EXAFS analysis of homonuclear metal clusters
Dear mailing list - This is a general EXAFS question I decided to post in regards to your collective thoughts and/or experience with this EXAFS analysis approach. I am currently attempting to model a number of Fe-S clusters with nuclearities of 2, 3, 4, 5, and 6. In some cases (where all the Fe atoms are chemically equivalent i.e. same oxidation state and coordination geometry) it is easy to approach this by assuming a single Fe absorber model. When the Fe atoms become inequivalent (oxidation states which may be a mixture of I, II or III, and different coordination geometry) I can average the environments (by including all interactions and ajusting coordination numbers for the different iron atoms accordingly) again into a single Fe absorber model. The fits go well, giving realiable bond lengths (when compared to available crystallographic data), DW's, and statistics (I am also keeping in mind the number of refined parameters to ensure a well over-determined fit for each of the models). My only hesitation, and main reason I am posting this question, is that I am not completely sure this is a reasonable assumption as the Fe atoms are in different oxidation states and will have different E0's. What I do know is there is a shift of roughly 2 to 3 eV in E0 to higher energy for every increase, by one, in oxidation state (i.e. Fe(I) to Fe(II)). Is there any way of determining if this small difference in energy, and hence E0, between the Fe atoms in the clusters can or cannot be neglected in modelling the EXAFS? Regards, Mark -- Mark Bondin PhD student School of Chemistry, University of Melbourne (03) 8344 7015 (Tower lab) 0409 252 983 (mobile)
On Thursday 29 July 2004 08:49 pm, m.bondin@pgrad.unimelb.edu.au wrote:
Is there any way of determining if this small difference in energy, and hence E0, between the Fe atoms in the clusters can or cannot be neglected in modelling the EXAFS?
The generic answer to this sort of question is that the fit should tell you what the data will support. Are the data well fit in the R-factor sense? Are the fitted values for e0 different outside their error bars? Does the reduced chi-square value decrease significantly when you add the many e0 values? Are the e0 values physcially justifiable? Are the other parameters physically reasonable? At the end of the day you may be inclined to believe a particular fitting model, but not solidly convinced. That is not a failure of analysis (either by the software or by its user), it just means that the data is supported by a particular model, but not conclusively. In that case, I would say it is good science to report your inclination so long as you also report an honest assessment of the data and an honest assessment of your uncertainty in the result. HTH B -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/
participants (2)
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Bruce Ravel
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m.bondin@pgrad.unimelb.edu.au