[Ifeffit] Large XAFS data sets

Edmund Welter edmund.welter at desy.de
Wed Apr 11 08:51:54 CDT 2012

Dear XAFS users

recently I read several mails on this list which were dealing with the
problem of large data sets, as they are produced by Q-EXAFS scans or by
dispersive XAFS. The question was if there are tools available to handle
data sets of several 1000 spectra and perform a linear combination fit
or even a full EXAFS evaluation on each of them. Evaluating 3000 spectra
is a heroic attempt, but I wonder if it is also economical.

In most (that means not in ALL!) cases, the vast majority of these
spectra is boring, because the spectrum with the number X looks exactly
like the spectrum with the number X-1 looked and how the spectrum with
the number X+1 will look and so on. Evaluating all these (basically
identical) spectra is in principle a waste of time and working memory.
The interesting spectra are those which were measured when something was
happening in the sample. Since we do not always know at which time, or
temperature or reactant concentration etc. interesting things will
happen it is without any doubt justified to measure x-thousand spectra,
but after that we should use a more sophisticated approach than brute

I think that it would be much more useful to find procedures (that means
develop computer programs) that search for the (usually relatively small
number of) interesting spectra. The most obvious parameter is how
similar is a particular spectrum to the spectra measured before and
after. The next step would probably be to identify clusters of related
spectra using statistical methods. This is a problem which had to be
solved in other areas like the automated analysis of images before and
should also be possible with our kind of data.

Anyway, how to handle thousands of XAFS spectra will become a very
important problem in the future. With all these beamlines that provide
10^12 photons per second we can measure a factor 100 -- 1000 faster than
we did with 10^9 photons per second. So, I wonder if anything beyond the
brute force approach is going on in the EXAFS software universe to make
effective and economical use of the measured data.

Best regards,
Edmund Welter

Dr. Edmund Welter      Deutsches Elektronen-Synchrotron

Notkestr. 85            Email: edmund.welter at desy.de
D-22607 Hamburg         Phone: +49 40 8998 4510
Germany                 Fax  : +49 40 8998 2787

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