[Ifeffit] Distance dependent phases and amplitudes

Bruce Ravel ravel at phys.washington.edu
Mon Dec 6 09:01:41 CST 2004


On Monday 06 December 2004 08:36 am, Hugo Carabineiro wrote:
> Hi,
>
> I've never quite well understood why do theoretical phases and amplitudes
> seem to be  dependent on the interatomic distance between the absorber and
> scatterer atoms. Shouldn't it only depend on the nature of the atoms
> involved? Nevertheless when trying to build theoretical references I'm
> always careful in choosing an appropriate distance. Otherwise the
> difference in phase shifts must be compensated at the expense of large E0
> values. Could someone please shead more light on this subject?

Hi Hugo,

That's good question.  The photoelectron is a curved wave, not a plane
wave.  There are lots of references on this, but one useful and
readable one is Rehr et al, J. Amer. Chem. Soc. v.113 (1991) p. 5135.
If you look at Eq. 1 (and the cited references), you will see that
there is a Hankel function involved in the computation of the
scattering amplitude.  This introduces the scattering distance into
the equation.

It turns out not to be a super sensitive dependence.  In my
experience, getting R wrong by a little bit can be compensated simply
by the delta R shift in Ifeffit.  But when R is vastly wrong (tenths
of an angstrom), then you are correct in stating that some other phase
portion of the fit might be used to compensate for the error.

HTH,
B

-- 
 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
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