[Ifeffit] a question about the white line

[Kelly, Shelly D.]

I just put a background tutorial on my web page.  Complete with project
files and a pdf showing screen shots and some instructions.  It used
MoO3 which I found to have a particularly tough background.  I used a
theory to remove the background and then showed how to reproduced the
"same" chi(k) data without the theory.  If anyone wants to play with the
background and add suggestions I will incorporate more info.  Here is a
link to the tutorial:  www.mesg.anl.gov/skelly.html 

Shelly


> -----Original Message-----
> From: Newville, Matthew G.
> Sent: Sunday, December 12, 2004 11:03 PM
> To: XAFS Analysis using Ifeffit
> Cc: John J. Rehr; Joshua Kas
> Subject: RE: [Ifeffit] a question about the white line
> 
> Hi John, Josh,
> 
> >   Certainly if a spline can do a good job of fitting the mu_0, then
the
> current
> > approach is more or less ok.  However in systems with strong white
lines
> > mu_0 can be strongly peaked and hence very difficult to fit with a
few
> > spline points. The broadening function lamda(E) can be constrained
to be
> small.
> > Also, for cases with large whitelines, the location of e0 is
difficult
> to
> > fit - the true e0 can lie well below the edge jump.
> 
> Oh, maybe I misunderstood. Is lambda(E) in
> >     mu_0 = mu^thy_0(E,E_0,Gamma)[1 + lambda(E)]
> 
> just a broadening term or a highly adjustable spline??  I guess
> the whole question is how much freedom this function has.
> 
> It definitely sounds interesting to use a theoretical mu_0,
> especially for challenging white lines, but it's not clear how to
> best make this accessible to users.  I'll probably have to think
> about this some more, but it would be nice to know the mechanics
> of how you're doing it, and how well you need to know the
> structure before you start.
> 
> >    In our view, mu0 does not depend very much on structure, since
it's
> > mostly determined by the local embedded atom potential. For this
reason
> > it's calculation would also be quite fast (only phase shifts and the
> > cross-section are needed) compared to a calculation of chi(k) which
> > requires multiple scattering paths.
> 
> Well, usually a standard for autobk only needs a decent guess of
> the first shell.  Getting a standard does require some prior
> knowledge of the system and calculation time, but I don't know
> that it would be a lot more knowledge and time than would be
> needed for a mu^thy_0(E).  Anyway, it would still be interesting
> to compare these two (autobk using calculated chi(k) v. starting
> with a mu^thy_0 and massaging it to match data).
> 
> On the other hand, it sounds as if the mu^thy_0(E) for a few dozen
> (or few hundreds) challenging systems could be tabulated.  That
> would make it very fast and very simple to use!
> 
> --Matt
> 
> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit