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 "(B) over bar (1)Sigma (+)(u)) in molecular hydrogen, lying above the ionization threshold, is reported. Rovibrational levels within a potential extended over internuclear separations of R = 7-25 a.u. are experimentally 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 different from similar outer well states of "g" symmetry, this phenomenon can be understood from the structure of doubly excited electronic states.