Re: [Ifeffit] How to distinguish whether the coordination element is heavy or light

Hi Prof. Bruce， Thanks for your reply. In the picture you recommended,it says "The variations in functional form allow Z to be determined (±5or so) from analysis of the EXAFS".But I don't find any publication use it. I simply try an example.I choose the second shell of MoS2 which is the Mo-Mo shell in R-space in Athena,then do the back FT to get amplitude of the second coordination shell,then I compare to amp described in paper http://pubs.acs.org/doi/abs/10.1021/ja00505a003.The peak is at the almost same wave number when q-space use k2 weight(when use higher kweight,q space peak shifts to high wavenumber ),see pic attached. I only get a peak without the vally described in paper when use kweight,but can see a vally without k weight,maybe stress the contribution in high k make the vally obscure.Can this method works in element determination correct to +-5 Z number? And is there any method to determine whether a coordination peak in R space have one element contribution or more? For example ([Ifeffit] path contribution to fit in low R-space position, but the fit bond length is much longer than that ):a cluster we expected it has both Ni-O Ni-S(normally Ni-S peak is in high position),and when spectrum has a peak between regular Ni-O and Ni-S and a shouler near Ni-O,can I ensure the expection of both Ni-O Ni-S？OR just maybe because of the multiplicity of EXAFS. Sincerely, Zhanfei

-----原始邮件----- 发件人: "Bruce Ravel" 发送时间: 2014年7月3日 星期四 收件人: "XAFS Analysis using Ifeffit" 抄送: 主题: Re: [Ifeffit] How to distinguish whether the coordination element is heavy or light

On 07/02/2014 01:06 PM, ZHAN Fei wrote:

...

Dear all,

when I try to solve my question{*[Ifeffit] path contribution to fit in low R-space position, but the fit bond length is much longer than that*},I find a method from Prof.Calvin to distinguish whether coordination shell is composed by light or heavy element:

http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2007-December/007983.html

A clue can perhaps be obtained by noting the relative height of the peak near 2.3 angstroms compared with the large peak you've fit. As k- weight is raised from 0 to 1 to 2 to 3, the peak at 2.3 angstroms does not grow relative to the first peak. That suggests the scattering may be from another low-Z element like oxygen.

But I still don't know ,why it works.

Hi Zhanfei,

First off, I am very pleased to see that you are using the archive of the mailing list as a learning resource. Well done! Lots of questions have been answered here over the years!

On this page, http://xafs.org/Tutorials, you will find a link to a document by Matt called "The Fundamentals of XAFS". Look at figure 3.3 on page 15. It's a plot of the back-scattering amplitudes of three different elements.

As you can see, they have very different behavior as a function of k. Light things don't scatter very strongly at high k whereas heavy element do.

When you change the k-weight, you change how strongly the different regions contribute to the Fourier transform. If you have different elements contributing to the spectrum, then changing the k-weight may give you a way of emphasizing one contribution over another.

HTH, B

-- Bruce Ravel ------------------------------------ bravel@bnl.gov

National Institute of Standards and Technology Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973

Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit