See below for clarification..

-----Original Message----- From: Kelly, Shelly D. Sent: Wednesday, October 13, 2004 3:10 PM To: XAFS Analysis using Ifeffit Subject: RE: [Ifeffit] delR and delE

Ritika,

delr and Eo are correlated. So you can (1) force delr to the same for two paths with different Eo-values, or (2) use two different delr-values with one Eo-value. Most prefer the second choice over the second choice.

This should read "Most prefer the second choice over the first."

Also, Eo is important at low k-values. If you move your k-range that you use in the FT of your data you might find a different Eo value, or that only one Eo is needed with a shorter k-range than with a longer k-range.

I do not think that you model is "okay" with three Eo values. How much does EO differ? Usually Eo is the last button I turn on when I try to make a model fit the data. Large Ezero values (> 10 eV) are common if you have a poor model. So use only one Eo at the beginning. Often I just set it to zero, and then when the model gets close, try to add one or two more to really get it right on.

I would try only one Eo first. Let delr vary for all the paths and change the k-range. See how small the k-range needs to be before you can fit the data with only one E0 value.

1.9 Angstroms is a short distance. It would seem more reasonable to me that you might need two Eo values. One for this short shell and then one more for all the other shells.

Give these things a try and tell us how it goes.

Shelly

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-----Original Message----- From: ritika.uppal@yale.edu [mailto:ritika.uppal@yale.edu] Sent: Wednesday, October 13, 2004 2:50 PM To: Segre, Carlo U.; XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] delR and delE

I have 3 different values of Eo for the fit. 2 for the first shell and the 3rd for the rest of the paths. Actually the bond distance of the 6 oxygens in the first shell differs. 3 of them have a bond length of around 1.9 A and the other 3 around 2.2 A. So using one value of Eo does not fit the second peak very well.

Ritika

Quoting "Carlo U. Segre" :

...

delR is just the deviation from the distance you used in

...

file to generate the paths. It can be positive or negative depending on

whether these atoms are closer or further away from the central atom.

By delE, do you mean Eo? When you say two values, do you mean different values for two different shells? Carlo

On Wed, 13 Oct 2004, ritika.uppal@yale.edu wrote:

...

Hi guys, I have a few questions about the fitting parameters.

the feff.inp the delR for

...

my

...

fit has a negative value. Is that wrong or its possible to have a negative value?? and also for the first shell, I have 2 values of del E. Does that mean that the model I am using is not a good one??

Thanks for all your help Ritika _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

-- Carlo U. Segre -- Professor of Physics Associate Dean for Special Projects, Graduate College Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 Carlo.Segre@iit.edu http://www.iit.edu/~segre _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

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