[Ifeffit] EXAFS above 5 angstroms

Matthew Marcus mamarcus at lbl.gov
Mon May 21 19:23:12 CDT 2018

One of the biggest inhibiting factors for seeing long distances is that there are generally many, many paths, which interfere with each other so that you don't see much.
OTOH, there are sometimes special circumstances such that one set of paths stands out above the crowd and you can sometimes see them.  One example that I had was a
Cr bis-arene complex (I think cyclopentadiene, so that Cr was sandwiched between two rings).  At RT, it was nothing special, but at 11K, the Cr-Cr distance popped out
at 7A.  That also shows the role of vibrations in causing shells to disappear.  One thing that can give let you see distant shells is focusing by atoms in a straight
line in between the absorber and scatterer.  This is sort of like the way a galaxy cluster can make it possible to see a very distant galaxy behind it due to
gravitational lensing.

Unless you give FEFF realistic MSRD's, then it will tend to overestimate the visibility of distant shells.

Depending on which edge you're on, core-hole lifetime can indeed compromise the visibility of high shells.  You can see it two ways: the uncertainty principle
shows that the energy resolution is limited by core-hole lifetime, and you can also calculate how long it takes for the electron to get to the scatterer and
back again.  If the core hole decays before the round trip is complete, then the absorption event doesn't contribute to the EXAFS.  As they say in textbooks,
it is left to the student to verify that these two approaches give you the same criterion for the visibility of a distant shell.  This effect is indeed significant
for heavy elements, either at the L-edge (5d and actinides) or K-edge (4d and up).

    Matthew Marcus
> Daniel,
> Those distances are really far away and I think it would really difficult to see in EXAFS even with pretty high quality data.  Is there are other information from diffraction that suggests these distances?
> Chris
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> Dr Christopher Patridge
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>> On May 21, 2018, at 7:13 PM, Daniel Sneed <sneedd3 at unlv.nevada.edu <mailto:sneedd3 at unlv.nevada.edu>> wrote:
>> Hello,
>> This is my first time posting to the mailing list, so please forgive me if I did not do it correctly.
>> I am currently working on EXAFS of tin(IV) oxide, and I have what appears to be high quality data out to 9 angstroms or so, but I am having some issues in fitting the data above 4 angstroms. I am attempting to verify a crystal structure with a= ~6 angstroms, and another structure with a=~9 angstroms. I know that there is some inhibiting physics at such long distances, such as inelastic scattering, and core-hole lifetime issues that can effect the data. My question for the group is, is there any methodical way to deal with these issues, other than incorporating Ei and the higher cumulants? Or is it even possible to get results from such large R?
>> Thank you for your time.
>> regards,
>> Daniel Sneed
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