DOUBLE-WELL STATES OF UNGERADE SYMMETRY IN $H_{2}$: FIRST OBSERVATION AND COMPARISON WITH AB INITIO CALCULATIONS.

Author Institution: Vrije Universiteit; Laser Centre, Department of Physics and Astronomy, Vrije Universiteit; Laser Centre, Department of Physics and Astronomy, Nicholas Copernicus UniversityThe observation of a new class of long-lived outer well states of ungerade symmetry $(B^{\prime\prime}\bar{B}^{1}\Sigma^{+}_{u})$ 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 involving a tunable extreme ultra violet (XUV) laser, two tunable infrared lasers, and a fourth laser for ionization. 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 behaviour (pre-dissociation and auto-ionization) 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