[Ifeffit] S02-Interpretation

Bruce Ravel ravel at phys.washington.edu
Wed Apr 14 09:49:40 CDT 2004

On Wednesday 14 April 2004 09:13 am, Norbert Weiher wrote:
> 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).
> 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?

I think there can be a subtle (and energy dependent) effect to S02
which depends on the fine details of the structural and energy
environment.  Perhaps John could comment on that.

There is an ambiguity when you use the word "S02".  There is a
physical quantity S02, which is exactly as you say -- a loss term.
More than that, it's a loss term which Feff8 makes a good stab at
calculating.  While S02 is clear in the EXAFS equation, it is not in
practice.  Ifeffit has no way in general of distinguishing between
S02, coordination, normalization errors, and empirical effects.  All
of these things are essentially constant amplitude terms in the
measured data.  In the fit, you are only measuring S02 if you have
reason to believe that you know the coordination, that you normalized
your data correctly, and that your detectors and electronics were all
behaving properly.  The extent to which S02 is different for two
nominally similar samples probably has more to do with coordination,
normalization, or empirical effects than it does with differences in
the physical quantity S02.

Given the information in your message, it would seem that coordination
is the most likely issue here.  While the might be subtle S02
differences between the crystalline and electrochemically prepared
samples, your difference in S02 is quite significant (although you
didn't mention error bars, which are always relevant!).  You once told
me that gold oxide is subject to oxygen vacancy.  You could interpret
your difference in S02 as the electrochemical sample missing about 1
oxygen atom in 6.  Or the distribution of oxygen atoms might be poorly
described by the cumulant expansion in the electrochemical sample,
which could result in an amplitude attenuation when measured by


 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
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 Naval Research Laboratory                          phone: (1) 202 767 2268
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 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/

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