Edward
Another method to get information on oxidation states from near-edges was used relatively commonly in the 1980s and early 1990s, but has gone out of fashion since (can someone tell me why? Is the underlying physics flawed?).
You determine the ionisation potential (IP) by fitting the arctan step background function simultaneously with Gauss/Lorentz lines for all the near-edge resonances. The inflection point of the arctan corresponds to
I agree with Bruce on this, and would like to add that in nanoparticles the very Fermi energy may be shifting around - as a function of size, metallicity, support-particle interaction, etc. For example, in bulk materials, the change in the oxidation state of an absorber by, say, -1, due to reduction of an oxide, for example, is accompanied by the shift to the lower energies of characteristic XANES features (peak position, max derivatives, or the energy at the half edge step, - these are most popular "reference" points for the chemical shift determination). The reason for this "chemical" shift is screening of the coure hole potential by the "extra" electron and the concomitant decrease in the ionization potential. However, in nanoparticles, the changes in the charge state of the absorber also noticeably change Fermi energy, which a few extra electrons per particle _increase_ Fermi energy compared to the initial sample. That, of course, causes the increase of the ionization potential. Which of this trends prevails is a very interesting question - we have seen both positive and negative shifts in nanoparticles, which means that these effects are at least the same order of magnitude. What is important, however, is that the energ shift in nanoparticles does not scale simply with the formal oxidation state. Anatoly -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov]On Behalf Of Bruce Ravel Sent: Monday, May 21, 2007 6:59 PM To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] white line intensity v. edge position indeterminingoxidation state of Re On Monday 21 May 2007, Sven L.M. Schroeder wrote: the
IP. Variations in the IP can be interpreted using the same considerations as for binding energy shifts in XPS...
For example, for Au there is an almost perfectly linear relationship between IP and oxidation state, from Au(-1) via Au(0) and Au(+1) to Au(+3) ! In this case the correlation with oxidation state is a lot stronger than for white line intensities and/or the now commonly used 'edge inflection points'...
Sven
My understanding of the physics behind the assumption that you can fit a XANES spectrum by an arctan and some peak-y functions is that you are assuming that the system is a perturbation from a simple, Drude-like gas of electrons. At zero kelvin, the electron density is a well defined, sharp step function with the step at the Fermi energy. At finite temperature, that sharp edge step is smeared into a Fermi function -- essentially an arctan. As a perturbation, there is some structure to the electron density that is handled by the peak-y functions. To the extent that you can point to the location of the Fermi energy on a XANES spectrum, the underlying physics of using an arctan is well-founded. In the sequence of pure-materials you describe, the arctan method should work well assuming you are talking about pure phase materials, even in the case of Au+3 where the "perturbation" (also known as a rather enormous white line) is not small. However, I don't think the use of one arctan is well suited to a mixed phase situation, which is a possibility for what Edward is seeing in his electrochemistry experiments. That is, at some intermediate voltage, a fraction of the sample might be oxidized and a fraction metallic. In that case, the XANES spectrum needs two arctans mixed in some ratio. Then the fitting of lineshapes has to include a mixing term to describe the amount of each species. The line shape fitting thus inherits some of the systematic uncertainty of the linear combination approach, along with all the rest of its sources of error. In the case of mixed species, tracking a feature in the XANES (inflection, white line position, what have you) can usually be shown to be as accurate as a linear combination solution assuming the correct feature can be identified. And in some systems, fitting a single arctan might work well also -- but that would not be generalizable to all systems. My US$0.02 worth, B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/ _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit