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!