ARTEMIS: Automated First Shell Theory

ARTEMIS - Automated first shell theory

Sometimes thinking about a fitting model is more than a problem merits. You just want a quick 'n' dirty stab at the first shell -- perhaps to measure the centroid of the distribution, perhaps to tell if a sample is 4- or 6-coordinated. Whatever.

Artemis is not extremely well suited to rapid-fire, first shell analysis. By design, Artemis tends to force the user to slow down and think hard about every step. Artemis is powerful, but she ain't simple.

The quick first shell (QFS) theory tool is an attempt at addressing this shortcoming. It works like this:

Import some data. Set the Fourier transform and fitting parameters to suitable values. Specifically, be sure to set the fitting range such that it encloses the first peak of the data.
Select ``Quick first shell theory'' from the Theory menu. This will display the QFS dialog.
The QFS dialog provides spaces for selecting the parameters for a simple first shell theory. These include the atomic species of the absorber and the scatterer, the absorption edge of the experiment, the approximate distance between the absorber and scatterer, and the coordination geometry to use in the Feff calculation.

Currently the following coordination geomatries are available:

4-coordinate crystal
6-coordinate crystal
octahedral molecule
tetrahedral molecule
square-planar molecule

The QFS theory is probably not highly sensitive to the choice of coordination geometry. Since the unknown sample is probably not well described by any of these geometries, they are all merely approximations for use in a quick 'n' dirty fit.

Once you have set up the parameters for the QFS theory, click the ``Do it!'' button. This will step through the following without pausing:

a.


Build an input file for the Feff calculation

b.


Run Feff

c.


Import the first path from the Feff calculation

d.


Create a set of guess parameters for the amplitude, the sigma^2, the e0, and the delta R. Also created are set parameters for the third and fourth cumulants, but they are set to zero. These higher cumulant set parameters are created to make it easy to consider higher cumulants in subsequent fits merely by changing them from set to guess.

If you have a mixed first shell, you might choose to repeat steps 2 through 4 two or more times.

At the end of this sequence, you are left with Artemis in its normal state. You may need to adjust the parameters used in the fit. The QFS dialog is really just a tool for initially setting up the project. It in no way changes the normal operation of Artemis.

If you import data from an Athena project file, the species of the absorber and the edge will be set correctly when you start the dialog.

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