Hello everybody,
Thinking about problems. You once told me that:
Fitting in unfiltered k-space also gives erroneous R-factors, as was recently discussed here.
would you give me a direction to the discussion about this point that I can read?
Pardon my poor knowledge of EXAFS fitting and reliability of the results, but more generally, I wondered if EXAFS fitting in raw unfiltered k-space is always relevant. Let me explain it; I do agree that in certain cases, when the whole structure is known and all EXAFS contributions well modelled by an ab-initio model (e.g., feff), you may well reproduce all oscillations of the EXAFS spectrum, and then fitting in raw k can be OK. However, more often than wanted, the fitting is done with a structural model including only the first few shells. With these shells, we model not only the actual contributions from the neighboring shells, but also that from the more distant shells. Physically, it can be demonstrated that this would not be a problem if the EXAFS signal had a constant amplitude and would spread over an infinite range in k space. However: (a) EXAFS contributions have not a constant amplitude, and (b) they can be recorded over only a few Å-1. So maybe (or even surely, I daresay), the contribution from non-modelled distant shells would affect the structural parameters from the modelled shells. Finally, another issue - for which I would not lay out my neck, though - is the noise. In EXAFS, the signal to noise increases with k, and of course fitting in the raw k space is another way of dealing conveniently with the noise -or relative uncertainty, as we may name it. However, the FT is a way of filtering out some of this noise, and so maximise the signal from a shell. Likewise, fitting in filtered q-space may yield more accurate results, because some of the noise is filtered out. However, I concur that somehow the fact that the uncertainty on the high-q part of the filtered contribution os greater than in the low q-part should be implemented somehow. So there are my own reflexions. Maybe some people have already worked on that, and there are some publications that I've missed (shame on me!). If that's the case, I would welcome any insight. Best regards, Michel Schlegel -- Michel Schlegel Commissariat à l'énergie atomique CEN de Saclay, DEN/DANS/DPC/SCP/LRSI Bat 391 - Piece 205B F91 191 Gif-sur-Yvette Cedex, France Ph: +33 (0)1 69 08 93 84 Fax: +33 (0)1 69 08 54 11