[Ifeffit] How to do with diffraction peak

[Bruce Ravel]

On Tuesday, December 11, 2012 04:08:03 PM Zhaomo Tian wrote:
> I got XAFS data for Ag with CO adsorption( using Ag K edge), Ag is thin
> film~300nm deposited on Si/SiO2 substrate. But in the original μ(E)
> spectra, from 25894-26044eV, four obvious diffraction peaks appear( I
> attached the file), and I guess they will influence the quality of fitting.
> Is there anyone who knows how to deal with these diffraction peak? Will it
> be corrected by smoothing or changing some origin data points in the
> original file? I want the modification that will not destroy data analysis
> later.

You have received some decent advice on how to post-process your data
to minimize the effect of the diffraction peak.  I think it bears
mentioning that not all prblems are best solved after the fact in
software.  Some problems should be addressed as the data are measured
or even beforehand.

I see from the data file you sent that you were at beamline 10C at
Pohang.  It is hard to tell for certain from the data file or from the
website, but it seems as though you were using a PIPS detector to
measure your fluorescence XAFS.

This is an experimental setup can be a difficult one to combine with a
sample that diffracts.  The sort of PIPS commonly used at an XAS
beamline tends to be of a very large surface area.  That means that
the likelihood of some diffraction peak from the sample hitting the
detector during the measurement is pretty high.  In fact, it happened
4 times for you.  That means you have a lot of data points to
deglitch, were you to follow Kicaj's advice.  Because you used a PIPS
rather than a multi-element detector, you don't have the option of
following any of Matthew's (excellent) advice.

I hate to say it, but I think you are screwed.  The best you can do is
deglitch as best you can.  Because you will be removing so many points
from the data, you will introduce substantial systematic error into
the data set that remains.  I don't really see what you can do about
that at this late date.

So, what might you do the next time you visit the synchrotron to
obtain better data?  In fact, there are number of things that you can
consider at the stages of sample preparation or of data collection.

1. You don't say a lot about the sample or its substrate.  Perhaps you
   have a reason that the substrate *must* be crystalline.  Perhaps
   not.  Putting your film on an amorphous substrate would obviate the
   problem of diffraction from the substrate.  That may be your best
   bet.

2. The large size of the PIPS detector is a contributing factor to the
   problem.  Simply using a detector with a smaller surface area
   reduces (but certainly does not eliminate!) the likelihood of
   diffraction peaks hiiting it.  Moving the detector farther away
   from the sample would serve the same purpose.  Of course, doing so
   would also serve to reduce your count rate, thus reducing the
   quality of your data.  There is a cost to everything!

3. Use the 13 element Ge detector instead of the PIPS.  Then you can
   simply eliminate channels hit by Bragg peaks or do the
   post-processing trick Matthew described.

4. At my beamline, we have users almost every cycle who measure stuff
   on crystalline substrates.  Our favorite trick is to mount the
   sample on a spinner (e.g. http://dx.doi.org/10.1063/1.1147815) At
   my beamline, we actually attach the sample the sort of small DC fan
   that is used to cool electronics.  Inexpensive, simple, and
   effective!  By keeping the sample constantly in motion, the energy
   at which the Bragg condition is met is constantly changing.  This
   serves to reduce the effect of the Bragg peak by a few orders of
   magnitude by spreading it out in energy.  Usually, it can be made
   smaller than chi(k), resulting in analyzable data.

The bottom line is that you have the sort of problem that I think
needs to be solved up front rather than after the fact.

I know that's not helpful right now, but hopefully it will be the next
time you go to the beamline.

Cheers,
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
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