[Ifeffit] Very large exafs amplitude in collected data - cause unknown

[Bruce Ravel]

On Wednesday, June 05, 2013 02:55:25 PM you wrote:
> Hi,
> 
> Sorry about that, I wasn't sure what the best format would be for these
> forums. I have converted the data to a different format that should
> hopefully be more friendly.
> 
> This has data in the following columns;
> 
> 1 = real time clock
> 2 = Mono angle (requested)
> 3 = Energy (eV)
> 4 = I0
> 5 = I1
> 6 = I2
> 7-9 = blank
> 10-44 - fluor channels (SCA)
> 45-79 - ICRs for the det elements
> 
> The weights are arbitrarily unity, and the dark currents were not recorded
> (assumed zero).
> 
> I have also attached an averaged file generated using average 2.0 if thats
> more useful for people.

Jason,

You replaced one large, unweildy file format with another large,
unwieldy file format.  You should consider yourself fortunate that I
have taken the time to bother with you.

The folks on this mailing list, myself included, are volunteers.  None
of us get anything for offering time to answer these questions other
than a sense of community spirit and community service.  It is
completely unreasonable that you have asked your question in a way
that required lots of work simply to understand the question.  Had you
been a little more thoughtful and a little less unreasonably demanding
of the time of volunteer help, you could have converted your data
files into simple, 2-column files -- energy and mu(E) -- that would
have been trivially easy for your volunteer to examine.  Instead, you
first sent the data in one basically inscrutable format, then in a
second similarly unwieldy format.  Awesome.  Such a good use of my
time.

Please read this:

  http://bruceravel.github.io/demeter/pods/help.pod.html

When you are done, please follow the link to the article by Raymond
and Moen and read that.

Now that I've had my rant, I'll address your question.  Attached is
your Au nanoparticle data overplotted with a spectrum measured on a
gold foil.  Clearly your data is awful and probably unsalvageable.

Without knowing all the details of your measurement, it is very
difficult to know what went wrong.  Given that the oscillations in
your data are commensurate with those in the gold foil, it is unlikely
to be a monochromator problem or a problem with pressure variations in
the I0 ionization chamber.

I also don't think you had an egregious problem with sample
preparation.  Problems that fall under the rubric of sample
inhomogeneity tend to damp the oscillations rather than enhance them.
Also sample inhomogeneity tends to fail to normalize mono glitches
away.  You data, while problematic, don't seem to have that problem.

My guess is that you have a problem of detector linearity, but not
saturation of the Ge detector.  Saturation (aka pile-up, dead-time)
also serves to damp the oscillations.

Perhaps there is some sort of coupling in the signal chain between the
Ge detector and I0?  I'm knd of running out of ideas here....

The bottom line is that this is the sort of problem that needs to be
recognized and addressed while still at the synchrotron.  I don't see
what you can do to correct this after the fact in a way that would
leave your data defensible for publication.  The slim silver lining is
that this teaches an important lesson about evaluating data quality as
it is being measured.

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

 Homepage:    http://xafs.org/BruceRavel
 Software:    https://github.com/bruceravel
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