Fast Neutral-Beam Photofragment Spectroscopy ofH2cΠu−3

A time- and position-sensitive detector has been used for the first determination of the momentum distribution of correlated photofragments. A fast beam of ${\mathrm{H}}_2$ $c{}_{}{}^3{}_{}{}^{}\Pi _{\mathrm{u}}^{-}{}_{}{}^{}$ is observed to undergo three types of photodissociation: (1) radiative dissociation of individual rovibrational levels; (2) predissociation by barrier tunneling of quasibound levels, for the first time observed in a neutral system with fragment energy analysis; and (3) direct photodissociation, which has been rotationally resolved for the first time.