Strong vibronic coupling in molecular Rydberg states

We consider the vibronic properties of molecular Rydberg states in the strong coupling (degenerate) limit of the ion core, and show how the observed optical Rydberg spectrum for such a molecule is closely related to the Franck-Condon spectrum of its Jahn-Teller ion core, regardless of the symmetry labels attached to the Rydberg electronic species. These relationships permit the resolution of a quarter-century old paradox in the Rydberg spectrum of benzene and give rise to easily verifiable predictions concerning benzene's vibronic spectrum, which are applied to a recently discovered new Rydberg series. We further suggest that observed deviations from the idealized Rydberg case discussed here will reveal additional details about the coupling of Rydberg electronic and core-vibronic motion.