Hund's cases for rotating diatomic molecules and for atomic collisions: Angular momentum coupling schemes and orbital alignment

The classification of rotational spectra of diatomic molecules according to the five Hund's cases provides a guide to the quantum mechanical formulation of alternative coupling schemes for the treatment of collisions between atoms having nonzero internal - spin and/or electronic - angular momenta. This paper presents an account of the corresponding diabatic representations and of the orthogonal transformations among eases; closure properties are derived from a pentagonal cyclic arrangement of angular momentum recoupling transformations and a limiting relationship between re-coupling and coupling coefficients. Adiabatic representations are also discussed, with reference to the description of orbital alignment in atomic collisions. Extensions to inelastic molecular encounters and reactions are briefly outlined.