Predissociation of H3 n=2 Rydberg states: Product branching and isotope effects

A translational spectroscopy technique is used to obtain predissociation kinetic energy release spectra from the lowest bound states of H₃, 2s ²A^'₁ and 2p ²A`₂. These H^*₃ states are formed in near-resonant electron capture by 3 keV H⁺₃ in Cs vapor. Their ground rovibrational levels are energetically about 1 eV above the H+H+H dissociation limit, thus all levels can yield both H₂+H and the three-body products. The spectra contain both three-body and two-body components and are deconvoluted to obtain the branching ratios. Data obtained from two different ion sources show that the three-body/two-body ratio increases with increased rovibrational energy in the H^*₃. The results are compared to recent theory and with previously reported ratios from dissociative recombination of H⁺₃. The comparison suggests that the ratio increases monotonically with the total electronic and rovibrational energy in the H^*₃. D^*₃ predissocation has a similar behavior. The H- and D-atom spectra from the two-body decay of HD^*₂ show that the ejected H atom is strongly favored on a per-atom basis.