As i understand, you only need to measure absorption coefficient for the thin film. For molar absorption coefficient, this is kind of same absorption coefficient for sample in solution.

Thanks Yu Bi. But I think you need to have a look to this article. From molar absorption coefficient one can calculate oscillator strength and electric dipole strength. But I am not sure how to achieve and use the article method.

Doi: 10.1016/j.optmat.2012.02.035

You're right. You need the molar absorption coefficient (or the absorption cross section, which is basically the same thing in a different language) to get the transition dipole moment.

However, in order to go from the absorption coefficient alpha (cm-1) to the molar extinction coefficient epsilon (M-1cm-1) you need to know the concentration of the absorbing chromophore. Otherwise what you want to do is impossible.

If you know the concentration (let's say it's C, expressed in chromophores/cm3), than you simply do this:

epsilon = alpha/(C * 3.82*1e-21)

If you DON'T know (and cannot calculate) the concentration, there is another way to estimate the electric dipole strength, which is using the radiative decay lifetime. But this is something you can only do if your sample is photoluminescent.

Hope this helps.