[Ifeffit] Self absorption questions

Richard Mayes mayes at ion.chem.utk.edu
Wed Nov 7 09:39:30 CST 2007

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?



Richard Mayes

Graduate Research Assistant
Barnes Group
450/452 Buehler Hall
Department of Chemistry
University of Tennessee
Knoxville, TN 37996

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