[Ifeffit] phase shifts

Bruce Ravel bravel at anl.gov
Wed Dec 14 11:13:49 CST 2005

On Wednesday 14 December 2005 10:38, Stefano Ciurli wrote:
> With Athena it appears that I can simply (maybe too simply) click on
> a button and plot the phase shifted spectrum in R-space. How can I
> try to do the same using IFEFFIT from the command line?

The answer to the generic question "how can I do in Ifeffit what I do
in A or A?" is always the same.  Look at the Ifeffit buffer
(Edit->Display Ifeffit buffer) as you are clicking stuff.  The commands
sent to Ifeffit are echoed in that space.

The Ifeffit buffer was originally a debugging tool, but it quickly
became clear that its great value was as a pedagogical tool for others
wanting to write their own Ifeffit scripts.

> In practice, using IFEFFIT we can plot the fitted spectrum and the
> FT'ed experimental spectrum, but the plot does not consider the
> phase shift. We need to extract the phase shift in oder to translate
> the plot to show the phase shifted spectrum. How can we do that?
> What is the parameter in IFEFFIT that allows one - as in Athena - to
> add a constant number and shift the plot?

The answer to this particular question is to add these arguments

   pc_edge="fe k", pc_caps=1

to the fftf() command.  Of course, the value for pc_edge should be
appropriate to your sample.

If you have a feff calculation you can use for a full phase
correction, the arguments are slightly different.  The "caps" in
"pc_caps" is an acronymn for "central atom phase shift".  If you know
the scatterer (which you might not when using Athena, but which you
certainly do when using feff/Artemis), then you can do the full
(central + scattering) correction.

Use the path() command to import the feffNNNN.dat file, then add this


to the fftf() command (assuming that you read the feffNNNN.dat in as
path #1).  Again, watch the Ifeffit buffer to see an example of this
in Artemis.

 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advance Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

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