[Ifeffit] Self absorption questions

Matthew Marcus mamarcus at lbl.gov
Wed Nov 7 10:30:59 CST 2007

You can find a writeup which explains overabsorption in great and gory detail at
http://xraysweb.lbl.gov/uxas/Beamline/Operation/Operation.htm .
----- Original Message ----- 
From: "Richard Mayes" <mayes at ion.chem.utk.edu>
To: "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
Sent: Wednesday, November 07, 2007 7:39 AM
Subject: [Ifeffit] Self absorption questions

> Greetings all!
> I'm trying to explain self absorption for our group's guide to EXAFS and I
> need some help understanding some of the finer details of self absorption.
> In the realm of fluorescence EXAFS, am I correct in that at a given
> energy there is a certain penetration depth of the X-rays that is expected
> (in a thick, dilute sample), but in a thick, concentrated sample, the
> penetration depth is actually less than the expected depth in thick,
> dilute samples (causing the decrease in the amplitude of the oscillation
> peak in Bruce's "wiggle up" of the XAFS oscillation)?  Then, in the
> "wiggle down" of the XAFS oscillation, the penetration depth is actually
> more than what would be expected so the depth of the oscillation valley
> decreases.  As a result, for thick concentrated samples like a metal foil,
> in the "wiggle up," the fluorescence accounts for fewer than expected
> absorbing atoms and in the "wiggle down" the fluorescence accounts for
> more than expected absorbing atoms.
> Next, in our amorphous silicates (doped with Ti, V, Cr, W, or Sn), is the
> self absorption effect greater in the lower Z elements (like titanium)
> than in the higher Z elements (like tin)?  Does the high background due to
> the silicon at the lower energies (4.9 keV) in relation to the higher
> energies (29.2 keV) have an effect or is it just an underlying,
> independent issue that is more pronounced with the lower Z elements when
> self absorption occurs?
> Thanks!
> -Richard
> -- 
> Richard Mayes
> Graduate Research Assistant
> Barnes Group
> 450/452 Buehler Hall
> Department of Chemistry
> University of Tennessee
> Knoxville, TN 37996
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