Re: [Ifeffit] How to fit "N" number of Neigbors in (D)Artemis?

On Monday, August 20, 2012 09:53:13 AM Wei Li wrote:

Hi, Bruce,

Does that mean each shell has a different S02 value?

No. It means that S02 must be evaluated for each path. Or, I suppose, I should say that the product of N and S02 must be evaluated for each path. No! No! Actually, I should say that the product of N and S02 must be evaluated correctly for each path where what I mean by "correctly" depends upon the details of your fitting model. That sounds right! If you are fitting your copper foil data, then "correctly" probably means to let N be the path degeneracy found by the path finder and to use a single S02 parameter for all paths. If you are fitting a crystal with different sites for the absorbing atom -- say the super conductor YBa2Cu3O7, which has 1 Cu atom in the Cu1 site and 2 Cu atoms in the Cu2 site -- then your parameterization of S02 must be scaled accordingly depending on which Feff calculation the path comes from. If you are fitting an amorphous solid or a nanoparticle or something else which unknown coordination, then you are probably best off setting N to 1 and parameterizing S02 for each path in some manner that captures the details of the coordination environment that you are trying to measure. Remember, N and S02 are multiplied together in the EXAFS equation. HTH, B

Wei

On Mon, Aug 20, 2012 at 9:44 AM, Bruce Ravel wrote:

...

On Monday, August 20, 2012 09:02:28 PM Huaming Guo wrote:

...

I am using the (D)Artemis for EXAFS fitting. However, the "N" is set to

be a

...

constant in the (D)Artemis. We can not set the variable for this

parameter.

...

How can I do that if I model non crystalline samples using (D)Artemis? Thanks a lot.

In Artemis, N is a pure number, but S02 can take a math expression.

In the EXAFS equation, N and S02 are multiplied together.

Thus, you might consider parameterizing the amplitude by setting N to 1 and putting all of the parameterization in the math expression for S02.

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

Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel

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-- 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 Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel