[Ifeffit] Question about transform windows and statistical parameters

[Shelly Kelly]

Hi Brandon:

I have a different idea on what might be happening. Take a look at the
window function and the data in k-space for both window functions. The
kaiser-bessel window works in a different way from the Hanning window. They
emphasize different regions of the data differently. The sill part of the
window is defined differently.  Usually use a dk = 1 or 2 A-1 with a Hanning
window. Take a look at the kq data showing the signal q that is used in R
for the fit.  The Kaiser-Bessel window works best with dk values of 3 to 4
A-1. This results in a slowly giving more importance to data in the k-range.
Again look at the kq data to see this effect.

If you use a Kaiser-Bessel window with a small dk then the data used in the
fit will extend more (as compared to Hanning) beyond the k-range set by kmin
and kmax.  Same for the Hanning window, but with a large dk value as
compared to K-B.
So by simply changing the window without changing dk, you are changing the
information that is Fourier transformed and that results in slightly
different values and all the rest.

If you need an example: I can send you my book chapter :).  It has an
excellent section on FT and all the buttons that go with them.

Kelly, S. D., Hesterberg, D. and Ravel, B. (2008). Analysis of soils and
minerals using X-ray absorption spectroscopy. Methods of soil analysis, Part
5 -Mineralogical methods. Ulery, A. L. and Drees, L. R. Madison, WI, USA,
Soil Science Society of America: 367-463.

Cheers,
Shelly

On Wed, May 11, 2011 at 2:46 PM, Brandon Reese <bjreese at gmail.com> wrote:

> Hello everybody,
>
> I am working on fitting some EXAFS of amorphous materials and have noticed
> an odd (in my mind) behavior when changing transform windows.  I settled on
> a fit using all three k-weights and the Hanning transform window obtaining
> statistical parameters of R=0.0018 and chi_R=361.  I decided to change the
> transform window to a Kaiser-Bessel to see what would happen.  The refined
> parameters came out more or less the same, well within the error bars, with
> the Hanning windows having slightly smaller error bars.  But my statistical
> parameters changed significantly to R=0.0022 and chi_R=89.354.  It seems
> that this large change may be related to why we can't use the chi_R
> parameter to compare fits over different k-ranges, but I am not sure about
> that.  Have other people seen this?  I would guess it means that when
> looking for trends in different data sets, it is more important to be
> consistent, rather than which specific window type is used.
>
> Thanks,
> Brandon
>
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