[Ifeffit] Anyone out there do SEXAFS?

Bruce Ravel ravel at phys.washington.edu
Mon May 12 17:01:07 CDT 2003

On Monday 12 May 2003 05:10 pm, k-kupiecki at northwestern.edu wrote:
> I'm trying to analyze my SEXAFS data...

Hi Kristine,

I'll take a stab at this question even though I have never personally
measured grazing incidence data.  I think, though, I can get you
pointed in the right direction.

> Inparticular I'm currently looking for suggestions for how to simulate
> with FEFF SEXAFS in the "grazing incidence" orientation (with the E
> vector of the synchrotron beam close (~0.2 deg) to the surface normal of
> the sample (incident beam ~.2 deg incident on surface).
> Thus far I've been trying to analyze my samples with SEXAFS taken from a
> "normal geometry" (~0.2 angle incident to the beam and the E vector of
> the synchrotron beam in the plane of the surface)... I managed to
> simulate this with FEFF by giving it a hemispherical cluster representing 
> my surface...  But I'm a little confused what I need to change in FEFF to 
> replicate the information coming from just the inter planar bonding (in
> "grazing incidence") instead of both inter and intra planar bonding as
> found in the "normal incidence" situation....

The hemispherical cluster is certainly a good start.  That gives a
physically reasonable cluster of atoms for the feff calculation.
(Wouldn't it be great if you could specify a plane in TkAtoms and have
it throw away all the atoms on one side or the other?  Someone should
suggest that to the lazy guy who wrote atoms ;-)

The incidence angle of the beam will have relatively little impact on
the feff calculation, although it may prove significant when you go to
fit the data.  The important bit is the orientation of the
polarization vercor of the beam relative to the crystal.  To make the
next couple of paragraphs simple, I will assume that you have a
tetragonal crystal with the c-axis parallel to the surface normal.
This means that when the E vector of the beam is in the plane E is
perpendicular to c.  If you have something ickier, say polymers
assembled and canted on a surface, you will have to think a bit
harder, but this should get you started.

To start, I'll tackle the somewhat simpler case of E parallel to c,
that is with the polarization of the beam perpendicular to the
surface.  In that case, FEFF will kindly compute a standard for you
that is appropriate to this measurement geomatry if you add this line
to the feff.inp file:
    POLARIZATION  0  0  1
Typically, feff computes spherically averaged polarization.  With that
magic line, feff will consider E//c.  This means that scattering from
atoms in the atomic planes parallel to the surface will be very small,
but scattering from atoms along the c-axis will be very large.  This
will show up in the feffNNNN.dat files (and in Artemis when you use it
to plot those feffNNNN.dat files from the polarized calculation).

The situation you want is possibly a bit more complicated.  If your
film really is a good single crystal and you know its orientation
relative to the beam, then just use the right value of the
POLARIZATION keyword in the feff.inp file.

However, it is quite common for films to be well textured in the
surface-normal direction but very poorly so in the plane of the
surface.  In that case the ab planes of the unit cells are in all
orientation in the bit of the sample illuminated by the beam.  So how
do we do a calculation in this case?

There is a conceptual mismatch here between the sample and the
feff.inp file.  The sample that I described above is polycrystalline
in the ab plane and the incident beam is linearly polarized.  The
list of atoms in a feff.inp file describes a snippet of a perfect
single crystal.  What to do?

Here comes the big concept....

You can specify a circularly polarized beam in the feff.inp file.  A
perfect single crystal with a beam which is circularly polarized in
the ab plane will give the exact same XAFS as a crystal which is
polycrystalline in the ab plane, well-textured in the c-hat direction,
and measured with a beam linearly polarized in the ab plane.

It's true.


Here is the magic incantation for your feff.inp file:
   POLARIZATION    1  1  0
   ELLIPTICITY  1  0  0  1
This says to rotate an E vector pointed in the (110) direction about
the 001 axis.  A circle has an ellipticity of 1, hence the first 1 in
the ELLIPTICITY keyword.

I described all this in more detail in Chapter 3 of:

Hope that helps,
 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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