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-boun...@millenia.cars.aps.anl.gov> On Behalf Of Matthew
Marcus
Sent: Saturday, October 2, 2021 9:04 AM
To:ifeffit@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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