Dear Fred,

Thank you very much for your help. Your explanation does help me understand
the theory behind. It also reminds me to be careful of using some
parameters for future fitting analysis. Thank you so much and have a great
weekend!

Best,

Yunyun

On Fri, Oct 27, 2017 at 2:24 AM, <fred.mosselm...@diamond.ac.uk> wrote:

> 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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