[Ifeffit] Debye factor and S02 correlation

[John J. Rehr]

Hi Everyone,

  I agree with Matt's assessment:

> ... my view is that the
>errors in the calculated mean-free path may have a much stronger
>energy dependence than the errors in the calculated So2.  From
>the analysis point-of-view, I prefer to think of So2 as
>k-independent, and put all k-dependent pieces go into lambda(k)
>or F(k).  Feff tries to include all loss terms as well as it can.

  There are two ingredients in the mean free path lambda(k)

     lambda_k approx k/[Im Sigma(E) + Gamma]

1) The core hole lifetime Gamma - FEFF estimates this by interpolating from
tabulated values. This rough estimate should not lead to serious errors,
in my view, since a 10% error in the lifetime translates to
only few percent error at worst in the loss factor exp(-2 lambda R).

2) The self energy Sigma - FEFF uses an electron gas plasmon pole approximation.
which can easily be off by 1-2 eV. This is the most serious error
and leads to large energy dependent errors in lambda_k.

>So2 _is_ calculated (if crudely in Feff6 and 7 -- I don't know if

 FEFF6-8 will make an atomic value of So2  if one sets S02 = 0
in the HOLE card.  This approximation is unsatisfactory since it
can't account well for the valence electron contributions or 
interference effects which dominate So2. I don't think it can be
trusted - at least to better than 10%.

>Luke Campbell's work on improved So2 is included in Feff8 yet).
>So one might even expect that So2 should be 1, and even find it a

 Luke Campbell's routines are not in FEFF8 yet as they are not
yet fully automated or ready to be integrated. This will us take some
months. Moreover, Luke still uses a plasmon-pole self energy,
which we want to improve.

>little strange that it's usually around 0.9.  Clearly, there are
>loss terms that are not taken into account well enough in Feff.
>My view is that it's not well known whether this is dominated by
>an k-dependent term (ie, should be put into F or lambda) or a
>k-independent term (So2).

  The remarkable bit of physics here is that interference terms
between extrinsic and intrinsic losses and hence tend to suppress
shake-up/shake-off effects. 

  In any case, my suggestion in fits would be to treat the loss
terms using a constant and an effective mean free path:

a) set So2 = constant approx 0.9
b) fit the mean free path, e.g., by fitting the core-hole effective lifetime. 

   I'd be interested in feedback on this suggestion. Can it
be automated within FEFFIT?

  Cheers,
  John Rehr