[Ifeffit] Ifeffit Digest, Vol 108, Issue 17

Bruce Ravel bravel at bnl.gov
Wed Feb 15 14:05:15 CST 2012

On Wednesday, February 15, 2012 01:52:50 am 陈娟 wrote:
> However, for the pure standards without dilution by BN, How to do the
> self-absorption correction? For example, I try one, but cannot get the
> credible spectra. ForDibenzyl Disulfide, the formula should be
> (C14H14S2)0.5435(BN)0, no dilution. I put (C14H14S2)0.5435(BN)0 in, but
> the spectrum I got is
> So can you tell me how to do the pure standards self-absorption correction
> using Athena?

The self-absorption corrections are not particularly robust
algorithms.  For pure materials, they tend to be quite sensitive to
errors in tabulated values, small errors in stoichiometry, and so on.

I don't really know what dibenzyl disulphide is or how you prepare the
sample for measurements.  I would guess that, when specifying the
stoichiometry, you will need to do something to account for the sample
holder.  For example, if you have a thin piece of polyimide tape
between the beam and the sample, that will have the effect of
"diluting" the sample, at least as far as the correction algorithm is

This (along with Matthew's point about particle size and many other
issues) is one of the reasons why self-absorption correction is such a
tricky business.  You could try making the coefficient for the (BN)
something non-zero to approximate the effect of the tape (or of any
water that might have been absorbed, or whatever).  I suspect that if
you do so, you will find that the correction will be numerically
stable.  Will it be right?  Who knows!



 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

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