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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<[email protected]>
To: XAFS Analysis using Ifeffit<[email protected]>
Subject: Re: [Ifeffit] schemes for delr and sigma2 for multiple
scattering paths
Message-ID:<[email protected]>
Content-Type: text/plain; charset=us-ascii; format=flowed;
delsp=yes
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<[email protected]
> 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, [email protected] wrote:
Send Ifeffit mailing list submissions to
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When replying, please edit your Subject line so it is more specific
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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)
---
-------------------------------------------------------------------
Message: 1
Date: Thu, 7 Oct 2010 09:38:26 -0400
From: "Frenkel, Anatoly"<[email protected]>
To: "XAFS Analysis using Ifeffit"<[email protected]>
Subject: Re: [Ifeffit] Multiple scattering paths in fitting
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
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
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Message: 2
Date: Thu, 7 Oct 2010 06:58:04 -0700
From: Scott Calvin<[email protected]>
To: XAFS Analysis using Ifeffit<[email protected]>
Subject: Re: [Ifeffit] schemes for delr and sigma2 for multiple
scattering paths
Message-ID:<[email protected]>
Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
Jatin,
IF the uniform expansion model is valid for single scattering paths,
then it is for multiple scattering paths as well. For some materials,
particularly those with cubic space groups, that's got a good chance
of being a useful model. Others tends to distort with changes in
temperature, doping, etc., and it may not work as well. But even in
those cases, if you've decided how to constrain the delr for single-
scattering paths, you'll do reasonably well by using some kind of
appropriate average of the delr's for related single-scattering
paths.
--Scott Calvin
Sarah Lawrence College
On Oct 7, 2010, at 3:05 AM, Jatinkumar Rana wrote:
Dear all,
It is reasonable to assign a constant fraction by which unit cell
expands at a given temperature of XAFS measuremnt and so the
variation
in the path lengths for every single scattering paths could be
assigned
as delr = alpha * Reff. Similarly, one can assign sigma2 value for
each
single scattering path depending on both type of scatterer and its
distance from the absorbing atom.
Now coming to multiple scattering paths, Sigma2 for multiple
scattering
paths can be constrained based on the sigma2 of related single
scattering paths and a definite path-geometry-dependent scheme
(Triangle, collinear, reversed etc.) could be applied.
Is there any such scheme for delr of multiple scattering paths ?
or we
can simply assume that all paths (single scattering and multiple
scattering) undergo uniform expansion by a factor alpha.
Thank you so much in advance for your valuable time...
With best regards,
Jatin Rana
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Message: 2
Date: Fri, 8 Oct 2010 10:50:52 -0400
From: Bruce Ravel<[email protected]>
To: XAFS Analysis using Ifeffit<[email protected]>
Subject: Re: [Ifeffit] schemes for delr and sigma2 for multiple
scattering paths
Message-ID:<[email protected]>
Content-Type: Text/Plain; charset="iso-8859-1"
On Friday 08 October 2010 09:30:14 am Jatinkumar Rana wrote:
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)
As always, it is important to consider the physical meaning of the
terms we use. \mu is the absorption coefficient, i.e. the thing that
goes up at the edge and up and down throughout the EXAFS.
Another way to say this is that \mu is some kind of measure of the
amplitude of the unoccupied (recall that electrons are fermions -- if
an electron already occupies a particular statem, the photoelectron
cannot transition into that state) portion of the density of states
projected onto the final state angular momentum. That's a mouthful.
In short, for a K edge, we measure the unoccupied portion of the p
band.
That portion of the density of states is not flat. It has structure
-- peaks and troughs. A peak is place where there is higher state
density and vice versa for a trough. Consequently, if the incident
photon has the amount of energy needed to raise the photoelectron to
an energy at which there is a peak in the density of states, then it
is relatively more likely to be absorbed than for a photon with an
energy that takes the photoelectron to a trough. The wiggles in the
XAS follow the ups and downs of the density of states.
Now suppose that one were interested in calculating an XAS spectrum.
Well, there are many theoretical frameworks for making such a
calculation. Feff (and, as a consequence, Ifeffit and Artemis) use an
approach called "real space multiple scattering". In this approach we
need to know two things -- the function that describes how an electron
travels between points in space and the function that describes how an
electron scatters off of an atom. Putting these two functions
together, we can now describe how an electon leaves the point in space
occupied by the absorbing atom, travels to a neighbor, scatters off
that neighbor, and continues traveling.
We are interested in computing the absorption at a particular atom.
The photoelecton starts in the deep core of the absorber and is
promoted to a higher lying state OF THE ABSORBER. Thus, the thing
that is relevant is to compute the density of states OF THE ABSORBER.
In the RSMS approach, the density of states is computed from the
overlap of the wavefunctions of the outwardly propagating
photoelectron with the functions of the various scattered waves.
The part of the overlap that is relevant to computing the density AT
THE ABSORBER is the bit that happens at the position OF THE ABSORBER.
So, the interaction in question is "a deep core electron is promoted
to an occupied state of the absorber". The computational tool used to
compute that interaction is RSMS. Happily, the parameters of the RSMS
approach (R, sigma^2, N) map readily onto the things that we want to
know when we do an XAS experiment.
Here are all the details, as implemented in Feff:
http://rmp.aps.org/abstract/RMP/v72/i3/p621_1
It's a dense read but it's also an excellent and very rewarding paper.
B
