[Ifeffit] Path problem in IFEFFIT

[Bruce Ravel]

On Saturday, April 21, 2012 02:20:17 pm you wrote:
> Dear Bruce,
> 
> Attached is the file whcih gives me the Path problem (I posted on the
> mailing list), I try to fot the br K edge for the AuBr4- complex. I also
> have fitted the Au edge which worsk fine by then adding the Br edge
> data, the problem start occuring, so I tried to putr it on its own....
> I might just miss something really easy, but I can t find it.
> Hope you can help and thank you for your effort!


Yvonne,

There are a number of problems with your project file.  Setting aside
the inherent difficulty in measuring the Br K edge of a gold-bromine
complex (the Au L2 edge is only 8.2 inverse Angstroms above the Br K
edge), you have made a few mistakes in your feff input data.

First, it would seem that you started from a feff.inp file you
prepared for use with Feff8 or Feff9.  The "XANES" keyword makes Feff6
(the version bundled with Ifeffit and Demeter) unhappy.  I believe
this is the proximate cause of the error message you initially
reported last week.  Unfortunately, Artemis does not provide a useful
warning that Feff stopped without running properly to completion when
it encountered the "XANES" keyword.  That needs to be fixed.  Thanks
for bringing that to my attention.

Second, you had an RMAX value in the Feff input data of 80.  That's
not a problem per se, but it does make Artemis issue a warning
message since it's an oddly large value.

Third, it seems that you started with Feff input data for the Au L3
edge then edited it to make one of the Br atoms the absorber.  You did
this 75% correctly.  You did the right thing with the potentials list
and the ipot values, but you forgot to change the HOLE value from 4 to
1.  4 is the value for the L3 edge, 1 is the value for the K edge.
That is a common mistake -- indeed one that I make regularly -- that
has come up on the list in the past.  It tends to result in large E0
values since the phase of scattering an L3 photoelectron is so
different from the K.

In the end, I couldn't make much out of your fit, however.  The data
range is so limited that you have very little information to work
with.  And I suspect that the presence of the L2 edge is dominating
the determination of the background function.  I bet that the peak at
1 Angstrom is a background removal artifact.  It is hard for me to
believe that a gold scatterer at almost 2.5 Ang would make a peak at
only 1 Ang in the Fourier transform.  You might try restricting the
range of the spline in Athena to something that does not include any
of the Au L2 edge step.  Even so, I am not so optimistic about being
able to interpret these data.  (Although, as part of multiple data set
fit they may prove useful....)

In any case, by making those fixes to the feff input data and
re-clicking the run Feff button, I was able to make the program behave
correctly.

B

-- 

 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

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/ifeffit/Demeter