[Ifeffit] Using the amplitude reduction factor as a linear combination fitting parameter
Bruggeman Christophe
cbruggem at sckcen.be
Wed Apr 30 01:50:45 CDT 2008
Dear Wayne,
Thanks for the reply! This is indeed a drawback; if I just fit the
spectra without constaint on the S02 values for the two U species (i.e.
giving them "x" and "1-x"), the sum does not add up to one, but rather
to 0.8 - 0.9. I would be a good idea to defend this value by comparing
with some standards.
Thank you,
Christophe
-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Wayne
Lukens
Sent: dinsdag 29 april 2008 16:44
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] Using the amplitude reduction factor as a linear
combination fitting parameter
Dear Christophe,
The one drawback with using S02 in the way that you describe is that
in so doing, you are assuming the true value of SO2 is 1. If you have
a good reference spectrum and your reference is a well-defined solid,
you can fit the reference spectrum to its known structure and determine
SO2 for the experiment. You could then define a parameter, x, in
artemis that is the fraction of UO2 then (1-x) would be the fraction of
uranyl. The coordination number of each UO2 shell would be multiplied
by x and each uranyl shell by (1-x).
For what it's worth, SO2 for uranium oxides using Feff7 always seems to
be around 0.9 for us if I recall correctly.
Sincerely,
Wayne
--
Wayne Lukens
Scientist
Lawrence Berkeley National Laboratory
email: wwlukens at lbl.gov
phone: (510) 486-4305
FAX: (510) 486-5596
Bruggeman Christophe wrote:
> I have recently collected EXAFS spectra of uranium on a FeS2
> surface. Using principal component analysis of the XANES and
k3-weighted
> EXAFS spectra, I have found that there are two uranium species which
> compose the spectra. As a first tentative guess, I believe these two
> uranium species are uraninite (UO2(c)) and a uranyl species. I would
> like now to fit the fourier transform functions (real parts and
> magnitudes) using the theoretical paths and path degeneracies created
by
> feff, and use the amplitude reduction factor S02 as a fitting
parameter
> to derive the relative amounts of the two uranium species in my
samples.
>
> Normally, this S02 is taken as a constant (between 0.7 and 1.0), and
the
> path degeneracies are fitted. So normally, S02 is not really a fitting
> parameter (some papers derive it even with theoretical functions).
> However, given the fact that S02 and N are completely correlated, I
> think it is justified to use this approach.
>
> Can someone comment on this?
>
> Many thanks in advance,
>
> Christophe
>
>
>
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