Multiphoton excitation of high singlet np Rydberg states of molecular hydrogen: Spectroscopy and dynamics

We have used two-step, two-color excitation to populate high np singlet Rydberg states of molecular hydrogen under electric-field-free conditions. Sub-Doppler resolution is obtained by exciting the X ¹Σ⁺_g-->E ¹Σ⁺_g two-photon transition with copropagating photons, then further exciting the molecules to high Rydberg states with a narrow-band laser. We have observed transitions to states up to n~90 for the Rydberg series which converge to the two lowest ionization limits. From an analysis of the energies of the observed transitions we have obtained new results for the first two ionization potentials of molecular hydrogen, with the highest accuracy reported to date. The new values are 124 417.61+/-0.07 cm^-1 for parahydrogen and 124 475.94+/-0.07 cm^-1 for orthohydrogen. Some of the Rydberg states predissociate strongly, apparently due to the influence of perturbing states of low principle quantum number. This is the first observation of such predissociation in Rydberg states of high principle quantum number.