[Ifeffit] Distance dependent phases and amplitudes

[Hugo Carabineiro]

Thanks Bruce for unravelling this mistery... so quickly!

However you have referred particularly to the scattering amplitude
dependence on the interatomic distance. How about the phase shift, which
seems to depend strongly on the this variable?

Hugo

----- Original Message ----- 
From: "Bruce Ravel" <ravel at phys.washington.edu>
To: "Hugo Carabineiro" <carabineiro at ist.utl.pt>; "XAFS Analysis using
Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
Sent: Monday, December 06, 2004 4:01 PM
Subject: Re: [Ifeffit] Distance dependent phases and amplitudes


> 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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>
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>
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>