[Ifeffit] EXAFS mean free path in small particles

Matt Newville newville at cars.uchicago.edu
Tue Jun 14 20:46:55 CDT 2005


Anatoly,

> The point is, to my opinion, that since In the most
> interesting range, below 2nm, the plasmon activity decreases,
> therefore its effect on EXAFS interpretation, if any, should
> be negligible.

I think I may not understand your point.  The Zhao and Montano
paper shows very pronounced changes in the plasmon spectra
between each of 10,3,2, and 1 nm for Al.  At 1 nm, the surface
plasmon is definitely dominating -- not going away.  So their
conclusion, that lambda changes substantially below 10 nm due to
the changing relative importance of surface-to-bulk plasmons
seems well-argued to me (not to say its correct).

I agree (I think) with Matthew that the Zhao and Montano work
shouldn't be too easily dismissed, though it might not be
completely correct.  I don't know of any other work on how
lambda might depend on particle size, and never really thought
about this.  The Rao paper seems broader in scope and not
necessarily addressing plasmons and mean free path, but I only
skimmed it, so maybe I missed something.  Their fig 8 definitely
shows a size dependence of O K-edge, though I didn't immediately
see a clear interpretation for this -- it could have several
explanations. Again, I only skimmed this paper, so maybe I
missed something: perhaps the electron diffraction literature
has more information??

> ...  There are other evidences that small particles have DOS
> similar to molecules, not typical metals. Most notable is the
> evidence that HOMO-LUMO band grows to more than 1 eV at sizes
> of 1 nm range.

Hmm, I'm not sure I see that becoming more molecular and less
metallic indicates that using the bulk mean-free-path is OK. .   
The 'universal mean free path' that Feff uses and the fact that
S02 is still has to be a fitting parameter even for excellent
data with Feff8 suggests to me that there may be many hidden
sins in Feff's mean-free-path calculation.

Sadly, we seem to be a little short on theorists these days.

--Matt





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