[Ifeffit] Re: Truncate, Post-edge Athena Issues

[Bruce Ravel]

On Tuesday 20 April 2004 11:56 pm, Barton, David (DG) wrote:
> One final question.  I often see a strong discontinuity near the edge
> energy in the background subtraction.  The attached Athena project clearly
> shows this discontinuity, any thoughts about this?  

Dave,

I am CCing the answer to this one to the mailing list.

The background spline is evaluated between the energies (or k values)
specified.  Outside that range the spline is not evaluated at all.

To make the arrays conatining the data and background spectra easier
to handle when interacting directly with Ifeffit, Matt wisely chose to
make the background array the same length as the data array.  At
energies within the spline range, the background array contains the
background spline.  Outside the spline range, that background array
contains the original data.  Not only does this make the arays the
same length, but it makes computation of chi(E) completely trivial --
you can just subtract the background array from the data array point
by point.

If the first or last few points of the spline are not near in value to
the data (which can happen since a spline can be poorly constrained at
the ends), then the plot of the data and background arrays will be as
you described.  The background array will have a discontinuity, but
that is not a bug.  Regardless of the constraint of the spline near
its ends, the criterion for choosing the spline is met.  That is, the
spline is always chosen to minimize the low-R Fourier components
(possibly with respect to a standard spectrum).  Sometimes this
minimization results in a spline that deviates from the data at the
ends.

Constraining that deviation is the purpose of the spline clamps.

> I typically have ignored this since it is outside the EXAFS fitting
> region and the XANES background subtraction routines are not
> affected by it.

That is completely correct.


HTH,
B


-- 
 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
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