[Ifeffit] beginner's question

Bruce Ravel bravel at anl.gov
Fri Nov 4 16:55:52 CST 2005

On Friday 04 November 2005 16:23, Ling Fei Zhang wrote:
> May I ask you do me a big favor to let me know when plan EXAFS experiment
> for comparison of local structural enviroments around certain say, Cu
> atoms, how to evaluate the accuracy in term of atomic spacing in angstroms
> for instance?

It's possible I don't quite understand your question, but.... I'll
give it a try.

In any experiment that relies heavily upon theory and upon non-linear
minimization (EXAFS certainly falls in that category, as do lots of
other measurement techniques), one is wise to question accuracy.  To
my mind, the only way to satisfy oneself is to test the analytical
apparatus against a well known standard.  To use your example, you
could test your theory and analysis software with copper metal and
copper oxide.  Let's assume that you can do the analysis on the
standards and obtain good fits with physically reasonable parameters.
If the interatomic distances then match what is known from, say,
diffraction within the reported error bars, then you can have some
confidence in the accuracy of the analytical apparatus.

What I described is exactly what those of us who write this stuff do
with our codes as we develop them.  The codes must give answers to
questions for which we know the answer before we can trust the answers
to questions for which the answer is previously unknown.

Of course convincing yourself of accuracy in the case of an unknown --
almost all research problems are unknown, that's why they are research
problems! -- can be daunting.  There are all sorts of empirical and
modeling errors which can have an impact on the accuracy of a
measurement.  Such concerns have to be addressed for each individual
analysis problem.  But certainly knowing that the codes work on the
standards is a good place to start.


 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advance Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel 
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

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