-----Original Message----- From: Kelly, Shelly D. Sent: Wednesday, December 15, 2004 10:06 AM To: XAFS Analysis using Ifeffit Subject: RE: [Ifeffit] Using more than one E0
Hi Scott,
See below:
The MS scheme you outline below seems to treat the E0 due to the scattering species (I'll call that the "bonus" E0 for lack of a better term) as being the average of the bonus E0's for all the scattering atoms in the path. Thus you give the Cu-O1-Cu1-Cu path as using the average of the O1 and the Cu1 bonus E0's, while the Cu-O1-Cu-O1 path uses just
O1 bonus E0. I would naively expect we should be using the sum, rather than the average. After all, if this bonus E0 is a correction to the shift caused by the muffin-tin potential FEFF has calculated, shouldn't a MS path that bounces off the same atom twice have to use twice the correction? I feel like there's something basic I'm missing here...
[Kelly, Shelly D.] The energy shift applies to the entire scattering event. The scattering event can not be broken down into the sum of
Hey Scott, One correction, I botched this part: the the
legs.
[Kelly, Shelly D.] Most of the time, it is good to think about the scattering event as being a sum of the legs. If you follow this through you can see why the energy shift is an average rather than a sum. Take two sine waves and displace one from the other by a few eV. Then add them up. The total will have an energy shift that is in between the two that you started out with. --Hence an average rather than a sum. Hence the energy shift for the entire scattering event can be
approximated as an average of the energy shift from each leg. I have used the average energy shift a lot and found that it works well. I have never tried using a sum of the energy shifts... I think that in an extreme limit you can convince yourself that the sum will break down for an 8 legged paths. The energy shifts are "never" greater than 10 eV. "never" in my experience. -Hence the answer to your next question.
Also, what is your experience as to the actual differences in bonus E0's that you get in ionic compounds? Right now one of my students is working with Sr(NO3)2... should we really expect the difference in bonus E0's between the Sr2+ and the oxygen atoms to be more than 12 eV, or does some kind of charge overlap from the nitrate ions actually make the strontium ions behave as if they are less than +2?
[Kelly, Shelly D.] One full electron of ionic bonding gives about 6 eV. So that is about the most you would expect from different ions in a solid. Usually, I have found 3 to 4 eV. You make up most of the valence state change from the shift in the edge position. I have found unrealistic energy shifts when I try to use too much chi(k) data at low k. I'd look there first for you 12 eV shift.
HTH Shelly
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