[Ifeffit] EXAFS mean free path in small particles

Matt Newville newville at cars.uchicago.edu
Mon Jun 20 13:44:30 CDT 2005

Hi John,

Thanks for looking at this!

> Clearly the coorrelations between S02, ThetaD and Ei make any
> assessment of possible problems with Ei problematic. Also, I
> don't know what the experimental resolution Gamma_expt is -
> probably some fraction of an eV, so Ei=Gamma_expt+Im delta
> Sigma implies a comparable but negative correction delta Sigma
> to the self-energy, which is evidence of excessive loss in the
> HL plasmon-pole self-energy for Cu.

I'd guess the monochromator resolution was around 0.75 to 1eV
for the Cu measurement.  And I agree that testing the effect of
Ei v. S02 is very challenging, especially with finite
monochromator resolution and finite thermal vibrations.

> Moreover, although your fits are in R-space, inspection of the
> k-space data shows that contributions from distant MS paths
> are suppressed by the theory, also suggesting that FEFF's PP
> self-energy overestimates loss.

Yep, it looks to me that in both k- and R-space the shapes of
the amplitudes for the best-fit result are slightly, but
perceptively different from the data.  At first look this is an
exampalry "great fit".  But looking more closely you can really
see that reduced chi-square should be ~10, as the best-fit
really is off from the data in ways that can't be easily
explained as a wrong value for S02, sigma2, or Ei.

I don't have a good explanation for the discrepcancy (and don't
want to guarantee it's not a systematic error in the data -- but
it sure doesn't seem to be).  This results seem very similar to
those of Bud Bridges several years ago.  I think it's fair to
say that neither you nor I disagreed with Bud at the time: we've
known that reduced chi-square for fits to very good data is much
too big for some time.  It would be very nice if we could nail
down the pieces that go into the EXAFS amplitudes better.

It will be interesting to see if you come up with anything
different.  I'd love to hear about it!


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