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 cumulants. HTH B -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/