[Ifeffit] amplitude parameter S02 larger than 1

Scott Calvin scalvin at sarahlawrence.edu
Fri Mar 20 09:59:24 CDT 2015

Hi Yanyun,

I am hesitant to promote a commercial project from which I directly profit on this list, but it seems to me you are asking a bigger set of questions than can comfortably and sufficiently be answered in this format, and they are questions which have been answered in detail elsewhere.

In my book XAFS for Everyone, I have four pages devoted solely to S02, along with related information elsewhere in the book.

Since you have a University of Toronto address, I am guessing you have access to their library. If you don't wish to purchase the book, you can request it via interlibrary loan, at no cost to you or your institution. 

In the mean time, a quote from the book that may be useful in thinking about S02:

"Alternatively, one can treat So2 as a phenomenological parameter that accounts for any amplitude suppression independent of k and R, regardless of physical cause (Krappe and 
Rossner 2004). Under this view, So2 does not have any particular physical meaning, and the k or R dependence of intrinsic losses can be assigned to other parameters."

That's the way I usually think about it--as not having a single physical meaning, but rather as being an empirically observed correction factor relative to simplistic theories which is indicative both of experimental effects and limitations in the theoretical model.

Hope that helps...

--Scott Calvin
Sarah Lawrence College

> On Mar 19, 2015, at 6:32 PM, huyanyun at physics.utoronto.ca wrote:
> Hi all,
> I know this question has been asked for many times. S02 is expected to  
> be around, but smaller than 1,  a fact that has been explained, such  
> as in the following previous emails, in our mailing list.
> http://www.mail-archive.com/ifeffit%40millenia.cars.aps.anl.gov/msg02237.html
> http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2003-February/000230.html
> However, I am continually get S02 value larger than 1 for a series of  
> similar samples when I fit data in Artemis. I think my fit is very  
> good, because my suspected model(based on other technique) could be  
> verified in XAFS analysis (i.e., defensible in physics), the  
> statistics is good ( R=0.01, reduced chi-square=31.4, fit-range:1.5~6  
> Angstrom, k-range: 3~14 angstrom-1) and all the parameters such as the  
> bond length, sigma2 are physically reasonable. The only thing makes me  
> uncomfortable is that parameter S02 keeps between 1.45 to 1.55 during  
> the fitting.
> In my system, the absorber atom occupies two crystallographic sites.  
> So I built a model with paths generated from two FEFF calculations.  
> For paths generated from the 1st and 2nd FEFF calculation, the  
> amplitude parameters are set to be S02*P% and S02*(1-P%) respectively,  
> where P% is the first site occupancy percentage. Both S02 and P are  
> free parameters during the fit, and P is an important conclusion I  
> want to extract from XAFS fitting.
> However, the fit result gives me S02=1.45 ~ 1.55 and P=0.51 ~ 0.56 all  
> the time (i.e., for each path the 'total amplitude' S02*P% or  
> S02*(1-P%) are about 0.7~0.8, smaller than 1). It looks to me that I  
> got a 'perfect' fit but I am not sure if S02 larger than one is  
> defensible. So I have to ask:
> 1) Is my current fit with S02 larger than one reasonable? If not, what  
> could be suggested to get around it?
> 2) What's the meaning of S02? It is interpreted in physics that it is  
> a reduced electron excitation parameter, but is it possible that S02  
> will be affected by any experimental condition?
> 3) Can anyone share whether you had the multiple site system that gets  
> S02 larger than one?
> Looking forward to your help.
> Best,
> Yanyun
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