[Ifeffit] CN and bond distances in Artemis

[Scott Calvin]

Hi Juan,

Sorry for the slow response; the end of the term gets busy for me!

I think there are still some loose ends in this 
discussion that are worth trying to tie up:

At 03:03 PM 12/11/2006, you wrote:


>First of all, I would like to thanks Anatoly for his file and everybody for
>useful comments. I have analysed Anatoly's data 
>and I have obtained a good value
>for S02 = 0.85 or 0.82 (depending on the number 
>of variables used). So, my data
>is the problem, and it is not my analysis, but 
>maybe my measurements need a more
>accurate analysis with Athena as Scott suggested.
>I was not at synchrotron measuring platinum samples and I only know that are
>measured in fluorescent mode. As Bruce said, I have no beamtime now for more
>measurements.
>
>I have more questions, related and not related to the last subject, but I am
>still thinking about them:
>The first one is easy, it is about the Nyquist theorem. I read in a paper that
>the formula is 2·deltak·deltaR/pi + 2. The last "+2" is new for me and I am
>afraid that Artemis does not consider it. I am sure that it is a silly thing.

For a while, arguing over this +2 (or +1 or +0) 
was a popular topic in the EXAFS community. 
Eventually it was realized that there isn't 
really as much information as implied by the 
Nyquist criterion anyway. Crudely, the Nyquist 
criterion assumes you have someone trying to 
convey as much information as possible in a 
signal. Nature isn't so obliging. So it's 
becoming more common to leave the +2 off, and 
even that is not conservative. If you're running 
out of independent points, introducing more 
constraints, extending the k-range, or extending 
the r-range can be better ways of getting 
yourself out of trouble than invoking the +2.

>I will try to correct again Ptfoil considering self-absorption in order to
>obtain a spectrum similar to Anatoly's or Bruce's one. And then, I will apply
>the same correction for supported platinum catalysts, right?

Since your samples have a low concentration of 
Pt, the self-absorption correction should not be 
necessary for them. You've talked about changing 
the variable from S02 to N in subsequent fits to 
obtain both variables...if I understand you 
correctly, that won't accomplish anything. If it 
were that "easy," Ifeffit would include it in its 
fitting algorithm! S02 and N for a single-shell 
single-sample fit are 100% correlated and values 
cannot be obtained for both no matter what you 
do. I think the best you can do is fix S02 at 
some plausible value (0.85, say, or the result of 
a FEFF calculation, or whatever), and then 
realize, and explicitly note in publications, 
that this assumption introduces an uncertainty of 
perhaps 10% in N, in addition to whatever uncertainties are found by Ifeffit.

>I also observed that Anatoly's Pt foil shows good signal even for large k (20
>A-1). Nevertheless, obviously platinum catalysts spectra possess lower signal
>and specially for high values of k where the noise is big. The question is,
>despite Pt foil has a good signal until 20 A-1, it is usually used a smaller
>k-range (i.e. 3-12 A-1), right? I normally use a k-range of 3-12.

As Matt said, use the data to guide you when 
choosing k-range; not some arbitrarily chosen 
range. I also find it useful to try varying my 
k-range a bit after the fit is done to check that 
the results are stable. Of course, if you are 
visually comparing Fourier transforms of 
different samples, rather than performing fits, 
you want to compare over the same k-range.

Finally, at the risk of repeating myself, I'm 
going to suggest that your system sounds like it 
would benefit from a multiple-shell fit. That's 
pretty easy to do for a metallic cluster like 
platinum. And it reduces some of your problems. 
It's hard, as you've noticed, to determine N for 
a single shell, in part because you have to know 
S02. But since S02 is the same for all shells, 
it's easier to determine the ratio between N for 
the first shell and N for the second shell. 
That's still a little dicey because sigma2 is 
likely to be far different for the first two 
shells, and sigma2 correlates to N (but not 100% 
correlation; the effect of sigma2 depends on k, 
and N does not). If you get to three shells, 
though, then the ratios of N3 to N2 to N1 start 
to get teased out from the other effects, and you 
can start to determine things like crystallite 
size, which it sounds like is the thing you're after.

That's the principle that both Anatoly and I have 
used in the past to find the size of 
nanoparticles or nanocrystals. Our methods differ 
in detail--Anatoly's is better for good data and 
highly uncertain morphology, because it assumes 
less; mine is probably better for iffy data and 
roughly known (e.g. "spherical") morphology, 
because mine has fewer free parameters. A search 
of the literature will reveal several articles by 
each of us detailing how to do this kind of 
analysis, including the APL I mentioned earlier 
on platinum nanoparticles and a JACS article of 
Anatoly's on platinum-ruthenium nanoparticles.

--Scott Calvin
Sarah Lawrence College