[Ifeffit] What does FEFF stand for?

[Bruce Ravel]

On Tuesday, May 10, 2011 03:03:23 pm Scott Calvin wrote:
> My understanding, although I could be wrong is that the "effective"  
> part came from an improvement of the theory to account for curved-wave  
> effects. In other words, early theories approximated the photoelectron  
> as a plane wave, but of course it spreads out radially from the  
> absorbing atom. That change necessitated tweaking the definitions of  
> the factors, so it became the "effective" f.

I think you are mistaken.  My memory of the etymology has to do with
the formalism dating back to Feff5 for computing MS paths.

For a purely single scattering theory, you have an F and a phi
(without the subscript eff).  That is, you can simply compute the
scatting function for the one scatterer and be done with it.

Feff's path expansion introduced two clever things to the EXAFS
business.  One is that it provided a formalism for computing a single
function that takes into account the angle-dependent scattering
functions of all atoms in an arbitrary-geometry multiple scattering
path.  This allows one to treat a MS path with the familiar SS EXAFS
equation only by replacing F and phi with F_eff and phi_eff.  That
innovation is central to how Ifeffit works.

The second clever thing is that it's really fast.  That's not such a
big deal today, but back in the mid-90s, when a Feff run could take
several minutes, a faster algorithm was very welcome indeed.

B

-- 

 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/