[Ifeffit] S02 selection from reviewer

[Jeff Catalano]

Peng,

I think what everyone has said is quite useful, but there is likely 
another aspect to consider as you make revisions. Based on the reviewer 
comments, it sounds like you are trying to interpret your fitted N 
values as being statistically distinct. My guess is that is not correct, 
as Matt suggested, and the fact that the N is lower in fluorescence 
suggests that overabsorption is attenuating the EXAFS amplitude, as 
Matthew suggested.

If you really want to make the claim of differences in coordination 
environment then you should also look at R. A lower coordination number 
should be accompanied by a decrease in R. How R shifts will be impacted 
by the resolution of the data, determined by your k-range, but I think 
if fitting a single Sb-O path then in most cases you should see R change 
if N is truly going down. It can get more complicated, clearly, and the 
S02 you are using may not be a perfect value, as Shelly's paper 
indicates, but if N is truly going down in one sample then R definitely 
should be changing.

I do not think about Sb very often, so if the only way for coordination 
to change is from a change in redox state then you should see a shift in 
the XANES spectrum. However, be cautious here because overabsorption 
will increase the intensity of a XANES spectrum for normalized 
absorbances below 1 and decrease the intensity above 1. If the XANES 
fine structure features of all samples are the same (in position) but 
you see what I describe in the overall absorbance, then your 
fluorescence samples is likely just suffering from overabsorption 
effects and your N will appear smaller. This gives a lower apparent S02, 
which is likely what the reviewer is getting at.

My sense is that what the reviewer is really criticizing is your 
interpretations based on differences in fitted N values, which may not 
be valid.

Jeff



On 10/2/2021 9:48 AM, Kelly, Shelly D. wrote:
> Hi Peng:
>
> It might be helpful to understanding some of Matt's points regarding S02 transferability, Ei and energy resolution by looking at this paper.
>
> Comparison of EXAFS foil spectra from around the world
> DOI: 10.1088/1742-6596/190/1/012032
>
> Kind regards,
> Shelly
>
> -----Original Message-----
> From: Ifeffit<ifeffit-bounces at millenia.cars.aps.anl.gov>  On Behalf Of Matthew Marcus
> Sent: Saturday, October 2, 2021 9:04 AM
> To:ifeffit at millenia.cars.aps.anl.gov
> Subject: Re: [Ifeffit] S02 selection from reviewer
>
> 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.
> 	mam
>
> 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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