Journal of Physics B: Atomic and Molecular Physics, 46, IOP, November 22, 2013.
Double-resonance laser spectroscopy was used to probe the energy region below the third dissociation limit of molecular hydrogen. Resonantly enhanced multi-photon ionization spectra were recorded by detecting ion production as a function of energy using a time-of-flight mass spectrometer. Energies and line widths for the v = 14–17 levels of the D state of H2 are reported and compared to experimental data obtained by using VUV synchrotron light excitation and fully ab initio non-adiabatic calculations of D state energies and line widths. Several high vibrational levels of the B′′B state were also observed in this region. Term energies and rotational constants for the v = 67–69 vibrational levels are reported and compared to highly accurate ro-vibrational energy level predictions from fully ab initio non-adiabatic calculations of the first six sigma levels of H2. While additional observed transitions can be assigned to other states, several unassigned features in the spectra highlight the need for a fully integrated theoretical treatment of dissociation and ionization to understand the complex pattern of highly vibrationally excited states expected in this region.