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

[Jatinkumar Rana]

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 at millenia.cars.aps.anl.gov wrote:
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>     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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> ----------------------------------------------------------------------
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> Message: 1
> Date: Thu, 7 Oct 2010 09:38:26 -0400
> From: "Frenkel, Anatoly"<frenkel at bnl.gov>
> To: "XAFS Analysis using Ifeffit"<ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] Multiple scattering paths in fitting
> Message-ID:
> 	<CCBF39F82283424BBB90EA208CD9DE80014175C7 at exchangemb5.bnl.gov>
> Content-Type: text/plain; charset="iso-8859-1"
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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
>
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> ------------------------------
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> Message: 2
> Date: Thu, 7 Oct 2010 06:58:04 -0700
> From: Scott Calvin<dr.scott.calvin at gmail.com>
> To: XAFS Analysis using Ifeffit<ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] schemes for delr and sigma2 for multiple
> 	scattering	paths
> Message-ID:<86AF0804-8418-4943-BBB3-163F7E63983D at 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
>> _______________________________________________
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