Bruce: We are definitely interested in the oxide component. We want to have a way of understanding the number of Ru atoms in the nanoparticle which "see" oxygen. Eventually we need to determine if this changes as a function of fuel cell operating conditions or particle size, etc. Carlo On Tue, 13 Apr 2004, Bruce Ravel wrote:
Stanislav,
Matt said:
The thing that makes me the most uneasy is the relatively large values of Rbkg/Rmin (that is rbkg in spline() and rmin in feffit(do_bkg=1,....) which also refines the background) and how they change around in your analysis. Normally, you'd want to keep Rbkg/Rmin below the 'first shell peak', which is sometimes difficult to define for the asymmetric peak common with short metal-oxygen distances, but if the peak is around 1.3Ang, the Rbkg/Rmin of 1.5Ang seems like it might be too high. Can you get decent results with a lower Rbkg/Rmin?
I have a question for you. Are you trying to determine the oxide content? That is, do you want to know what fraction of the Ru atoms are in an oxide environment? Or are you trying to account for the oxide so that you better measure the metallic component?
If you don't need a good measurement of the oxide, then perhaps the thing to do is to consider the oxide to be inseparable from the background, and just let the spline to remove that oxide contribution from the fit.
That's not as silly as it might sound. As Matt explained, the purpose of the spline is to remove components below Rbkg. Another way of saying that is that the components above Rbkg are the interesting ones and the components below Rbkg are not part of your analysis problem. One often use the AUTOBK spline to remove the step function, the so-called AXAFS, and the low-frequency contributions due to multi-electron excitations. One removes all those things because they are not the portion of the XAS spectrum that you want to fit. All I am suggesting is that you might lump the oxide in that category as well, if you are only concerned about the metallic component.
Of course, you may find that the Ru-Ru path from the oxide contributes in a frequency range similar to the metallic first shell. But that's another story.
B
-- Carlo U. Segre -- Professor of Physics Associate Dean for Special Projects, Graduate College Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 Carlo.Segre@iit.edu http://www.iit.edu/~segre