Rotational autoionization and energy levels of triplet nf, v=0 Rydberg states ofH2s

We have measured the energy levels and purely rotational autoionization rates of 25 different triplet Rydberg states of the ${\mathrm{H}}_{2}$ molecule with quantum numbers n=14--29, L=3, v=0, and R=2--4. Electron impact excites the ground-state molecules in a beam to the metastable c(2p)${\mathrm{}}^{3}$ ${\mathrm{\ensuremath{\Pi}}}_{\mathrm{u}}$ states. Two-color laser excitation from two counterpropagating single-mode cw dye laser beams excites the metastable molecules through an intermediate (3d)${\mathrm{}}^{3}$ \ensuremath{\Sigma} or (3d)${\mathrm{}}^{3}$ \ensuremath{\Pi} state to the triplet nf Rydberg states. The linewidths of the transitions to the Rydberg states give the autoionization rates, which vary widely and agree satisfactorily with the quadrupole-moment-polarizability long-range Coulomb interaction model. The laser transitions have been measured to 0.002 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ . We use these measurements, the model, and an extrapolation of the Rydberg series to derive the ionization potential of the metastable c(2p)${\mathrm{}}^{3}$ ${\mathrm{\ensuremath{\Pi}}}_{\mathrm{u}}$ v=0 states with an accuracy of 0.020 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the energy difference between the singlet ground state and the triplet metastable c(2p)${\mathrm{}}^{3}$ ${\mathrm{\ensuremath{\Pi}}}_{\mathrm{u}}$ states with an accuracy of 0.025 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ . This allows the whole singlet and triplet manifolds of ${\mathrm{H}}_{2}$ to be linked together with higher accuracy: our correction to Dieke's ${\mathrm{H}}_{2}$ triplet term values is -149.704(25) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ .