On Thursday 12 May 2005 05:03, Gerrit Schmithals wrote:
I have learned that it is quite possible that multiple scattering paths do not contribute to the spectrum because of their large mean square displacement. Is that due to the fact that the mean square displacements are large for all atoms and for multiple scattering paths with more atoms involved the mean square displacements are multiplied?
Multiplied, eh? That doesn't sound quite right to me. When you measure the sigma^2 you are, as you say, measuring the mean square displacement about the nominal length of the path. A three-body path has many more modes of vibration (or degrees of freedom, if you prefer that language) than a two-body path. Each leg of the path has a mean square displacement, but there are also angular modes of motion. That is, the atoms are vibrating along the directions parallel to the line between them, but they are also vibrating in angle about the scattering angle. So you can imagine that atoms are connected by springs and there are springs restoring the angle between bonds. Picture something like this (this silly little picture won't make sense unless you look at it with a monospace font): O O \ / \-----/ \ / \ / X The springs represented by the slashes might be quite stiff, but the spring represented by the dashes might be quite floppy. This situation might result in a sigma^2 for the three-body path that is quite large compared to the two body sigma^2s.
One easy way to obtain a fit that is not too bad was to exclude all multiple scattering paths.
That might be ok. One way to explain that is to say that between the effects of sigma^2 and the sum of a large number of paths with different phases, the net effect of all those MS paths is to be reduced to small background signal. In general, what you have done so far is a good start to a complicated fitting problem. I am a big fan of the strategy whereby you start with the simplest model, see how it fits the data, then add complexity as the problem warrents. For a similar approach to the problem of a highly complex system, you might want to look through Shelly's presentation from last year's NSLS EXAFS school: http://cars9.uchicago.edu/xafs/NSLS_2004/Kelly.pdf The bit around page 25 is particularly relevant to this conversation. Good luck! B -- Bruce Ravel ----------------------------------- bravel@anl.gov -or- ravel@phys.washington.edu Environmental Research Division, Building 203, Room E-165 Argonne National Laboratory phone: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/