[Ifeffit] rbkg

[Silvio Levy]

Hi, I'm a new reader of the list.  Thanks for all this very useful
software and impressive documentation.

I'm having trouble understanding the role of rbkg in spline (or
autobk).  Ravel's Exafs Analysis with FEFF and FEFFIT describes it as
"the cutoff between the portion of the Fourier transformed spectrum
dominated by the background function and the portion containing
information about the local structure".

This would seem to imply that after removal of the background spline,
the signal will have NO frequencies below rbkg -- that is, in the R
domain, it would be 0 (or close to 0) for R < Rbkg.  But this is not
quite what I see; varying Rbkg has a large effect throughout the plot
of chir_mag versus R. In particular I observe sometimes that with Rbkg
= 1.3 (say) I get a very different R-plot than with either Rbkg = .9
or 1.5 -- not just at low R but throughout.  And yet the Rbkg = .9 and
1.5 R-plots look similar to one another, much more than to R-plot for
the intermediate value of R.  Moreover changing Rbkg shifts the peaks
in the R-plot noticeably.

What does the algorithm actually do with this parameter?  It would be
nice to understand that without having to decipher the code.  Forgive
me if this information is given somewhere, but I couldn't find it.

Note: I'm working with compounds for which I don't yet have a good
theoretical model, so I don't know how to choose between Rbkg values
based on the position of the peaks.

Thanks

Silvio Levy