We measured the survival probability of the metastable (N=0,K=0) level of the 2p2Alsquo;2of H3as a function of time elapsed from formation of the molecule by probing its population with photoionization. We find lifetimes tgr;=640+300minus;100ns for the ground vibrational state and tgr;=740+300minus;100ns for the symmetric stretchhyphen;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 2p2Alsquo;2rarr;2s2Arsquo;2. We attribute the faster decay channel to weakly allowed radiative transitions between the metastable state and the degenerate modehyphen;excited repulsive ground state of H3, as well as to predissociation of the metastable levels by the repulsive ground state of H3induced by spinndash;orbit coupling.
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