Measurement of the lifetime of metastable triatomic hydrogen.

We measured the survival probability of the metastable (N=0, K=0) level of the 2p2A‘2 of H3 as a function of time elapsed from formation of the molecule by probing its population with photoionization. We find lifetimes τ=640+300-100 ns for the ground vibrational state and τ=740+300-100 ns for the symmetric stretch-excited level. Equally short lifetimes are obtained from an analysis of the photoinduced bleaching of the spontaneous dissociation signal of metastable H3. These lifetimes are about 2 orders of magnitude shorter than those expected on the basis of the allowed radiative transition 2p2A‘2→2s2A’2. We attribute the faster decay channel to weakly allowed radiative transitions between the metastable state and the degenerate mode-excited repulsive ground state of H3, as well as to predissociation of the metastable levels by the repulsive ground state of H3 induced by spin–orbit coupling. [ABSTRACT FROM AUTHOR]