Hi Charles,
Here's what I would try for your situation:
1) Use Artemis to create FEFF files for bulk crystalline Pt. Also
generate FEFF files for some plausible Pt-surface interactions.
2) Constrain the amplitude of the Pt-Pt paths to fall off with
reff in the way expected for a homogenous sphere (I have a couple of
publications on this, including a recent one in Physica Scripta). If
you are confident you know the crystallite size, then use that for the
size of the sphere; if not, you can leave it as a parameter for
Artemis to fit. If you were really confident you knew the size
of the crystallites, then it might be worth it to do something more
precise (e.g. use a magic number cluster), but that doesn't sound like
your situation. In clusters of this size that are "roughly"
spherical, I've found that the best-fit results aren't all that
different if you just use a sphere, although the uncertainties
in the results may be considerably higher.
3) Use Artemis to guess the fraction of Pt's having a surface
interaction and start trying the different candidate
interactions.
In advance, and knowing very little about your system, I'd warn
you that the problem will be much more tractable by the method I
describe if the bonding to the surface is somewhat random and floppy;
i.e. if there is not a fixed orientation and distance of the Pt
particle from whatever the substrate is. Then you can be more
confident that the high-R part of the FT is entirely due to the Pt-Pt
interactions, and can thus gauge the relative contributions of Pt-Pt
vs. surface interactions more easily.
--Scott Calvin
Sarah Lawrence College
Anatoly,
I did not consider magic numbers & don't know for certain what the
exact # is. The 95 atom cluster is only an estimate of the size of the
composition of particles that was observed in TEM images of Pt
tethered onto a a surface with -COH, -C=O groups; the exact nature of
the bonding is what I'm trying to determine. The Pt seems to be
stabilized by its surface interactions; the exact nature of the
binding is what I'm trying to figure out. The EXAFS data that I have
"sees" both the Pt-Pt interactions as well as the Pt-surface
interactions. I realize that it would be best to simple scan a
sample containing Pt clusters of the same size as observed in the TEM;
but, unfortunately, Pt has a tendency to agglomerate and it is
difficult to get a monodispersed size in colloidal suspension in
solution. Comparing with Pt foil doesn't work because of the
quenching problems in the fluorescence since there are so many
atoms.
I wanted to get a simulated EXAFS spectrum of something that just
represents the bulk Pt-Pt only to compare with the data that I have.
Differences in the spectra (in principle) would be attributed to
Pt-surface interactions present in the TEM. I would like to try
to get an idea of the # of Pt interacting with the surface as opposed
to the Pt-Pt interactions.
Charles