[Ifeffit] fitting sulphur using standards by Athena

Matthew Marcus mamarcus at lbl.gov
Tue Feb 14 09:33:38 CST 2012

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.
----- Original Message ----- 
From: "Bruce Ravel" <bravel at bnl.gov>
To: "XAFS Analysis using Ifeffit" <ifeffit at 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  ------------------------------------ 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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