[Ifeffit] amplitude parameter S02 larger than 1

Fri Mar 20 11:48:52 CDT 2015

Hi Scott,

Thank you. Our group has one copy of your book, I'll read it again  
after my colleague return it to shelf. I still want to continue our  
discussion here:

If we treat S02 as an empirically observed parameter, can I just set  
S02=0.9 or 1.45 and let other parameters to explain the k- and R-  
dependence? Because S02 is not a simplistic parameter which may  
include both theory and experimental effects, I feel that S02 is not  
necessarily to be smaller than 1, although I admit S02 smaller than 1  
is more defensible as it represents some limitations both in theory  
model and experiment, but I have a series of similar sample and all  
their S02 will be automatically be fitted to 1.45~1.55, not smaller  
than 1. Could this indicate something?

I actually found in my system, when I set S02=0.9 (instead of letting  
it fit to 1.45), other parameter will definitely change but the  
fitting is not terrible, it is still a close fit but important site  
occupancy percentage P% changed a lot.  So how should I compare/select  
from the two fits, one with S02=0.9 and one with S02=1.45 with two  
scenarios showing different results?

Best,
Yanyun
Quoting Scott Calvin <scalvin at sarahlawrence.edu>:

> 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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>
>
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