[Ifeffit] large Eo's and neg. D-W factors
scalvin at slc.edu
Wed May 2 11:02:31 CDT 2007
It depends on the information you want out. Here's where it's good to
think about correlated effects. Sigma2's affect primarily the
amplitude. They therefore correlate highly with coordination number,
S02, normalization, sample quality (e.g. variable thickness in
transmission; self-absorption in fluorescence), beamline effects
(e.g. harmonics), and the like.
Other variables are primarily a question of phase: E0, delr, and the
third cumulant, to name some of the more important ones.
Of course, some factors, such as the elemental identity of the
scattering atom or the fraction of various phases that are present,
affect both amplitude and phase.
If you're interested in knowing about the phase variables, then the
amplitude variables are less crucial. A slightly wonky value can come
from something else being slightly wonky; e.g. normalization. That's
not ideal, and should make you nervous enough to poke at it a bit
(for example, go back and look carefully at the normalization in
Athena). But it's not nearly as serious as if you are interested in
an amplitude variable. For example, if you're trying to fit
coordination numbers, then a weird sigma2 is a big big red flag.
Finally, this is the kind of place where analyzing a standard of
known structure is immensely helpful. Likewise, a temperature series
can be very helpful for sorting out some of the correlated variables.
Hope that helps.
Sarah Lawrence College
At 10:52 AM 5/2/2007, you wrote:
>Also, should I be worried if my Debye-Waller factors get small
>(0.0005-0.001) if I optimize my model in FEFF to a point that my delr's
>are on the order of 0.005-0.01 Angstroms? This has been a concern of
>ours since we started doing EXAFS, especially since all of our
>measurements are at room temp. We haven't seen this as much with our
>vanadium on silica samples, but it seems to be true with all of our
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