Re: [Ifeffit] schemes for delr and sigma2 for multiple scattering paths

12 Oct 2010

      Hi Matt,

Thank you very much. I have gone through the chapter in your Tutorial
and things are more convincing now.

With best regards,
Jatin Rana
HZB Berlin

On 09.10.2010 19:00, ifeffit-request@millenia.cars.aps.anl.gov wrote:
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Today's Topics:
1. Re: schemes for delr and sigma2 for multiple scattering	paths
       (Matt Newville)
    2. Re: schemes for delr and sigma2 for multiple scattering	paths
       (Jatinkumar Rana)
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Message: 1
Date: Fri, 8 Oct 2010 13:13:23 -0500
From: Matt Newville
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] schemes for delr and sigma2 for multiple
  scattering	paths
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Dear Jatin,
I tried to answer both of these question in chapter 3 of
     http://xafs.org/Tutorials?action=AttachFile&do=view&target=Newville_xas_fundamentals.pdf
Does the explanation there help?   If not, do you have any suggestions
for making it clearer?
The basic physics is that an absorbed x-ray creates a photo-electron
which will scatter from its neighbors.   A portion of the scattered
portion (so, a small amount) of that photo-electron will return to the
absorbing atom.  But Pauli's exclusion principle tells us that if an
electron is at the absorbing atom, and has the same energy (and other
quantum numbers) as the outgoing photo-electron, the outgoing
photo-electron cannot be created in the first place.   Thus, the
scattering of the photo-electron from the neighbors affects whether
the photo-electron can be created, and so whether the x-ray will be
absorbed.
The effect can be either destructive or constructive, as with nearly
any wave-like phenomenon.    It does seem a bit circular that the
photo-electron scattering affects the existence of the photo-electron,
but quantum mechanics leads to many much stranger behaviors.
--Matt
------------------------------
Message: 2
Date: Sat, 09 Oct 2010 09:41:12 +0200
From: Jatinkumar Rana
To: ifeffit@millenia.cars.aps.anl.gov
Subject: Re: [Ifeffit] schemes for delr and sigma2 for multiple
  scattering	paths
Message-ID:<4CB01C98.4020103@helmholtz-berlin.de>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hello Chris, Bruce,
Thank you so very much for your detailed explanations.
With best regards,
Jatin Rana
HZB Berlin
On 08.10.2010 19:00, ifeffit-request@millenia.cars.aps.anl.gov wrote:
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Today's Topics:
1. Re: schemes for delr and sigma2 for multiple scattering	paths
        (Chris Patridge)
     2. Re: schemes for delr and sigma2 for multiple scattering	paths
        (Bruce Ravel)
     3. Re: schemes for delr and sigma2 for multiple scattering	paths
        (Bruce Ravel)
----------------------------------------------------------------------
Message: 1
Date: Fri, 8 Oct 2010 10:09:34 -0400
From: Chris Patridge
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] schemes for delr and sigma2 for multiple
  scattering	paths
Message-ID:<68FC92BB-B842-48F2-8596-7675D168EB16@buffalo.edu>
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Hello Jatin,
I don't confess expertise but i think the absorption coeff. relates to
the overlap between the initial and final wavefunction. Therefore the
interference between outgoing and backscattering modulates the final
wavefunction which manifests as the exafs fluctuations past the edge
which in turn changes the It signal so we can use the difference
between Io and It to measure exafs data.  Any additional comments are
welcomed if I am mistaken as I said I am not an expert.
Buena salud,
Chris Patridge
On Oct 8, 2010, at 9:30 AM, Jatinkumar Rana
...
wrote:
...
Hi Scott,
Nice to see you back-in-action from holidays. Thank you very much
for your suggestions.
Since last few days i was trying to visualize the physical process
going on inside an absorbing atom when we measure EXAFS, i met few
really basic questions, which of course, could be due to lack of my
understandings. Therefore, I tried to go through literatures and
EXAFS books but i am not satisfied.
Therefore, i decided to put them to mailing list
My first question is :
In EXAFS, the oscillations are due to the intereference between the
outgoing photo electron from absorbing atom and incoming
backscattered photoelectron from the scatterer. Depending upon, how
much they are out of phase w.r.t each other we get oscillations in
EXAFS curve. These oscillations are nothing but the variation in the
absobtion coefficient of sample as the energy of incident photon is
varied.
I wonder, how, the intereference between the photo electrons is
related to absorbtion coefficient of sample ??
because, It = I0 * exp (-ut)
My second question is :
While measuring EXAFS at beamline, we only measure the intial
intensity (Io) and the transmitted intenstiy from sample (It). In
other words, we only measure the number of photons before and after
the sample and variation in abs.coeffi.(u) is related to variation
in It.
How interference between photoelectrons will be reflected as
variation in abs.coeffi. (u) of sample while we never measure photo
electrons in EXAFS??
We only measure Io and It.
Also, whether the intereference between photo electrons is
constructive or destructive, the incident photon will lose its
energy once it knocks an electron from say K-shell of absorbing atom
and hence photon is said to have absorbed.
I will be really thankful to you for your valuable explanation.
With Best regards,
Jatin Rana
HZB, Berlin
On 07.10.2010 19:00, ifeffit-request@millenia.cars.aps.anl.gov wrote:
...
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Today's Topics:
1. Re: Multiple scattering paths in fitting (Frenkel, Anatoly)
     2. Re: schemes for delr and sigma2 for multiple scattering
paths
        (Scott Calvin)
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Message: 1
Date: Thu, 7 Oct 2010 09:38:26 -0400
From: "Frenkel, Anatoly"
To: "XAFS Analysis using Ifeffit"
Subject: Re: [Ifeffit] Multiple scattering paths in fitting
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For relationships between sigma2 for MS and SS paths, some are
listed in the Appendix to this article:
P. Shanthakumar, et al, Physical Review B 74, 174103 (2006).
Anatoly