Energy structure and dynamics of the $B"\overline B { }^1\Sigma _u^+ $ and D${ }^1\Pi _u^+ $state of molecular hydrogen near the third dissociations limit

Two-color, resonantly-enhanced, multiphoton ionization spectroscopy is used to probe highly excited vibrational levels of the $B"\overline B { }^1\Sigma _u^+ $ and D${ }^1\Pi _u^+ $states of molecular hydrogen near the n=3 dissociation limit. Transitions are observed via two-photon excitation of the $E,F{ }^1\Sigma _g^+ ,v'=6,J'$ state from the ground state. Both molecular and atomic ion production are detected as a function of wavelength by using a time-of-flight mass spectrometer. Term energies of multiple rovibrational levels the $B"\overline B { }^1\Sigma _u^+ $ and D${ }^1\Pi _u^+ $ states and lifetimes of the J = 1-4, $v $= 12, 13 and 14 levels of the D${ }^1\Pi _u^+ $state are reported. The trend of lifetime with vibration is strongly suggestive of a new dissociation channel opening up for the high vibrational levels. Recent theoretical calculations of the rotational interaction of the D${ }^1\Pi _u^+ $ state with the 6 lowest ${ }^1\Sigma _u^+ $ states of H$_{2}$ are invoked to explain the dynamics of these highly excited vibrational levels.