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