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"
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@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642
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