sigma^2 values for multiple scattering paths
Hello, I briefly read through the FAQ about this but I'm not sure if it answers my question. Are there situations where the sigma^2 for a multiple scattering path can be smaller than the direct paths? So small that they're on the order of 0.001-0.003 for a degeneracy of 12 such paths? I'm working on a fitting model that does not work well with additional shells but it looks almost perfect with a multiple scattering path included. I'm skeptical however, because of the small sigma^2 values. I am also not discounting the fact that the data quality may be poor. But I would appreciate any physical reasons for small sigma^2 values. Thanks! han sen
Hi Han Sen:
I agree that the small sigma2 values for the MS paths is suspect. The
data is saying that it wants more amplitude from these paths than is
physically possible. One way to increase the amplitude of the paths is
to make the scattering angle closer to 180deg for a linear path. It
is a bit tedious to edit the feffit input file to get that to run, but
can be done with a bit of geometry.
This will not work for a triangle MS path. In that case I would look
for additional contribution from other MS or SS paths at the same
distance.
Another way to force sigma2 to be larger would be to use a Debye or
Einstein Model to model both the SS and MS path. That will make
sigma2 for both of them at least reasonable. Although the fit will not
be as perfect.
HTH,
Shelly
On Wed, Oct 6, 2010 at 12:26 AM, Han Sen Soo
Hello, I briefly read through the FAQ about this but I'm not sure if it answers my question. Are there situations where the sigma^2 for a multiple scattering path can be smaller than the direct paths? So small that they're on the order of 0.001-0.003 for a degeneracy of 12 such paths? I'm working on a fitting model that does not work well with additional shells but it looks almost perfect with a multiple scattering path included. I'm skeptical however, because of the small sigma^2 values. I am also not discounting the fact that the data quality may be poor. But I would appreciate any physical reasons for small sigma^2 values. Thanks! han sen _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
On Wednesday 06 October 2010 01:26:55 am Han Sen Soo wrote:
Hello, I briefly read through the FAQ about this but I'm not sure if it answers my question. Are there situations where the sigma^2 for a multiple scattering path can be smaller than the direct paths? So small that they're on the order of 0.001-0.003 for a degeneracy of 12 such paths? I'm working on a fitting model that does not work well with additional shells but it looks almost perfect with a multiple scattering path included. I'm skeptical however, because of the small sigma^2 values. I am also not discounting the fact that the data quality may be poor. But I would appreciate any physical reasons for small sigma^2 values. Thanks! han sen
Han Sen, It is always useful to remember the physical meanings of the parameters use in the EXAFS equation. sigma^2 is a mean square variation in the distance between the absorber and a scatterer. Suppose we have this configuration: X------O------Y that is, absorber X, scatterer Y and a colinear O atom in between. The path length for the path X-Y-X is the same as for X-O-Y-X and for X-O-Y-O-X. If we just consider thermal motion of the atoms along that axis, then the mean square variation in paths lengths for those three paths must also me the same. That's neither deep nor complicated -- its just geometry. The argument in the last paragraph neglected the prospect of the O atom experiencing thermal motion perpendicular to that axis. That effect means that sigma^2(X-O-Y-O-X) > sigma^2(X-O-Y-X) > sigma^2(X-Y-X) A common approximation made in data analysis is that this perpendiular effect is small compared to the uncertainties in sigma^2 and so those three sigma^2 values are constrained to be the same. In no case can I understand a physical explanation for the the MS sigma^2 being smaller than for the SS. That said, you have a fit and a result. When you float the MS sigma^2 it comes out smaller. I would suggest that is telling you something about the fitting problem rather than something about the physics of the atomic configuration. A smaller sigma^2 means that the contribution from those paths is being enhanced. That might be due to a correlation with an amplitude parameter. It might be due to a data quality problem. It might be due to a mistake in the implementation of your fitting model. This sort of thing happens all the time. I frequently analyze data and come up with a curious, unphysical result like this. It hasn't yet meant that I have discovered some wonderous new physics. Very occassionally, it means that I have uncovered a shortcoming in Feff, but that is exceedingly rare. Usually it means that I have a problem with my data or I have made a mistake filling in all the boxes in Artemis. B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
participants (3)
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Bruce Ravel -
Han Sen Soo -
Shelly Kelly