[Ifeffit] McMaster correction
John J. Rehr
jjr at uw.edu
Fri Jun 17 06:14:24 CDT 2011
Hi Scott et al.,
Thanks for bringing up this issue. Whether or not McMaster corrections
are useful does seem to depend on details of the measurement. But
my question is: for the cases where they are useful, can one do
better? As the data & theory get better and better, perhaps we should try
to extract more accurate cross sections mu(E). For example, is it at all
of interest to have embedded atom cross-sections to replace the atomic
based Cromer-Liberman cross sections or empirical tables?
On Thu, 16 Jun 2011, 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
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