[Ifeffit] McMaster correction

Bruce Ravel bravel at bnl.gov
Thu Jun 16 19:50:33 CDT 2011


Is this a discussion topic or a feature request? :)


On Thursday, June 16, 2011 08:28:18 pm Scott Calvin wrote:
> Hi all,
> I've been pondering the McMaster correction recently.
> My understanding is that it is a correction because while chi(k) is
> defined relative to the embedded-atom background mu_o(E), we almost
> always extract it from our data by normalizing by the edge step. Since
> mu_o(E) drops gradually above the edge, the normalization procedure
> results in oscillations that are too small well above edge, which the
> McMaster correction then compensates for. It's also my understanding
> that this correction is the same whether the data is measured in
> absorption or fluorescence, because in this context mu_o(E) refers
> only to absorption due to the edge of interest, which is a
> characteristic of the atom in its local environment and is thus
> independent of measurement mode.
> So here's my question: why is existing software structured so that we
> have to put this factor in by hand? Feff, for instance, could simply
> define chi(k) consistently with the usual procedure, so that it was
> normalized by the edge step rather than mu_o(E). A card could be set
> to turn that off if a user desired. Alternatively, a correction could
> be done to the experimental data by Athena, or automatically within
> the fitting procedure by Ifeffit.
> Of course, having more than one of those options could cause trouble,
> just as the ability to put sigma2 into a feff calculation and in to
> Ifeffit sometimes does now. But wouldn't it make sense to have it
> available (perhaps even the default) at one of those stages?
> --Scott Calvin
> Sarah Lawrence College


 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

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