Rydberg Radicals. 1. Frozen-Core Model for Rydberg Levels of the Ammonium Radical

Rydberg radicals are transient polyatomic species stable with respect to dissociation in excited electronic states but dissociative on the ground-state surface. This paper proposes that these systems be treated theoretically by Rayleigh-Schrödinger perturbation theory (RSPT) that reduces to a frozen-core model in zeroth order. Special computational techniques permit the use of very large Gaussian basis sets for the Rydberg orbital space. The zeroth-order equations are solved to high accuracy, probably to within 10 cm^-1, for Rydberg orbital energies of NH₄ as well as for the isoelectronic sodium atom. Energies, force constants, Coriolis coupling constants, Jahn-Teller parameters, orbital radii, and transition moments are reported for Rydberg states of the ammonium radical up through the ²A₁(5s) level. Serious conflicts arise in comparing theoretical, spectroscopic, and molecular-beam results for NH₄ and ND₄. Some key areas for further investigation are outlined, e.g., resolution of the conflict between spectroscopic and molecular-beam values for the lifetime of the metastable ground state, and a 3000-cm^-1 discrepancy between experimental and best theoretical estimate of the frequency of the Schuster band.