Fwd: average first shell position and width
Moneeb,
I'm in the middle of beam run, so I am forwarding this along to the
Ifeffit mailing list for commentary.
B
---------- Forwarded Message from Moneeb Shatnawi
Hi Moneeb, I'll bite at this one (and promise to carefully inspect the Send-To line!)
Dear Dr. Ravel, Hello, I am Moneeb Shatnawi from physics department at Michigan State University,
I have a PDF and XAFS data on GeSe system, and I would like to check if the PDF and XAFS results are consistent with each other in terms of the first PDF peak position and width.
The first PDF peak in this system is a mixed shell of Se-Se bonds as well as Ge-Se bonds.
The PDF data gave a width of the first PDF peak of about 0.06 A, and the XAFS data gave sigma^2 for Ge-Se bond is 0.006 A^2, and for Se-Se bond is 0.003 A^2.
How is sigma in XAFS results (I was using Artemis program) related to the width of the first PDF peak?
It sounds like you're asking what the width of the PDF peak means.... As with any "width of the peak" measurement, this will depend on details of both the measurement the physics. Since the PDF "G(r)" (which is what I assume your "peaks" are in) I normally see is both positive and negative, I think defining exactly what a "peak" is might be subtle. So: as far as I understand, the g(R) that PDF measurements sample are broadened by both the k-range of the measurement and the width of the distribution atomic distances. For a system (perhaps such as yours) with a mixed distribution of atoms, that would include both the distribution of all atoms. Whether the total of the partial g(r)'s or the k-range dominates the width of the PDF peak probably depends on several experimental details. The sigma2 gives the second moment of the Gaussian distribution that best matches the partial g(R) to match the data. The width of the XAFS peak has essentially no physical meaning. But, perhaps I misunderstood your question?
Also, when obtaining the XAFS fits to the data, I was using: Theory ---> first shell fit. In this, I was giving r_eff for Se-Se bond to be 2.32 A and that for Ge-Se to be 2.368 A. Through the XAFS refinement, I obtained r, dr as well as r_eff. which of these should be considered as the actual bond length (r or r_eff)? and how are these related to the average position that I get from fitting the first PDF peak in this system?
The bond length is r_eff + dr. That's the centroid of the distribution that best matches the data. How that relates to the g(R) measured by PDF is much a more interesting question. At some level, it will depend on how you model the g(R) to best match the PDF data. --Matt
On Thursday 28 June 2007, Matt Newville wrote:
Also, when obtaining the XAFS fits to the data, I was using: Theory ---> first shell fit. In this, I was giving r_eff for Se-Se bond to be 2.32 A and that for Ge-Se to be 2.368 A. Through the XAFS refinement, I obtained r, dr as well as r_eff. which of these should be considered as the actual bond length (r or r_eff)? and how are these related to the average position that I get from fitting the first PDF peak in this system?
The bond length is r_eff + dr. That's the centroid of the distribution that best matches the data.
How that relates to the g(R) measured by PDF is much a more interesting question. At some level, it will depend on how you model the g(R) to best match the PDF data.
Well.... it has as much to do with how you model the distribution in EXAFS as in PDF. Using the Artemis' quick first shell utility with no further modification conflates structural and thermal disorder. How those two terms get mixed together will have significant bearing on how you might relate the parameters of the distributions in EXAFS and PDF. If you are using a very simple-mided model to interpret the EXAFS ( Artemis' quick first shell theory utility certainly provides you withj a *very* simple mided model) but a rather sophisticated model to interpret the PDF, then I suspect it will be very hard indeed to relate the parameters of the distributions in a physically meaningful way. B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/
participants (2)
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Bruce Ravel -
Matt Newville