Rotational Spectroscopy

This chapter presents a basic treatment of rotational spectroscopy. The rotational degrees of freedom of a diatomic molecule are most simply described through the rigid rotor model. The rotational motion of a diatomic molecule, or any other linear molecule, is defined by a single moment of inertia. A nonlinear molecule has a more complicated distribution of masses and its rotational motion is described by three mutually perpendicular principal inertial axes A , B , and C . Nonlinear molecules are classified as spherical, symmetric, near-symmetric, or asymmetric tops. An asymmetric top has three different moments of inertia, and its rotational dynamics are complicated to calculate even with classical mechanics. When the Schrödinger equation for the rotational kinetic energy of a symmetric top is solved, the wavefunctions turn out to involve three quantum numbers: J , K , and M .