Autoionization ofH2Induced by a Doubly Excited Triplet State

A fast beam of ${\mathrm{H}}_2$ molecules in the metastable $c{}_{}{}^3{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{-}$ state is photoionized with visible laser light in the region 540-670 nm. Very strong photoionization from high vibrational levels is observed with a conversion of up to ten vibrational quanta into electronic energy. This extreme non-Franck-Condon behavior is attributed to avoided crossings between the doubly excited $2p{\sigma }_{\mathrm{u}}2p{\pi }_{\mathrm{u}}{Q}_1{}_{}{}^3{}_{}{}^{}\Pi _{\mathrm{g}}^{}{}_{}{}^{}$ state and the singly excited $1snd{}_{}{}^3{}_{}{}^{}\Pi _{\mathrm{g}}^{}{}_{}{}^{}$ Rydberg states with $n\ge 3$.