Spectroscopic study of the second 3Πg state of H2 : A bound triplet state perturbed by a doubly excited state

A double-resonance technique is employed to measure the positions of energy levels of the second 3Πg state of H2. Vibrational levels of this state with v>3 lie above the ionization threshold of H+2, and are observed both to autoionize and dissociate. This state, 23Πg, arises from the homogeneous interaction between the singly excited 1sσg3dπg and 1sσg4dπg diabatic Rydberg states with the core-excited 2pσu2pπu state. The energy-level spacings are in good agreement with a simple model calculation for the interacting potentials. We also include a listing of autoionizing levels lying in the same energy range.