[Ifeffit] amplitude parameter S02 larger than 1

[huyanyun at physics.utoronto.ca]

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