Dissociation Energies of Molecular Hydrogen and the Hydrogen Molecular Ion

We have obtained improved values for the dissociation energies of molecular hydrogen and its ion by using a high-resolution pulse-amplified laser to probe the second dissociation limit. The onset of the vibrational continuum is observed by state-selective detection of the atomic products of dissociation, and several auxiliary measurements link the results to the ground state. The dissociation energies are accurate to $0.010–0.026{\mathrm{cm}}^{-1}$, improving previous measurements by a factor of 3–7. Agreement with ab initio calculations is good for ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and their ions, but not for HD and ${\mathrm{HD}}^{+}$.