To reiterate a point which is often not understood - dilution won't fix overabsorption if the particles of the concentrated material
which are mixed in with the diluent are too big. For concentrated S compounds, too big is in the low microns.
When I run a S standard, I do it in total electron yield to avoid this problem. When I fit a S spectrum, I use a crude
oveabsorption model with a single parameter which is treated as an unknown fitting parameter, with the assumption
that the reference spectra were collected in a way which avoids overabsorption.
mam
- Original Message -
From: Bruce Ravel bra...@bnl.gov
To: XAFS Analysis using Ifeffit ifeffit@millenia.cars.aps.anl.gov
Sent: Tuesday, February 14, 2012 6:07 AM
Subject: Re: [Ifeffit] fitting sulphur using standards by Athena
On Tuesday, February 14, 2012 07:55:54 am 陈娟 wrote:
As I known, the spectra for standard diluted with BN and non-diluted are
different. The peak of the non-diluted is low due to self-absorption, as
shown in the figure below. S1 is non-diluted and s1_10 is diluted sulphur
to about 3%. For dilution, the position of the peak is also changed.
Therefore the non-diluted s1 spectra isnot the real spectra of
dibenzothiophene. The question is what percentage should I dilute the
standard to get the credible spectra for the standards?and how to explain
the peak position change?
Although this sort of thing can be approximated from tables of
absorption coefficients, the advice I usually give to my users is to
prepare a series of samples of diminishing concetrations and measure
then.
Having a good intuition for sample preparation and the effects of poor
sample preparation is always important and even more so for low energy
edges. The best way to develop that intuition is by doing the
experiment yourself.
Can I use Athena to counteract the self-absorption and get the
credible spectra for sdandards?
Yes it can -- in principle. Doing a good job of self absorption
correction requires a lot of knowledge about your sample and its
matrix -- often more than one has.
The use of the self absorption tool is explained in the Athena
document. I have also written a lecture on this topic. Go to
https://github.com/bruceravel/XAS-Education/downloads
and download the selfabs.pdf file.
B
--
Bruce Ravel bra...@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
___
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
___
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
