[Ifeffit] High SO2

[Scott Calvin]

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