I Spectroscopy: vibronic and rotational transitions

[pisluca99]

In spectroscopy, the highest peaks in the absorption spectrum are those that are associated with the most probable energy transitions in a molecule. The most probable transitions are those in which the best superposition between the wave function of the vibronic level of the fundamental state and the wave function of the vibronic level of the excited state in which the molecule arrives after absorbing radiation occurs: in the image this corresponds to the transition v'' = 0 --> v' = 2, so this transition is associated with the highest peak. However, this is an argument that applies only to vibronic transitions, but each vibrational level in turn has many rotational sublevels (J0, J1, J2, etc.) at which the molecule can arrive. That said, how do you figure out which rotational transition is the most probable, again within the second vibronic level of the excited state?

 

[Delta Prime]

This as you know! Is the Franck-Condon principle & is based on the Born-Oppenheimer approximation, which allows separation of the electronic q and nuclear Q
wave functions given the total wave function. If you would have read a little more you would have answered your own question. Enjoy!
https://chem.libretexts.org/Courses...ectroscopy/13.07:_The_Franck-Condon_Principle