[Ifeffit] Using more than one E0

[Kelly, Shelly D.]

Hey Scott,

One correction, I botched this part:


> -----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
the
> 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
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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