The second dissociation threshold bound levels of hydrogen molecule

The near-threshold highly bound states of all three stable isotopic variants of molecular hydrogen have been studied. Numerous perturbations and unexpected transitions are observed as far as 1cm^-1 just below the second dissociation threshold. This complex structure may arise from a combination of nonadiabatic coupling between B, B', C electronic states, perturbations due to fine and hyperfine interactions, and strong shape resonances. The perturbed near-threshold states and vibrational continuum exhibit finegrained structure, differing greatly between isotopes because of varying nonadiabatic coupling.