[Ifeffit] How to do with diffraction peak
Matthew Marcus
mamarcus at lbl.gov
Wed Dec 12 17:00:41 CST 2012
Hmm. Another possible way to do it is to delete the bad data points and then do a "slow" FT, which would be a fit
of the data, at the points given with no interpolation, to a sum of sines and cosines. This would have the
nice feature of using the data as it is and ignoring the bad stuff. Filtering would involve multiplying the
sines and cosines by some window function (in R space) and evaluating them *at the given k-points*, not on a regular
grid. This of course means that evaluating FEFF paths and the like is likely to be slow because you don't get to
use recursion relations to evaluate sin(2*R*k(i)+delta) as you would if k(i) are uniformly tabulated. Now that computers are
a bazillion times faster than they were when EXAFS analysis traditions were established, maybe that's the way to go. What do you think?
mam
On 12/12/2012 2:16 PM, Bruce Ravel wrote:
> On Wednesday, December 12, 2012 11:09:29 PM Dr. Dariusz A. Zając wrote:
>> coming back to the problem with Bragg peaks, does anyone knows if it is
>> possible to analyse EXAFS with more than 1 interest regions? like 2 or
>> more FT windows - before, between and after Bragg peaks, or is possible
>> to introduce one own FT window function in Athena?
>
> Interesting question. Some days I really love this mailing list.
>
> A similar question came up recently on the mailing list, although I
> think here you are asking whether regions can be chosen in k-space.
> (The previous discussion was about regions in R space.)
>
> I am commenting off the top of my head here, so I may be neglecting
> something. But I think what you are suggesting to have a windowing
> function that goes up, then down, then up again, then down again. I
> think the problem with that is that it would introduce low frequency
> components into the transformed data. Granted, the transform of the
> theory would have the same effect, but I worry about how that kind of
> windowing function would be correlated with the parameters of the fit.
>
> As Scott suggested in the earlier conversation, another option is to
> do a multiple data set fit where the data is imported twice. For one
> instance of the data, the FT window is set over the first range. For
> the other instance, the window is set over the second range. Again, I
> worry about how the choice of windows may be correlated with the
> parameters of the fit, given the limited information content used in
> each part of the MDS fit.
>
> I don't plan on exploring this myself. But if someone wanted to do so
> and attempt to convince me that it should be implemented in the
> software, I am -- as always -- willing to listen.
>
> Cheers,
> B
>
>
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