[Ifeffit] Too many points in pre-edge

[Bruce Ravel]

On Thursday 02 December 2004 02:17 pm, Barton, David (DG) wrote:
> Carlo and Matt:
> > With these files, the location of the edge (and element
> > identification as well) using the first derivative
> > does not work.
>
> I also had a similar problem when we had a somewhat noisy pre-edge and edge
> data.  Often our analysis of the XANES requires high confidence in the edge
> energy to determine energy shifts and to accurately align our spectra using
> metal foil references.  Of course, I can always manually assign the edge
> position in Athena, but it would be nice if Ifeffit was a bit smarter about
> how it chooses Eo since it is critical for Athena operation when a
> reference standard is used to calibrate a sample spectra.  I would guess
> that there is some robust algorithm that one could implement which would
> more accurately determine either (1) the maximum in the first derivative or
> (2) the zero of the second derivative for data with any number of data
> points or noisy data.  I imagine that such an algorithm would include some
> sort of adjustable "smoothing" parameter to filter the signal from noise.

Carlo's problme seems to be a little bit different.  His data are not
particularly noisy.  The problem appears to be correlated to the
length in steps of the pre-edge.

I, too, would be interested in a perfectly robust against noise
algorithm for finding the edge energy.  However, I suspect that when
someone builds a better algorithm, somoeone else will build noisier
data ;-)

Athena does have a feature that might be some help in this case.  In
the "bkg" section of the preferences, one of the parameters is "e0".
This is set to "derivative" by default, telling Athena to use
Ifeffit's first derivative based algorithm to determine e0.  Unless
the noise in the pre-edge is really large, the "half" option for that
parameter might be helpful.  It's described right there on the
preferences page.

Another things that might help is Athena's pre-processing of data as
it's imported.  If all goes well, then you will have to set e0 by hand
for the first data set, but choosing the "set all parameter values"
option in the processing dialog should set all the rest to reasonable
values.

In any case, if you have noisy data, you really must be prepared to do
hands-on data analysis.  Ifeffit and Athena will only ever be so
smart, but data can be arbitrarily crappy!

B

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