On the internal partition functions for the diatomic molecules N2, O2, NO, and their ions—I. Symmetry and spin considerations

When excited vibrational states are encountered, the rotational states are dependent upon the vibrational states. When excited electronic states are encountered, the vibrational states, in turn, become dependent upon the electronic states, thereby also coupling the rotational states to the electronic states. The task of determining the internal partition functions of diatomic molecules and their ions is influenced by the restrictions imposed by symmetry and spin considerations and predissociation phenomena. A factor is derived which represents the symmetry and spin considerations necessary to calculating the internal partition functions for Bose-Einstein and Fermi-Dirac diatomic molecules. The factor is employed computationally as a multiplier on the rotational degeneracies, 2J + 1. Two forms of the factor are presented, one for use at lower temperatures and the other for use at higher temperatures.