Re: feff polarization

[Matt Newville <newville@cars.uchicago.edu>]

Hi Alex,

Thanks for getting back on this.  For now I'll just respond to
the 'regular/generic' polarization (ie, ignoring the exact
details of the angular dependence of L-edge polarization).

The confusion about polarization dependence is due to the
nature of EXAFS -- not FEFF.  But maybe FEFF's implementation
could make it easier to work with polarized data.

> > The outputs from from Feff's pathfinder are confusing because
> > 'degeneracy' ("number of equivalent paths"??)  becomes poorly
> > defined when a polarization vector is defined. If one could
>   I have to disagree. E.g. for linear polarization along
> arbitrary axis FEFF uses the following symmetry operations
> in the pathfinder: all rotations about the polarization vector
> and reflection in plane perpendicular to that axis. If one path
> can be transformed into another using these operations they are
> considered to be equivalent. However, if one changes orientation
> or polarization with respect to crystal axes, the degeneracy
> will depend on polarization vector.

I think the issue is whether polarization should be included in
the path-finder.  That is, the path-finder _could_ be (nearly)
purely geometrical.  This has some appeal for a few reasons,
such as that the paths would be the same for all polarizations.

I think historically that the path-finder included scattering
for it's filtering criteria.  If we stick to EXAFS only (or
expect that XANES calcs won't rely on very high numbers of
paths), then the problem of path explosion may be less
important, and we may be able to get by with a nearly-pure
geometrical path-finder based only on the cluster geometry and
tabularized plane-wave criteria.

Then there would be no polarization dependence to 'degeneracy'.
Though more paths would end up with very little amplitude due
to polarization, it would be more obvious what was happening --
and have a more highly predictable set of paths.

I don't have a strong opinion one way or the other on this, but
I do think this is where part of the confusion comes from.

> > If you replace the oxygen at (1.8,0,0) with a sulfur, should
> > Feff report the oxygen coordination is 1, 3, or 5?  It is
> > definitely confusing.
>
>   Analyzing polarization average data again will remove confusion.
>   Polarization dependent data will help to analyze orientation of
>   MeSO_5 molecule on a surface.

But analyzing polarization average data is not the issue!
Sometimes you _want_ to use the polarization to better
illuminate certain directions (say, normal to a surface).  If
that 1 sulfur was in-plane or out-of-plane, you would get very
different EXAFS!

For L-edges, I'm still confused what Feff is doing.  I'll look
into this a little more, and write a separate message on that,
but maybe not for a week or so.  But for now:

 - Which version(s) of Feff have a working MULTIPOLE card?

 - You said you thought that LeFevre's results were more
   reliable than Heald/Stern, but your value for c=|M21|/|M01|
   (= 0.24) is much closer to Heald/Stern's (0.2) than
   LeFevre's (0.4).  Any thoughts on this?

 - You said that this ratio should have very little
   k-dependence.  But the plots of scattering amplitudes
   from the feff.dat files shows clear k-dependence.  Can
   you explain this?  My guess is that the strong k-dependence
   could explain the high value of c that LeFevre got:  I see
   much less angular dependence at low k (which would give a
   higher value of c), and their data stops at k=5 or so.

Thanks,

--Matt