Near-threshold continuum structure and the dissociation energies of ${\mathrm{H}}_2$, HD, and ${\mathrm{D}}_2$

Using laser double-resonance spectroscopy, we have studied the second dissociation limit of all three stable isotopic variants of molecular hydrogen. Just above threshold, the vibrational continuum exhibits considerable fine-grained structure, differing greatly between isotopes because of varying nonadiabatic couplings between the B, B’, and C states. Significantly improved values of the dissociation energies are obtained from the continuum onsets. We also report preliminary investigations of atomic 2S:2P branching ratios very near threshold.