[Ifeffit] fitting questions

Kelly, Shelly D. SKelly at anl.gov
Wed Jan 7 12:24:36 CST 2004

Hi Kristine,

I feel like I can add something to question number 3. Please see below

> (3)  Also is there anyway to have Artemis determine the best 
> k-range (or perhaps a way to have it step through different 
> values of kmin and kmax) ?  Just wishful thinking on my part 
> probably...

No, Artemis does not do this automatically.  Here is a recipe that I use
to determine the k-range.

1.  back-up to the Athena stage.  Look closely at the edge of the xmu(e)
data and the background.  Count the number of oscillations until you get
to the flat region of the sepctra (about 30 eV away from the edge or
so).  Then plot the chi(k) data and count the oscillations to figure out
where this node is in the chi data and set that to kmin.  Usually the
value for kmin is a node around 2 to 3 1/Angstroms.

2.  Make about 6 copies of the chi(k) data.  In each of the copies make
sure that all the parameters are the same.  Set kmin to the value from
step 1.  Then choose a node for kmax that looks really clean around 8
1/Angstroms.  Then for the following copies choose the next and next
nodes for kmax.  Then compare the Fourier transform of all the copies.
As you include more data the peaks should become more well defined but
smooth.  If you see a bunch of oscillations with a high frequency all
over then you have gone too far for kmax.  

3.  Model the data with these conservative choices for the data range.
Once you have a model that you like, try including another oscillation
or two on both sides and see if the theory agrees.  At this point you
can also include a theory in the bkg subtraction step to get down to
lower k.


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