[Ifeffit] Debye factor and S02 correlation
Matt Newville
newville at cars.uchicago.edu
Mon Nov 4 17:18:04 CST 2002
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!
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