[Ifeffit] phase problem copper
Neeb, Matthias
matthias.neeb at helmholtz-berlin.de
Wed Jul 15 10:24:18 CDT 2015
Dear Bruce,
thank you for your rapid answer and sorry for my unprecise question.
I thought the answer to my question must be very trivial and therefore I didn´t want to make an excessive story.
In fact the answer is trivial as I did not realize that the literature displays the DIFFERENCE between the phase function of the measured sample and a reference sample. The phase spectrum in Demeter, i.e. [Pha[Chi(q)] versus k displays the total phase shift of the backtransformed measured spectrum, not the phase difference of a model and the actual measured sample.
Again sorry for boothering you with this needless question.
Matthias
-----Ursprüngliche Nachricht-----
Von: ifeffit-bounces at millenia.cars.aps.anl.gov [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] Im Auftrag von Bruce Ravel
Gesendet: Mittwoch, 15. Juli 2015 16:09
An: XAFS Analysis using Ifeffit
Betreff: Re: [Ifeffit] phase problem copper
On 07/14/2015 10:18 AM, Neeb, Matthias wrote:
> I have a standard exafs spectrum of copper K-shell and everything
> looks as expected (Chi(E), Chi(k), Chi(R), Chi(q) ...) when using
> Demeter 0.9.21 .
>
> However the phase spectrum of (Chi(q)) appears quite different to
> literature (e.g. B.K. Teo, Inorganic Chemistry Concepts 9; Martens et
> al, PRB 17, p1481 (1979), i.e. positive slope, a factor of ~10 larger
> values on y-axis.
>
> Any reason why? Thanks for your answer in advance.
>
I imagine that the reason no one has attempted an answer to your question is because it is not clear what your problem is. You did not provide a picture to help explain the problem, you did not you provide data or an Athena project file. So I don't know what you are looking at, although I suppose it has something to do with figure 1 in the Martens paper. Vague, open-ended questions tend not to get prompt, useful answers.
That said, I can venture a few guesses. Some possibilities:
1. Is the k-weighting different?
2. Are you comparing the real part of chi(q) to the real part of
chi(q)? Or imaginary to imaginary?
3. The Fourier transforms in EXAFS are complex transforms. chi(k) is
a real measurement. A decision has to be made about whether the
chi(k) measurement occupies the real or imaginary part of the
function that is transformed. I do not know which convention
Martens used. In 1979, that convention may not have been
established yet, but it certainly is in Ifeffit, Larch, and
Demeter. The different convention would result in phase shifts.
B
--
Bruce Ravel ------------------------------------ bravel at bnl.gov
National Institute of Standards and Technology
Synchrotron Science Group at NSLS-II
Building 535A
Upton NY, 11973
Homepage: http://bruceravel.github.io/home/
Software: https://github.com/bruceravel
Demeter: http://bruceravel.github.io/demeter/
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