[Ifeffit] Self absorption questions

Matthew Marcus mamarcus at lbl.gov
Wed Nov 7 23:23:15 CST 2007


MessageThanks.  It was never  intended as a comprehensive review.  It was intended to be what you described it as, a summary.
It's actually part of a larger review article on microprobe for soil science.  I broke it out for users to read because all
too many of them haven't a clue about this stuff.  In my work, samples are generally inhomogeneous
and of unknown composition, so inverse methods don't work.  What I do in XANES fitting is to use the simplest model of
overabsorption and have the one parameter that results be a fitting parameter.  This works better than it has any right to.
Inverse methods might, however, be useful for standards.  What I do now is to map a sample which has been ground fine and
deposited on tape or similar and look for small particles on which to do fluorescence.  The sneaky thing about overabsorption
is that even if you look at transmission, you can get fooled by hole effect, which distorts the transmission signal in a way
very similar to what overabsorption does to a fluorescence signal.  Thus, you might think you're OK because the transmission
edge jump is small and the fluorescence looks just like the transmission when both modes have been compromised by the same amount.

I'll look at the papers you cite.
    mam
  ----- Original Message ----- 
  From: Anatoly Frenkel 
  To: 'XAFS Analysis using Ifeffit' 
  Sent: Wednesday, November 07, 2007 5:56 PM
  Subject: Re: [Ifeffit] Self absorption questions


  Matthew:

  It is a beautiful summary. Thanks for putting it together.

  Here is another reference, for your collection:

  A. Ryazhkin, Y. Babanov, M. Takafumi
  "Thickness inhomogeneity and fluorescence effects in EXAFS spectroscopy for powder samples: solution of the inverse problem".
  J. Synchrotron Rad. (2001), 8, 291-293.

  as well as the earlier one:

  D. L. Brewe, D. M. Pease, J. I. Budnick, 
  "Corrections of residual fluorescence distortions for a glancing-emergence-angle x-ray-absorption technique "
  Phys. Rev. B 50, 9025-9030 (1994).

  and the later one:

  A. I. Frenkel, D. M. Pease, J. I. Budnick, P. Shanthakumar, T. Huang 
  Application of Glancing Emergent Angle Flourescence for Polarized XAFS Studies of Single Crystals 
  J. Synchrotron Rad. 14, 272-275 (2007)


  Anatoly


   

    -----Original Message-----
    From: ifeffit-bounces at millenia.cars.aps.anl.gov [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Matthew Marcus
    Sent: Wednesday, November 07, 2007 11:31 AM
    To: XAFS Analysis using Ifeffit
    Subject: Re: [Ifeffit] Self absorption questions


    You can find a writeup which explains overabsorption in great and gory detail at
    http://xraysweb.lbl.gov/uxas/Beamline/Operation/Operation.htm .
        mam
    ----- 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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