Argon Plasma , Excitation temperature. Help!

soufi16

Hallo!
I am analyzing the spectrum of Argon only plasma (from Optical emission spectroscopy method). I try to find the excitation temperature using boltzmann plot method. The plasma was in vacuum. But, I get problem. I make a linear plot of upper level energy of Ar I vs ln(Iλ/gA), but I got positive slope instead of negative slope. based on boltzmann, the slope should be negative.
Anyone can help me????

Thank You !!!