Hi Todd, One thing to try is to set s02 to a positive value and all the other variables to reasonable values and then look at the real or imaginary parts of the Fourier transform and also look at the chi(k) spectra. You should "see" the affect that Scott mentioned. Shelly

-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit- bounces@millenia.cars.aps.anl.gov] On Behalf Of Scott Calvin Sent: Tuesday, January 09, 2007 10:27 AM To: 'XAFS Analysis using Ifeffit' Subject: Re: [Ifeffit] Is there a physical meaning to a negative SO2

Hi Todd,

Your paragraph below is correct; i.e. you do not have a physically realistic model. One common cause of a negative S02 is that the model also is off by half an oscillation. After all, the negative S02 turns the signal upside down, shifting it by half an oscillation can then cause a rough (but spurious) alignment between model and data.

In any case, the model needs work; depending on your system and degree of prior knowledge, you might be assuming the wrong species of scattering atom, for example.

--Scott Calvin Sarah Lawrence College

At 10:41 AM 1/9/2007, Todd Luxton wrote:

...

I am very new to XAFS analysis, I am wondering if there is a physical meaning to a negative value for SO2. Based on the XAFS equation I can't understand what the physical significance of a negative SO2 value would be, therefore I am assuming that my modeling results using Athena are not representative of a physically realistic model due to the negative value calculated fro SO2 or more specifically the amp variable in the guess set. Any insight would be greatly appreciated.

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