[Ifeffit] spline fitting

[Matt Newville]

Hi Matthias,

I don't see a problem with this background removal (using Rbkg=1.0 and
kweight=3 as in your project file).  The mu0 spline does shoot up (to
perhaps a value of 3 for the normalized XAFS) whereas the data goes to
about 1.8 (again, for normalized XAFS).    Mu0 then drops down into the
range of the data within 5 eV.

This is fairly normal for the AUTOBK algorithm.  It's not even looking at
mu(E), but rather at the Fourier transform.  So to first order it doesn't
care (and, I'll argue, neither should you) about what happens at the end
points, especially at low k.  You're going to k-weight the chi(k) spectra
anyway.  That will multiply the low k portion of the spectra by a very
small number over the first few eV -- asserting that chi(k) below 1 or 2
Ang^{-1} doesn't matter.

You can use the "low energy spline clamp" to force the mu0 spline to vary
less at low energy.  Even setting this to "weak" in Athena should change
mu0 to be much closer to the data value at E0.  If anything, that probably
over compensates.

--Matt



On Fri, Aug 29, 2014 at 2:13 AM, Matthias Filez <Matthias.Filez at ugent.be>
wrote:

>   Dear mailing list users,
>
> I am trying to fit the spline background function in a proper way to a
> spectrum (Ga K edge, see attached).
> However, it doesn't seem to work out, but I don't see why.
>
> I already tried the following:
> * Vary the Rbkg value
> * Change spline k-weight
> * Change k-range of spline fitting
> * Alter the strength of spline clamps
> ... and the combination of these effects.
>
> Since there is a strong white line, I suspect this causes the spline
> misfit. The data quality is OK.
> If you have any idea, please post it.
>
> Thanks in advance,
> Matthias
>
> _____________________
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> Matthias Filez
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> Laboratory for Chemical Technology
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