Born-Oppenheimer approximation

The approximate time required for the various transitions are as follows:

            Electronic: 10^–15s;                     Vibrational: 10^–13s;                    Rotational: 10^–10s.        

Since the time scales and energies required for various types of transitions are very different, it may be safely assumed that one type of transition does not interfere with the others. This assumption is known as Born-Oppenheimer approximation. And since the energy range for each transition is also very different, we can have separate, independent spectra for rotational, vibrational and electronic transitions. Each of these spectra will give us different kinds of information about the molecule.