Pure rotational spectrum of a rigid diatomic molecule would consist of a series of equally spaced lines corresponding to the transitions: (J=0) to (J=1), (J=1) to (J-2), (J=2)to (J=3), .. The transition probability calculations show that the probability of all changes with ΔJ =1 is almost the same, i.e., all such transitions are equally likely to occur. Despite the above facts, all the spectral lines do not have the same intensity. This is because there will be different numbers of molecules in each level and therefore the numbers of molecules that undergo transition from different levels will also be different. Thus, since the intrinsic probabilities are identical, the line intensities will be directly proportional to the initial numbers of molecules in each level.
The factors that decide the numbers of molecules present in different levels (i.e., the population of the levels) and, thereby, the intensities of spectral lines are
(i) The Boltzmann distribution in the levels and
(ii) the possibility of degeneracy in the energy states.