[Ifeffit] amplitude parameter S02 larger than 1

[huyanyun at physics.utoronto.ca]

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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