On Apr 10, 2009, at 12:28 PM, Matt Newville wrote:
In these cases, It is generally better to tell the fit to look only at the higher shell so that the inherent misfit in the first shell (inevitable, and generally of little importance to the result) does not dominate the total fit uncertainty. In short, it speeds up the process (both for the computer and the human) to fit only the second shell.
I find myself flipping back and forth right now between completely agreeing (I've done things somewhat like what you describe in my own fitting) and thinking that we've changed the discussion to a different kind of system. Let me take a stab at some circumstances in which I would use a fit to the first shell to fix parameters for analyzing other shells: * When there are a couple of distinct possibilities for the first shell, and I want to distinguish between them. For example, it might be tetrahedrally or octahedrally coordinated. But in a circumstance like that, I still don't quite use the results from the first shell fit directly. If the first-shell fit yielded a coordination number of 6.3 +/- 0.7, I'd use 6, figuring it was octahedral. Using 6.3 would seem to me to be a pointless introduction of error. * When the first shell is not important to me, but the second shell is. This often happens in my work because there may be a little oxidation on the surface of a metal nanoparticle. The monolayer of oxide may be very puzzling structurally and not of much interest. If I can get some rough value for it and then fit from the second shell up, I can get at the metal structure I'm interested in. But in that case, I wouldn't use the S02 and E0 from the first shell, since the second shell is better understood. * When the first shell is dominant in the spectrum, and thus the fit tries to use the second-shell paths to fine tune the first-shell feature rather than to fit the second-shell feature. I think this is essentially what you're describing. In that case, I tend to not only set the parameters for the first shell when looking at the second, I also move the fitting range so that it doesn't start until above most of the first-shell feature. In those cases, I'd agree with "valid and recommended." But this discussion started with a copper foil. And I've seen students at workshops applying it to systems of their own that are more like the copper foil (e.g. metal alloys) than the system you described. And it's always puzzled me. For cases like that, it seemed like the person was making more work for themselves by the back-and-forth of guessing and setting than by just extending the fit in a straightforward fashion. --Scott Calvin Sarah Lawrence College