[Ifeffit] High-R data

Bruce Ravel ravel at phys.washington.edu
Thu Aug 26 08:28:25 CDT 2004

On Thursday 26 August 2004 01:54 am, Peter Southon wrote:
> Hello all,
> Athena and Artemis do not plot chi beyond 20 Å-1 or the Fourier transform
> beyond 10 Å. Is this a limitation of Athena/Artemis or of Ifeffit? Can it
> be changed?

This only seems to be half true.  The fe.060 file that comes with the
Athena examples has data out to 22 invAng and gets plotted correctly
by both programs.

On the other hand, you are correct that data only gets plotted in R up
to 10 A.  That was not a conscious decision by me and seems to be some
kind of default in Ifeffit.  Matt, how does one change that default?

> The 10.5 Å peak raises another issue. I understand that Ifeffit by default
> assumes that the high-R region is pure noise in order to make its estimates
> of uncertainty. (I know the actual R-range that is used was mentioned
> recently but I can't find it in the list archives or any of my manuals.)
> Can this range be changed? Does anyone have any advice on how to deal with
> this issue?

In Ifeffit the chi_noise command is used to determine this.  Here is
its doc page:

As you can see, the rwgt1 and rwgt2 parameters are used to change the
high-R range.

Artemis does not provide widgets for setting these, but you can do
this rather more hands-on approach:
  1. Open up the Ifeffit buffer from the Edit menu
  2. Enter the chi_noise command using the command line at the bottom
     of that window.  To see an example of the chi_noise command,
     select "What is epsilon_k?" from the Data menu.  The chi_noise
     command used to answer that question (using the default R range)
     will be displayed in the Ifeffit buffer.
  3. The value for epsilon will be diplayed in the Echo area and
     preserved in the Echo buffer.  Enter this number in the box
     labeled "Epsilon" on the data page.


 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
 Code 6134, Building 3, Room 405
 Naval Research Laboratory                          phone: (1) 202 767 2268
 Washington DC 20375, USA                             fax: (1) 202 767 4642

 NRL Synchrotron Radiation Consortium (NRL-SRC)
 Beamlines X11a, X11b, X23b
 National Synchrotron Light Source
 Brookhaven National Laboratory, Upton, NY 11973

 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/

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