[Ifeffit] S02 selection from reviewer

Matthew Marcus mamarcus at lbl.gov
Sat Oct 2 09:03:46 CDT 2021

Since S02 is a parameter in the description of EXAFS and not of the 
experiment, it's independent of technique.  Overabsorption (misnamed 
'self-absorption') can reduce the *measured* amplitude, an effect which 
can be fudged in analysis by reducing S02.  If the sample is truly 
homogeneous (on the scale of the absorption length), then you can 
calculate the amount of overabsorption to see if it's significant. 
However, many kinds of samples, such as concentrated powders mixed with 
a diluent such as BN, this condition is not met.  If the particles are 
large enough for each to have significant absorption edge jumps, then 
diluting them in BN doesn't fix the problem.

On 10/2/2021 12:49 AM, Peng Liu wrote:
> Dear IFEFFIT members,
> I am sorry to bother you again. I asked about S02 selection for the 
> first major revision. I just received the second revision. The reviewer 
> is not satisfied with one S02 value for all our samples.
> "
> 1. I am still not satisfied with selected SO2 value (it is set to 0.85). 
> SO2 is not transferable between different samples and detection methods. 
> It is not possible to use a value obtained from different compound using 
> transmission measurement mode to completely different other compound 
> measured using fluorescence mode. One method to fix SO2 value is to 
> measure diluted solution (to avoid self-absorption) of reference 
> material in fluorescence mode. Other is to use multiple spectra fitting 
> for all samples of interest (e.g. with Sb(V)) measured using 
> fluorescence mode where SO2 parameter is the same for all samples.
> At the same time I am confident that CN values 5.6, 7.1 and 6.9 
> correspond to CN(Sb-O)=6. I suggest reconsidering the SO2 value for 
> measurements in fluorescence mode.
> "
> We do get the S02 from a similar reference material measured in 
> transmission mode, and our samples were all measured in fluorescence 
> mode. It is not possible to measure the diluted reference material in 
> fluorescence mode in one or two months. If you could give me some 
> suggestions, that would be great.
> -- 
> Best Regards,
> Peng Liu
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