[Ifeffit] Is there a physical meaning to a negative SO2

Scott Calvin SCalvin at slc.edu
Tue Jan 9 10:26:31 CST 2007

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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