[Ifeffit] Question about Io during data collection

[Maxim Boyanov]

Hi Todd,

 

I have some familiarity with the beamline where these data were collected so
I will take a stab at what might have gone wrong and what you may be able to
do to salvage some of the data. 

 

Ideally, you want an Io(E) that is as flat as possible over the scan range,
or at least over the region where the amplitude of your EXAFS signal starts
rapidly declining, say above 8 A-1. We typically try to get a 10-20% smooth
variation in Io intensity over the scan range in our measurements, which
works for the setup and the systems we are interested in. The reason for
this "requirement" is that while normalization of If or It to Io should take
care of all variations in your incident photon intensity, your detectors
respond differently to large changes in intensity (even if your detectors
were exact replicates, which they are not, they are still using different
gases in Io, It, and If). This non-linearity in the detector responses will
result in any noise and EXAFS-like variations present in Io to also show up
in your normalized If/Io or log(Io/It) spectrum and to interfere with the
EXAFS signal you are trying to analyze. The problem will be exacerbated for
elements like Pb at the L3 edge, where the EXAFS signal is of smaller
amplitude to begin with so the noise and features resulting from
non-linearity will be a bigger proportion of your measured signal.

 

What seems to have happened in your measurement is that vibrations or
temperature drifts or other mechanical instability caused the second
monochromator crystal to move away from the Bragg condition, so your Io was
likely travelling randomly along the rocking curve, causing large changes in
measured Io intensity and changes in Io(E) structure from scan to scan. This
beamline has a feedback loop that attempts to keep the incident intensity at
the top of the rocking curve, but it may not have been doing its job at the
time of your measurement or it may have been turned off to reduce the noise
in Io resulting from the induced oscillation in the second crystal that
creates the difference signal used in the feedback loop. These problems in
Io could then be transferred in your data due to non-linearities in your
detectors' response. It is also possible that the harmonic rejection mirror
was not set up ideally and allowed harmonics in the incident beam, which
would also result in non-linear response and spectral artifacts,
particularly when your beam travels up and down the rocking curve.

 

I guess the bottom line is that you now have a data set with individual
scans that appear different from each other. I took a look at the project
file you sent and it seems that the data are relatively repeatable scan to
scan when you normalize and extract the EXAFS, despite what you show as
large variation in Io and in the un-normalized data in your email. For
datasets like that, I would not use the scans that differ significantly from
the others and the scans that show large noise at high k (e.g. 006, 008,
010). When you average the relatively repeatable remaining scans you should
be able to get workable data up to 9-10 A-1 or so. Whether you can tackle
the research question you set out to answer with data like those depends, of
course, on the system and the question.

 

Hope that helps,

Max.

 

 

From: Ifeffit [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf
Of Luxton, Todd
Sent: Thursday, March 31, 2016 9:28 AM
To: ifeffit at millenia.cars.aps.anl.gov
Subject: [Ifeffit] Question about Io during data collection

 

All:  

 

Recently our group was at the APS collecting Pb L(III) spectra on an ID line
using quick XAFS.  Data was collected from -200 to +800 eV for Pb L(III) at
0.2 eV steps with a count rate of 0.025 seconds.  Each scan took about 2.5
minutes to complete.   During our measurements we began noting issues with
the linearity of scans collected in the extended region of the same sample.
After poking around in the data we noticed that Io was not linear throughout
the measurement for a portion of the scans.  This was not always the true
(see Athena project attached to the email, and images attache to the email).
I was always under the impression that Io should ideally remain linear
throughout the energy region scanned, or at least remain unchanged between
replicate scans.  We worked with the beamline scientists to try
alleviate/fix the issue, but it kept persisting.   I realize this is not an
IFEFFIT issue, but I was hoping someone might be able to help me understand
what was going on and why this was happening?  If I have not included enough
information for the question please let me know.



 



 

All the Best,

 

todd

 

 

All the Best,

 

Todd Luxton, Ph.D.

Office of Research and Development

National Risk Management Research Laboratory

Land Remediation and Pollution Control Division

Waste Management Branch

 

Mailing Address:

5995 Center Hill Ave

Cincinnati, OH 45243

 

Phone:

Office: (513) 569-7210

Cell: (513) 319-5104

Fax: (513) 569-7879

 

Email: Luxton.todd at epa.gov <mailto:Luxton.todd at epa.gov> 

 

"A nation that destroys its soils destroys itself"

Franklin D. Rosevelt

 

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