed valence state metals.  However, one must be careful to not overinterpret these fits because of the sensitivity of the XANES edges to coordination changes as well as oxidation state.  With the appropriate set of standards this fitting can be a useful tool especially when the data quality does not permit a full blown EXAFS analysis.

In regards to the other peak shapes (Lorentzian or Gaussians)....use what fits best.  This fitting is just a tool to help describe your datasets and is not a fundamental approach to XANES interpretation.  

One other suggestion, be patient with the fitting and play around.  Try multiple initial guesses, don't let all variable float in the initial fit, consider fitting the energy first and then fix it and fit the other variables.  Once you get a good fit, use this solution as the initial guesses for all your other datasets.

And....one other feature request for Bruce for Athena version 2.3.  It would be nice if we could put constraints on each of the fitting variables.  This would help prevent fits where the variables quickly spin off into meaningless territory.  Of course, implemenation of this feature could get complex given the number of variables to be fit.  An easier implemenation of constraints would be a checkbox and a % deviation field that would let the variables chosen float within that %deviation.

Dave
  _________________________________ 
  David Barton 
  The Dow Chemical Company 
  Catalysis R&D 
  1776 Building 
  Midland, MI 48674 USA 
  Email: dgbarton at dow.com