[Ifeffit] Debye factor and S02 correlation
John J. Rehr
jjr at leonardo.phys.washington.edu
Sat Nov 2 12:11:17 CST 2002
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
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?
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