On Monday 12 May 2003 05:10 pm, k-kupiecki@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. Really. 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: http://leonardo.phys.washington.edu/~ravel/course/notes.pdf Hope that helps, B -- Bruce Ravel ----------------------------------- ravel@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/