[Ifeffit] schemes for delr and sigma2 for multiple scattering paths

[Matt Newville]

Dear Jatin,

I tried to answer both of these question in chapter 3 of
    http://xafs.org/Tutorials?action=AttachFile&do=view&target=Newville_xas_fundamentals.pdf

Does the explanation there help?   If not, do you have any suggestions
for making it clearer?

The basic physics is that an absorbed x-ray creates a photo-electron
which will scatter from its neighbors.   A portion of the scattered
portion (so, a small amount) of that photo-electron will return to the
absorbing atom.  But Pauli's exclusion principle tells us that if an
electron is at the absorbing atom, and has the same energy (and other
quantum numbers) as the outgoing photo-electron, the outgoing
photo-electron cannot be created in the first place.   Thus, the
scattering of the photo-electron from the neighbors affects whether
the photo-electron can be created, and so whether the x-ray will be
absorbed.

The effect can be either destructive or constructive, as with nearly
any wave-like phenomenon.    It does seem a bit circular that the
photo-electron scattering affects the existence of the photo-electron,
but quantum mechanics leads to many much stranger behaviors.

--Matt