Knowledge Bank

The bound excited states of molecular hydrogen can decay in three ways: through radiation, ionization, and dissociation. Which decay path the molecule chooses depend on symmetry and the available energy, but most importantly on the region of internuclear distance sampled by the vibrational wavefunction, which controls the extent of interaction with the dissociative doubly-excited configurations. We report experimental studies of competition between ionization and dissociation in both singlet ($EF1\Sigma g+,B1\Sigma u+$), and ($C1\Pi u$ intermediate) and triplet ($c3\Pi u,g.h3\Sigma g+,i,23\Pi g$ and $j3\Delta g$ final) excited states. A special feature of this work is control of the internuclear distance (i.e. high vibrational levels) in the final state by selecting a specific intermediate state by multiphoton excitation (singlets) or by double-resonance labeling (triplets). Predissociation occurs by barrier penetration, rotational, spin-orbit and spin-spin, and radial coupling mechanisms. Both rotational and vibrational autoionization are investigated, in the latter case with large $\Delta v$ changes mediated by doubly-excited states.