[Ifeffit] Questions about fitting of EXAFS

Fuxiang Zhang zhangfx at umich.edu
Thu Aug 4 13:50:36 CDT 2016


Thank you all for the answers.
The Kmax effect on the shape of magnitude of FTs of one of my samples may
come from the low signal/noise ratio.
I still not sure if there's any physical meaning of DeltaE0, Does it relate
to the shift of absorption edge?

Fuxiang

On Thu, Aug 4, 2016 at 1:35 PM, Scott Calvin <dr.scott.calvin at gmail.com>
wrote:

> I’ll add to Bruce’s comment that if the choice of kmax has a great effect
> on the parameters found by the fit, that’s an indication of a problem! Good
> fits should be stable to modest changes in kmax (e.g., an inverse angstrom
> or two one way or another). The statistics may suggest one kmax or another
> is somewhat better, but the fitted parameters should not be drifting
> outside of the ranges defined by their uncertainties. If they are, you have
> an unstable fit. In that case, there are several possibilities: perhaps you
> are including an artifact in your k-range (e.g. the beginning of another
> edge) or a lot of data dominated by noise. Or perhaps your model is having
> trouble distinguishing between two fitting minima, and is flipping back and
> forth between them (this is more likely if your fitting model is fairly
> complicated, with many free parameters). In any case, if you’re fit is
> highly sensitive to kmax you should investigate to try to determine why.
>
> —Scott Calvin
> Sarah Lawrence College
>
> > On Aug 4, 2016, at 11:07 AM, Bruce Ravel <bravel at bnl.gov> wrote:
> >
> >>
> >> 2. The cutting range of Kmax (FT transform parameters) has great effect
> >> on FTs of EXAFS, how do I know to use the best value of Kmax;
> >
> > If you have measured data with signal well above the level of noise, why
> would you choose to use less data?
> >
> > Similarly, if, at some point in your data, the signal becomes dominated
> by noise -- either statistical or systematic -- why would you include it in
> the analysis?
>
>
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