[Ifeffit] Re: Amplitude and the EXAFS equation

[John J. Rehr]

Dear FEFF and IFEFFIT users,


 As Matt notes,

>Feff tries to estimate many of the the physical processes
>identified as So2.  It also estimates lambda, but much more
>crudely: it interpolates a fit to the *figures* from Rahkonen and
>Krause (1974).  Feff definitely does not include the energy
>resolution of a Si(111) monochromator at 9keV.

  One of the biggest problems with the current FEFF calculation is that
the mean free path lambda is calculated from the Hedin-Lundqvist
electron gas self energy in the plasmon-pole approximation. This
often overestimates the inelastic losses (too short mean free paths).
The consequence of this is that the effective So2 must often be
increased to compensate. For this reason, it is not unusual to
see fits of So2 that that can exceed unity.

  Fortunately improvements are in sight. Luke Campbell's thesis
focused in part on calculations of So2, and he was able to explain
the So2 for Cu (about 0.9) to within a few percent.  Also efforts
are being made to improve the self energy. However, at present
the theoretical So2 is typically 0.9 +/- 0.1. This of course,
does not take into account possible errors in the value of 
the edge jump and corrections for experimental resolution which
can affect fits to So2.

  John Rehr