Jahn-Teller enhanced fundamental bands in the infrared spectra of degenerate electronic states

Abstract

Scharf and Miller have shown recently that the symmetry selection rule of Child and Longuet-Higgins for infrared vibrational transitions in degenerate electronic states is incomplete, and that vibrational transitions that do not satisfy their selection rule can be exceptionally strong because the intensity is enhanced by the Jahn-Teller effect. The present paper provides a compact set of symmetry rules for the Jahn-Teller-enhanced vibrations, and considers the general rotational structure for the three types of Jahn-Teller-enhanced fundamentals, namely symmetric-top perpendicular bands, symmetric-top parallel bands, and spherical-top bands. The detailed equations of Child and Longuet-Higgins for the symmetric-top perpendicular case are valid, including the Jahn-Teller enhancement, but their generalization to other cases does not take account of the distinctive symmetry properties of the Jahn-Teller enhanced contribution. In the present calculations the use of an effective dipole moment operator to allow for the mixing of states by the Jahn-Teller interaction shows that the symmetry of the enhanced transitions is associated with the antisymmetric square of the electronic symmetry species, whereas the normal contributions considered by Child and Longuet-Higgins are associated with the symmetric square.