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

[Chris Patridge]

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 <jatinkumar.r...@helmholtz-berlin.de 
> 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-requ...@millenia.cars.aps.anl.gov wrote:
Send Ifeffit mailing list submissions to
   ifeffit@millenia.cars.aps.anl.gov

To subscribe or unsubscribe via the World Wide Web, visit
   http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
or, via email, send a message with subject or body 'help' to
   ifeffit-requ...@millenia.cars.aps.anl.gov

You can reach the person managing the list at
   ifeffit-ow...@millenia.cars.aps.anl.gov

When replying, please edit your Subject line so it is more specific
than "Re: Contents of Ifeffit digest..."


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"<fren...@bnl.gov>
To: "XAFS Analysis using Ifeffit"<ifeffit@millenia.cars.aps.anl.gov>
Subject: Re: [Ifeffit] Multiple scattering paths in fitting
Message-ID:
   <ccbf39f82283424bbb90ea208cd9de8001417...@exchangemb5.bnl.gov>
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



-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/ms-tnef
Size: 3479 bytes
Desc: not available
URL:<http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20101007/35cd3550/attachment-0001.bin 
>

------------------------------

Message: 2
Date: Thu, 7 Oct 2010 06:58:04 -0700
From: Scott Calvin<dr.scott.cal...@gmail.com>
To: XAFS Analysis using Ifeffit<ifeffit@millenia.cars.aps.anl.gov>
Subject: Re: [Ifeffit] schemes for delr and sigma2 for multiple
   scattering    paths
Message-ID:<86af0804-8418-4943-bbb3-163f7e639...@gmail.edu>
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
_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit



------------------------------

_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit


End of Ifeffit Digest, Vol 92, Issue 6
**************************************




_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit


_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit