[Ifeffit] why ss_2 is negative?

[hw26 at njit.edu]

Dear all,
 
I am doing model fitting using Artemis. My sample is uranium solution with an 
ionic liquid,  1-methoxyethyl-3-methyl imidazolium tetrafluoroborate 
(MOEMIMBF4). The Fitting seems ok except that the ss_2 is negative. I don't 
know how can I fix this problem. Can you get a chance to take a look? My 
fitting is simple, only three single scattering pathes which were produced by 
Six Pack. The fitting file is attached. I really appreciate it if you could 
take a look and give some advice!

Best regards!
Hao

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