[Ifeffit] Node in chi(k) envelope for a single shell

[Matt Newville]

Dear Silvio, Wayne,

Yes, what Silvio is seeing is a 'perfectly normal' minimum in
backscattering amplitude for scattering from heavy atoms.  It actually
shows up for many elements with Z>35, but at such low k-value that it
is lost in the XANES features, and is really only noticeable in EXAFS
for Z>40 or so.  It is definitely visible in Ag (Z=47), and for Z>60,
it is very strong, with the minimum in scattering at ~5 to 6 Ang^-1.
This often gives rise to a 'double peak' in |chi(R)| even thought
there is really only one shell.

This effect is a function of Z for the *backscatter* -- not the
absorbing atom.  So it will show up for metallic Hg, but not in the
first shell for the Hg edge of HgS.

As Wayne said, this is normally called a Ramsauer-Townsend resonance,
though that term was originally about electron scattering from noble
gases.  See
  http://en.wikipedia.org/wiki/Ramsauer-Townsend_effect
for a little more information.

The way I think about this is that when the outgoing photoelectron has
a wavelength near 1 Ang (ie, k ~=6 Ang^-1), it may be able to "tunnel
through" the very deep, very small scattering potential from a very
heavy element.   This will only happen at a fairly distinct
wavelength, and so gives a 'resonant dip in scattering'.

Hope that helps,

--Matt