[Ifeffit] Re: Calculating Phase Corrected Re and Im

Bruce Ravel ravel at phys.washington.edu
Mon Apr 14 08:55:38 CDT 2003


On Sunday 13 April 2003 03:06 pm, Matt Newville wrote:
> This will do only the central-atom portion of the phase-shift,
> so will be incomplete, but is often most of the correction,
> will work for the whole spectra, and doesn't require knowing
> much about the system.  Athena (the excellent GUI to ifeffit
> from Bruce Ravel) can do these central-atom phase-corrections
> very easily (ie, no typing commands). 

Hi,

I just wanted to quickly chime in on this topic.  For what it's worth
-- I am not a huge fan of phase corrections.  While they do, in many
cases, make the chi(R) look more like a radial distribution function,
they do require some more explanation when showing your data to a
non-xafs person.  And, while a pc-chi(R) looks more like the
corresponding RDF, it most certainly is not an RDF.

Athena does only a central atom correction for the reason that Matt
gave.  Athena assumes only that your data is XAFS data and does not
assume any knowledge of the system.  Thus, the only thing that Athena
is sure to know is what the absorbing atom is.  That is determined
from the edge energy.

Artemis, on the other hand, has more information to work with.  When
you choose to do phase corrections in Artemis, you choose one of the
feffNNNN.dat files included in the fit as the phase correction
standard.  Thus, in Artemis, phase correction includes the scattering
portion as well as the central atom portion.  In this screenshot:
    
http://leonardo.phys.washington.edu/~ravel/software/exafs/images/artemis/artemis_main.png
the "Path to use for phase corrections" menu consists of "None"
(i.e. no pc), and each of feff0001.dat through feff0005.dat.  You
might then choose to use feff0001.dat (i.e. the first shell in the
example shown) for phase corrections.  Note that the corrections are
only for plotting.  Fitting in artemis is done with the
non-phase-corrected data and theory.


Upcoming news flash:  I have spent the last week getting multiple data
set fits working in Artemis.  Ifeffit has supported them for a long
time, but now Artemis will provide an interface to that extremely
useful tool.  I hope to have the new release out soon.

B



-- 
 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
 Code 6134, Building 3, Room 222
 Naval Research Laboratory                          phone: (1) 202 767 5947
 Washington DC 20375, USA                             fax: (1) 202 767 1697

 NRL Synchrotron Radiation Consortium (NRL-SRC)
 Beamlines X11a, X11b, X23b, X24c, U4b
 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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