Double-well states of ungerade symmetry in H-2: First observation and comparison with ab initio calculations

The observation of a new class of long-lived outer well states of ungerade symmetry (B "(B) over bar (1)Sigma (+)(u)) in molecular hydrogen, lying abo ve the ionization threshold, is reported. Rovibrational levels within a pot ential extended over internuclear separations of R = 7-25 a.u. are experime ntally investigated in a triple resonance scheme. Good agreement (<0.5 cm(- 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 dif ferent from similar outer well states of "g" symmetry, this phenomenon can be understood from the structure of doubly excited electronic states.