Hi Scott,
Thanks for your suggestion. I find that I need to generate FEFF
files for larger clusters than I had initially started off with. In your
recent Physica Scripta paper [T115 (2005) 744-748], you had looked at
some pretty large Pd clusters, particles 0.1 to 200nm in diam. (page 745,
col 2, para 2, lines 5-6). Some of these clusters are well over 500
atoms. How did you manage to create the FEFF? When I try to do so
for a cluster this large in Artemis, the program flags an error saying
that I have too many atoms (in my *.inp file generated from
TkAtoms).
Charles
At 10:36 AM 5/17/2005, you wrote:
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