[Ifeffit] iron carbide fitting with Artemis

fred.mosselmans at diamond.ac.uk fred.mosselmans at diamond.ac.uk
Fri Oct 27 01:24:57 CDT 2017


Dear Yunyun,
I don't think Mike has written anything particularly incorrect , but he didn't point out why this approach is necessary. 
I believe there are two things you should definitely be aware of.
An EXAFS dataset has a very limited amount of information. Approximately deltak.deltaR no of variables can be fitted 
To any spectrum as an absolute maximum and preferably quite a lot fewer than this (where deltak and deltaR are your fitted data range in k and R space respectively.
Because of this if say you know you have N C atoms at ca G angstroms around your central atom. It is best not to use a variable for fit the degeneracy for that path but set it at the number you know is correct.

And secondly because an EXAFS spectrum is effectively a collection of Sine waves there is a limited resolution, i.e. you cannot resolve paths of the same atom type that are less then Pi/2k(max) A apart, where k(max) is the maximum range of your dataset in k space, and Pi is Pi. Hence to separate two paths of the same atom that are 0.1 A apart you need about 15k worth of data. 

Regards
Fred



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