[Ifeffit] Re: Artemis Question

[Bruce Ravel]

On Sunday 18 July 2004 11:38 pm, John Butler wrote:
> Basically, the problem that I'm having is that the system I'm working
> on is a solution (to start I'm trying to look at a solution of Zinc
> Sulfate and then look at data with other components we're interested
> in).  Unfortunately, the part of the manual dealing with solution
> analysis hasn't been written yet, and the people I know who use EXAFS
> don't do anything with solutions.
>
> I was hoping that you could point me in the right direction for how to
> proceed with the analysis (e.g. is there anywhere I'm likely to find
> the FEFF files ... is there a 'standard' way to attack this kind of
> problem).  Of course I assume you're a pretty busy guy and have
> neither the time nor inclination to give me a detailed step by step
> guide or anything ... I was just hoping you could point me in the
> right direction, give me a couple of tips, and/or show me where to
> look.

John,

As you can see, I have forwarded you question along with my answer to the 
Ifeffit mailing list.

You are correct to assume that I'm a busy guy.  This week (like each of the 
other weeks this month) I am at NSLS collecting some data and helping our 
users.  Fortunately, the mailing list is a solution to the "busy expert" 
problem.  Enough people read and respond to the mailing list that someone 
should be available to answer just about every question.

I doubt, actually, that you problem is really so hard to get started with.  
Lots of problems in XAFS do not fall into nice little pigeon holes.  One 
knows how to deal with data on an iron foil because one knows what the 
structure of iron metal is.  Most real research problems, obviously enough, 
are problems precisely because the structure is not perfectly well known.

In the case of a solvated complex, there is probably some sort of analogue 
which can be used as a starting place.  Perhaps there is some similar 
compound whose structure *is* known.  Perhaps your solvated complex has a 
crystalline form that is similar, at least in the local environment, to the 
solvated form.  In any case, find something that is similar to your real 
material.  Run feff and compare to the data.  Perhaps your analogue structure 
will be close to your data.  In that case, you should be able to parameterize 
the feff calculation in such a way that you can begin to understand the ways 
in which your complex differs from your analogue.

Perhaps the analogue structure is a very poor match for the data.  That in 
itself is information, in that it tells you to go look for a different 
analogue.

The common theme to what I have said here (and indeed to any analysis problem) 
is that you make some educated guesses as to what you real sample looks like.  
You test those guesses.  If they seem to be leading you in the right 
direction, super.  If not, make new (and newly educated!) guesses.

My other piece of advice would be to spend some time fitting known spectra.  
If you are new to XAFS, then you are learning two things at the same time: 
how to interpret XAFS data and how to use my and Matt's screw-ball analysis 
programs!  I consider it a good idea to spend some time fitting data for 
which you know the answer so that you can learn how to use the codes before 
plunging into the harder but more interesting problem of your own research.

Good luck and join the mailing list,
B


-- 
 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
 Code 6134, Building 3, Room 222
 Naval Research Laboratory                          phone: (1) 202 767 2268
 Washington DC 20375, USA                             fax: (1) 202 767 4642

 NRL Synchrotron Radiation Consortium (NRL-SRC)
 Beamlines X11a, X11b, X23b
 National Synchrotron Light Source
 Brookhaven National Laboratory, Upton, NY 11973

 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/