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Dissociative recombination and excitation of H 2 O ϩ and HDO ϩ in the vibrational ground state have been studied at the heavy-ion storage ring ASTRID. Absolute cross sections have been measured in the energy range from ϳ0.1 meV to ϳ50 eV. The total cross sections for dissociative recombination are essentially the same for the two molecular ions. Complete branching ratios for all possible product channels in dissociative recombination at Eϭ0 have been determined. Three-particle breakup accounts for ϳ60% of the recombination events. With HDO ϩ , recombination into ODϩH is twice as probable as recombination into OHϩD. An isotope effect is also evident in the cross sections for dissociative excitation of HDO ϩ , where H ϩ production is more likely than D ϩ production.

Using the ASTRID heavy-ion storage ring, absolute cross sections for dissociative recombination and dissociative excitation of vibrationally cold H 3 ϩ have been measured as a function of energy in the region Eϭ0Ϫ34 eV. A thermal rate coefficient for dissociative recombination of (1.0Ϯ0.2)ϫ10 Ϫ7 cm 3 s Ϫ1 at 300 K was obtained from cross sections at low energy. At high energy, capture into a group of doubly excited states of H 3 gives rise to resonances in the dissociative recombination as well as in the dissociative excitation cross sections.

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