[Ifeffit] S02-Interpretation

[Matt Newville]

Hi Norbert, 

Bruce and Shelly already answered most of this, I think, but I'd  
like to add a couple of points.  You said:

> To my knowledge, S02 accounts for intrinsic as well as extrinsic
> losses in the sample.

Yes.  It also is used in the analysis to generally take up any slop
in normalization or differences in energy resolution, which is why
it can depend on 'beamline'.

> Now, if I have two samples and determined the S02 value according to 
> the way which Bruce has in his supplement to the FEFFIT course, 

How do you mean this?  Is this the 'plot three curves of sigma2 v.  
S02 curves for three different k-weights' again?  I find this
approach puzzling and dangerous.  I do not understand why the slope
(correlation) of sigma2 v. S02 should depend on k-weight in any
systematic way -- does anyone else know why it should??  What you
want is the S02 and sigma2 that gives the lowest chi-square, not
where these lines cross.

> I find two differing values for electrochemically prepared oxide
> (0.74) and crystalline gold oxide (0.92). This difference is
> independent of beamlines I measured and I can exclude quite safely
> that it is just a measurement error - it comes up with any
> spectrum I recorded on these systems.
>
> When discussing this with my colleagues, we interpreted the S02
> difference as an indication of more disorder in case of the
> electrochemical system (which would well fit to my picture of the
> whole thing).

Yes, a consistently low S02 can indicate vacancies or a component to
the disorder that is so large that it cannot be expressed with a
simple sigma2 (or even higher order cumulants).
 
> BUT: Is there any physical reason to assign a difference in S02
> (for "chemically equivalent" systems, where just the preparation
> is different) to a structural disorder?
 
In general, no.  There are some very subtle loss effects that might
differ, but these would be well within the 'normal' slop in the
measurement and data reduction that show up as changes in amplitude.

--Matt