Theory of the Rydberg spectrum of triatomic hydrogen

Energies and properties of the fifteen lowest Rydberg states of triatomic hydrogen are computed using Koopmans theorem with a large Gaussian basis set constructed to represent both core and Rydberg orbitals. Tests indicate that computed virtual orbital energies have converged to within a few wave numbers of the spd SCF limit. AU excited electronic states are computed to have geometries and vibrational force constants close to those of H₃⁺. Jahn-Teller effects are weak. Small quantum defects and nearly integer values of electronic angular momenta indicate atomlike character for the Rydberg states. Predicted vibronic transitions are in remarkably close agreement with rotational band spectra recently observed by Herzberg. Results using our model are in complete accord with Herzberg's assignment of the strong emission bands observed near 5600 and 7100 Å, and offer a reasonable interpretation of the previously unassigned, weak emission bands observed near 6200 and 8500 Å.