[Ifeffit] amplitude parameter S02 larger than 1

[huyanyun at physics.utoronto.ca]

Hi Chris,

The site percentage is fitted to be 0.53+/-0.04 if let S02 to be free  
parameter. If I fix S02=0.9, the site percentage is fitted to be  
0.72+/-0.06. So the uncertainties are pretty small in both cases.

Best,
Yanyun
Quoting Chris Patridge <patridge at buffalo.edu>:

> The two sites would mostly likely have very similar SO2 values so it  
> should not matter that much within uncertainty
>
> You do get a value but what are the uncertainty for value of the  
> site percentages?  This floating value would have an uncertainty  
> associated with it in the fit.
>
> Chris
>
> Sent from my iPhone
>
>> On Mar 20, 2015, at 12:04 PM, huyanyun at physics.utoronto.ca wrote:
>>
>> Hi Chris,
>>
>> Thank you for your suggestion. But I don't have a standard. Also, I  
>> wonder whether a multiple-site situation could be different from  
>> the normal one-site case with respect to S02.
>>
>> Best,
>> Yanyun
>> Quoting Chris Patridge <patridge at buffalo.edu>:
>>
>>> Hi,
>>>
>>> One thing that could be considered is transferring the SO2 factor  
>>> from a reliable source such as a standard and then use that value  
>>> in the fit. Chemical transferability of SO2 to similar systems is  
>>> often acceptable. You could also try constraining the value in the  
>>> fit as well.  SO2 and Debye are also correlated so this may also  
>>> affect the value.
>>>
>>> Hope that helps,
>>>
>>> Chris
>>>
>>>
>>>
>>> Sent from my iPhone
>>>
>>>> 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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>>
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