[Ifeffit] phase corrections

[Grant Bunker]

Following Bruce's commentary - it is indeed true that if you have
sufficiently heavy backscatterer (think Au or Pt, or even Ag/Pd) the
modulations in the backscattering phase and amplitude will mess up your
transform peaks, as Ian suggests. Basically you get "sideband modulation"
of your transform peaks. To get rid of them you need to not only do a
phase correction, but also an amplitude correction (divide out the
wiggles). I thinkd oing one without the other doesn't actually make a lot
of sense since they are related (because the real and imaginary parts are
related by kramers-kronig relation the phase and amplitude are connected).
Obviously this is not very difficult to do - I put it into the
VAX version of the UW programs in 1985 but that was dropped from the
simplified UW/NRL version later. Short term, it would be very easy to do
by exporting from ifeffit and using  any data massaging program that knows
complex numbers.

thanks - grant

On Mon, 4 Aug 2003 ifeffit-request at millenia.cars.aps.anl.gov wrote:

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>    1. Re: Phase Correction using IFEFFIT (Bruce Ravel)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Mon, 4 Aug 2003 09:56:14 -0400
> From: Bruce Ravel <ravel at phys.washington.edu>
> Subject: Re: [Ifeffit] Phase Correction using IFEFFIT
> To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Message-ID: <200308040956.14156.ravel at phys.washington.edu>
> Content-Type: text/plain;  charset="iso-8859-1"
>
> On Saturday 02 August 2003 04:32 pm, Ian Drake wrote:
> > At this time, my particular interest is in attempting to identify clearly
> > next-nearest neighbors in the RSF before attempting to do fitting.  It is
> > my hope that a positive identification of the next-nearest neighbor by
> > means of IM analysis/ and or k-weighting will add validity to my final
> > fitting results.
>
> Well... I don't know what an "RSF" is or what "IM analysis" is, but I
> think I can suggest a way to isolate the scattering atom phase shift
> in a way that requires only a small amount of work and can be used
> with either ifeffit or artemis as they exist today.
>
> The second column of the feffNNNN.dat file is the central atom phase
> shift.  The third and fourth are the scattering atoms magnitude and
> phase.  Copy your feffNNNN.dat file of interest to another name, say
> feff_next_neighbor_no_caps.dat.  Edit the new file, replacing the
> second column with zeros.  Read this into ifeffit or artemis and use
> it as the path for phase correction.  In artemis, this is done in the
> data view using the pull down menu labeled "Path to use for phase
> correction".  Obviously, you should choose your edited path.  In
> artemis, you may need to set the amplitude for this path to zero so
> that it doesn't really get used in a fit.
>
> That said, it's not clear to me how phase correction will really do
> what you seem to want.  It seem unlikely to me that the signal beyond
> the first shell will magically become clear simply by doing phase
> correction.  If the signal is a superposition of various scatterers
> before phase correction, it will still be a superposition of various
> scatterers afterwards.  Removing the scattering atom phase shift does
> not affect the terms in the EXAFS equation that make peaks in a
> Fourier transform broad, nor does it magically separate signals which
> contribute similar Fourier components.  Understanding how various
> paths contribute similar Fourier components is the reason that Artemis
> allows you to easily overplot data with the Fourier transforms of
> various paths.  Then you can inspect the contributions from each path
> before setting up your fitting model.  But if playing tricks with
> phase correction helps *you*, then great!  That is why Artemis allows
> you to do this.
>
> 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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> End of Ifeffit Digest, Vol 6, Issue 2
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