I use integral PL intensity vs 1/T graph (Arrhenius plot) to find Ea and I know to find activation energy using this formula I(T)= I(0K)/ (1+ A exp (-Ea/KT), but I don't know how to find the constant. I'm using Origin.
SImple ... use the values of the I at one temperature (T1) from the extrapolated Arrhenious plot and T1 in your formula ... Constant "A" is the only remaining unknown parameter.
I suppose you have to rearrange the equation to the following form: [ I(0)/I(T) - 1 ] = A exp (-Ea/KT). Then, after taking a natural logarithm, you obtain: ln [I(0)/I(T) - 1] = lnA - Ea/KT. Now, you can make a plot of ln [I(0)/I(T) - 1] vs. 1/T and ln A is the intercept of the linear fit to the data.
Fit with non-linear regression instead of mangling the Arrhenius equation by linearizing it. Then you will get truer values for the uncertainties of your fitting parameters rather than some sloppy, meaningless estimates.
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