The observation of a new class of long-lived outer well states of ungerade symmetry $\left(B^{\prime \prime }\overline{B}{}_{}{}^1{}_{}{}^{}\Sigma _{\mathrm{u}}^{+}{}_{}{}^{}\right)$ in molecular hydrogen, lying above the ionization threshold, is reported. Rovibrational levels within a potential extended over internuclear separations of $R=7\mathrm{}–25\mathrm{a}.\mathrm{u}.\mathrm{}$ are experimentally investigated in a triple resonance scheme. Good agreement ( $<0.5\mathrm{}{\mathrm{cm}}^{-1\mathrm{}}$) with updated ab initio calculations is found for vibrational levels up to $v=26\mathrm{}$, 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.

• Received 23 October 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.2988