Rotational Spectroscopy, Theory

The rotational spectra of gas phase molecules are explained using the quantum theory of angular momentum. The spectra of simple rigid molecules are explained on the basis of selection rules and energy level expressions which depend on the moments of inertia of the molecules. This article considers also how the theory needs to be extended to include effects of non-rigidity, and also small effects due to interactions between the angular momentum of the nuclear framework, electron spins, and nuclear spins. A calculation of relative intensities using spherical tensor techniques is given for a diatomic molecule using Hund’s case a.