[Ifeffit] McMaster correction

[Scott Calvin]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20110616/e62d2114/attachment.html>