Double-Well States of Ungerade Symmetry in H₂: First Observation and Comparison with Ab Initio Calculations

The observation of a new class of long-lived outer well states of ungerade symmetry (B^′′B̅ ¹Σ_u⁺) in molecular hydrogen, lying above the ionization threshold, is reported. Rovibrational levels within a potential extended over internuclear separations of R = 7–25a.u. are experimentally investigated in a triple resonance scheme. Good agreement ( <0.5cm^-1) with updated ab initio calculations is found for vibrational levels up to v = 26, demonstrating that such calculations can now be extended to this energetic range above ionization, as long as interaction with the Rydberg manifolds is shielded by a barrier. The dynamical behavior (predissociation and autoionization) of this class of “ u” symmetry states is remarkably different from similar outer well states of “ g” symmetry; this phenomenon can be understood from the structure of doubly excited electronic states.