[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: FEFF question?
> On Fri, 15 Feb 2002, "John J. Rehr" <jjr@leonardo.phys.washington.edu> said:
>> What is the expected polarization effect on an oriented fcc
>> lattice measured at the L3 edge?
JJR> There is *no* polarization effect in any perfect cubic lattice
JJR> due to symmetry. You can see this by writing the absorption in
JJR> terms of the polarization tensor
JJR> mu = sum_ij epsilon_i* epsilon_j mu_ij
JJR> Since mu_ij is diagonal for a cubic system (any edge) the
JJR> epsilon dependence drops out.
To put it another way... consider a K edge, linear polarization, a
polarization vector at some angle theta from the a-axis, and situated
in the ab plane. In that case, your spectrum will have a contribution
from the a-axis attenuated by cos^2(theta) and from the b-axis by
sin^2(theta). Since a and b are the same, it does not matter what
theta is.
Now, the L2,3 edges have a different polarization dependence, but you
will find the same result for a material with four-fold symmetry about
the incident direction. (e.g. for an fcc lattice).
I suspect, though, that Patrick's question may have been motivated by
something non-fcc. For something with two-fold or less symmetry
about the incident, there will probably (but not strictly) be an
attenuation of amplitude in the first shell due to the effect of
polarization. For L2,3, though, this is probably a smaller effect
than for K.
B
--
Bruce Ravel ----------------------------------- ravel@phys.washington.edu
U.S. Naval Research Laboratory, Code 6134 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/