Probing Long-Range Configurations of Molecular Hydrogen Near the Third Dissociation Energy

Double resonance spectroscopy via the$EF{ }^1\Sigma _g^+ ,v'_{EF} =6,J'$ state has been used to probe the H(n=1) + H(n=3) dissociation threshold region with quantum state selectivity and high energy resolution. Above threshold the continuum is detected by ionizing H(n=3) to produce H$^{+}$, which is then detected by using a time-of-flight mass spectrometer. The ion yield above threshold has several features. Broad structures due to an extension of the series of vibrational levels belonging to the inner and outer wells of the \textit{ungerade} double-well $B"\overline B $(3)${ }^1\Sigma _u^+ $state into the continuum are observed. Also present are sharp features due to vibrationally excited Rydberg states of H$_{2}$. Several unassigned features may be explained by transitions to extremely long-range rovibrational levels of the 6th member of the ${ }^1\Sigma _u^+ $ series of states in H$_{2}$. The new measurements are in good agreement with the energies predicted by \textit{ab initio} calculations of nonadiabatic couplings between the ${ }^1\Sigma _u^+ $ and ${ }^1\Pi _u^+$ series of states of this fundamental system.