[Ifeffit] Ifeffit Digest, Vol 170, Issue 19

[Matt Newville]

Hi Chris,

On Thu, Apr 20, 2017 at 5:02 PM, Christopher Thomas Chantler <
chantler at unimelb.edu.au> wrote:

> Note that in general any and every smoothing operation reduces the
> information content of the data and its ability to reveal structure.
>

Well, maybe.  If one has mu(E) measured every 0.01 eV over an 600 eV EXAFS
scan to k=12A^-1, one does not really 60,000 independent measures of the
structure.  Yuji's data wasn't that absurd, but it did have close to 4000
measurements for the full EXAFS spectrum out to k=18^-1.  And to be clear,
there is not anything wrong with that, it's just a matter of how you decide
to treat it.

As the plots attached in Yuji's original message and my replies show, the
resulting chi(k) definitely has is amplitude suppressed when doing a simple
boxcar average of data onto the 0.05 Ang^-1 grid (Athena with Ifeffit).
But when using cubic spline interpolation (Larch), the amplitude of the
EXAFS oscillations are not noticeably suppressed, though the high frequency
noise is also much higher.  Applying a Savitzky-Golay filter prior to the
cubic spline interpolation did not appreciably suppress the EXAFS
oscillations though the high frequency noise was reduced.

Very finely-spaced energy data might reveal is at much higher R than we can
hope to model with EXAFS.  A k-grid of 0.05 Ang^-1 can give frequencies to
31Ang, and so is probably accurate to 16Ang without significant signal
loss.   That is, out to k=18Ang^-1, you really only need 360 samples, but
you'd like these as noise-free as possible.  Having 3600 measurements on a
grid of 0.005 Ang^-1  might give you data out to 160Ang in principle, but
the photo-electron tends to not cooperate.

--Matt
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