[Ifeffit] question about Eo and simultaneous fitting

Kelly, Shelly D. SKelly at anl.gov
Wed Feb 14 11:31:36 CST 2007

Hi Scott,

The change in frequency is a bigger affect at high k than the energy
shift parameter.  The energy shift parameter affects the data more
strongly at low k.  So I align the data to each other at low k, and then
they become out of phase with each other at high k.  A similar effect is
seen for pressure dependent data where the oxidation state doesn't
change but the bond length changes.  I also look at how well the data
are both aligned to the theory used to describe the data sets
independently.  The energy shift values should overlap within the
uncertainty to justify using a single parameter for them.  I suppose
that my method might not work for all systems.  But if the data are
similar enough so that a multiple data set fit has significant overlap
in the models used to describe the spectra, then it has worked.  


> -----Original Message-----
> From: ifeffit-bounces at millenia.cars.aps.anl.gov [mailto:ifeffit-
> bounces at millenia.cars.aps.anl.gov] On Behalf Of Scott Calvin
> Sent: Wednesday, February 14, 2007 9:55 AM
> To: XAFS Analysis using Ifeffit
> Subject: Re: [Ifeffit] question about Eo and simultaneous fitting
> Hi Shelly,
> Interesting--I've never tried that method with samples that I expect
> to differ in oxidation state. If it's differing in oxidation state,
> the local environment is probably different too...particularly,
> nearest-neighbor distances should be changed significantly. So how do
> you align the chi(k) data with each other, when they oscillate at a
> different frequency?
> --Scott Calvin
> Sarah Lawrence College
> At 10:38 AM 2/14/2007, you wrote:
> >The approach that I like to take, is to vary the choice of E0 in the
> >background subtraction step so that the chi(k) spectra are well
> >with each other and the theory.  Then only one Ezero parameter is
> >needed.
> >
> >Cheers,
> >Shelly
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