Spectroscopic observation of bound ungerade ion-pair states in molecular hydrogen

Frequency-resolved observations of heavy Rydberg states in molecular hydrogen are reported in the ungerade manifold of states. Double-resonance spectroscopy via the $E,F$ ¹Σg⁺, v^'=6 state has been used to probe the energy region above the H($1s$) $+$ H(3l) dissociation threshold. Resonances are observed by ionizing H(3l) to produce H⁺, which is then detected by using a time-of-flight mass spectrometer. The kinetic energy of the H⁺ ion confirms that the observed signal is due to the photoionization of neutral H(3l) atoms, indicating that dissociation is a significant decay channel for the ion-pair states. The pattern of energies of the resonances agree well with the predictions of a mass-scaled Rydberg formula for bound quantum states of the H⁺H^- ion pair. Energies and quantum defects have been determined for principal quantum numbers in the range of $n=130$ to 207.