[Ifeffit] why ss_2 is negative?
bravel at anl.gov
Fri Feb 23 16:49:21 CST 2007
On Friday 23 February 2007 16:24, hw26 at njit.edu wrote:
> I am doing model fitting using Artemis. My sample is uranium solution with
> an ionic liquid, 1-methoxyethyl-3-methyl imidazolium tetrafluoroborate
> (MOEMIMBF4). The Fitting seems ok except that the ss_2 is negative. I don't
> know how can I fix this problem. Can you get a chance to take a look? My
> fitting is simple, only three single scattering pathes which were produced
> by Six Pack. The fitting file is attached. I really appreciate it if you
> could take a look and give some advice!
Let me ask you a couple questions that I think are closely related to
yours. You seem to be modeling an uranyl ligand and two equatorial
ligands. Focussing on the second path, the one with a negative
sigma^2 and the one that (I am guessing) represents either a hydration
shell or a bidentate ligand:
1. Is 35.996 a reasonable value for e0?
2. Is 2.59 Angstrom a reasonable value for an equatorial oxygen
ligand in a uranyl moiety?
It would seem that your fit has quite a few problems beyond the
sigma^2 value. I question whether your parameterization of these
three paths is physically defensible.
Bruce Ravel ---------------------------------------------- bravel at anl.gov
Molecular Environmental Science Group, Building 203, Room E-165
MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007
Argonne National Laboratory phone and voice mail: (1) 630 252 5033
Argonne IL 60439, USA fax: (1) 630 252 9793
My homepage: http://cars9.uchicago.edu/~ravel
EXAFS software: http://cars9.uchicago.edu/~ravel/software/
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