Re: [Ifeffit] sigma^2 values for multiple scattering paths (Scott Calvin); Re: Ifeffit Digest, Vol 92, Issue 4

Hello Scott, Just to make sure I understand what you mean, are you saying that in your 3 atom system, the S1 and S2 atoms have relatively fixed locations but A may have large vibrational amplitudes in the A-S1 and A-S2 directions? So the round-trip 3 atom MS path has a small sigma^2 value since the variation in the A-S1-S2-A path is dictated by the more or less fixed S1 and S2 end-points (with minimal perpendicular contribution), whereas the 2 individual SS paths have large sigma^2 value due to the large A-S vibrations? I tried setting the sigma^2 value to a reasonable number for the MS path and it appears to increase the R factor slightly and tries to maximize the floating degeneracy I set (with a restrain to be physically reasonable based on my model). It does not look as good but at least it seems more plausible. I will try out Shelly's suggestions to see if they work too. I guess what I wanted to find out is whether the model I included is telling me that something is terribly wrong. Thank you all again for your responses! han sen On Oct 6, 2010, at 10:00 AM, ifeffit-request@millenia.cars.aps.anl.gov wrote:

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1. Re: sigma^2 values for multiple scattering paths (Scott Calvin)

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Message: 1 Date: Wed, 6 Oct 2010 09:39:49 -0700 From: Scott Calvin To: XAFS Analysis using Ifeffit Cc: "Dr.Scott.Calvin@gmail.com" Subject: Re: [Ifeffit] sigma^2 values for multiple scattering paths Message-ID: Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes

Although I agree with the main points that Bruce makes, I do want to comment on one piece:

On Oct 6, 2010, at 7:03 AM, Bruce Ravel wrote:

...

.

In no case can I understand a physical explanation for the the MS sigma^2 being smaller than for the SS.

Actually, there is a physical situation where something like that can occur, although it sounds like it's not the one that Han Sen has.

Consider an absorbing atom rattling around in a relatively fixed cage or lattice. And then consider a linear (or near-linear) arrangement:

S1 -- A -- S2

One multiple scattering path that can sometimes have a sizable contribution is A --> S1 --> S2 --> A. This path will have a sigma^2 that is a bit larger than the single-scattering path S1 --> S2 --> S1, because of the perpendicular component of the motion of A.

But it's quite frequently the case that S1 --> S2 --> S1 is not modeled in a fit, because the S edge is not measured.

On the other hand, the single scattering paths A --> S1 --> A and A --

...

S2 --> A ARE included in the fit. Those two have high sigma^2's, because A is rattling around a lot.

Under that circumstance, a multiple-scattering path included in the fit may indeed have a lower sigma^2 than the single-scattering paths included in the fit.

The moral, of course, is that it's not hard to think physically about what sigma^2 means for a multiple scattering path. If one appears to have an "unphysically" small sigma2, then the explanation is probably one of the ones given by Bruce or Shelly.

One more thought on this. How much does it change your fit, Han Sen, if you set the sigma^2 for the multiple-scattering path to some "reasonable" value. If the scientific information you want from your fit is not sensitive to exactly what sigma^2 the MS path gets, and is not significantly different when given a "reasonable" value than when allowed to find its "best-fit" value, then there's probably no need to resolve the issue. In my experience, this is often the case with low- amplitude MS paths: the fit is improved by their inclusion, but may not be particularly sensitive to the details of their path parameters.

--Scott Calvin Sarah Lawrence College

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End of Ifeffit Digest, Vol 92, Issue 4 **************************************