[Ifeffit] Debye factor and S02 correlation

[Matt Newville]

Hi John,


> >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?

This seems like a fine approach to me, and it could be automated,
at least somewhat.  Feffit/Ifeffit have Ei as a path parameter,
so setting So2 to 0.9 and floating Ei is possible right now.
Using Ei instead of So2 has a couple possible benefits:
  1.  Not completed correlated with N (though see below!)
  2.  Can accomodate measurement resolution issues.
  3.  Might be a little easier conceptually on the new people.

For one variation on the canonical 'Cu 10K first shell fit',
with sigma2 varying as well, I get these values:
    So2              Ei            chi_square  r-factor
  0.93(0.03)      0.00(fix)          119.      0.0017
  1.00(fix)       0.55(0.25)         128.      0.0018
  0.95(fix)       0.14(0.24)         120.      0.0017
  0.90(fix)       -.29(0.23)         116.      0.0016
  0.85(fix)       -.75(0.23)         116.      0.0016

  0.87(0.14)      -.58(1.27)         116.      0.0016

The last one had both So2 and ei (and sigma2!!) floating. The
correlation between them was 0.98, higher than that between So2
and sigma2, which was a mere(!) 0.94.  So it looks like, for this
data, So2 really does want to be ~0.90 +/ 0.05, and Ei wants to
be 0.00 +/- 0.25.

I guess these results aren't all that convincing.  Still, this is
one simple test, and this approach may be worth considering,

--Matt

PS: I'll respond to more ifeffit mail soon!