Complete branching ratios for the dissociative recombination of H2O+, H3O+, and CH3+

Dissociative recombination of the polyatomic ions H2O+, H3O+, and CH3+ with electrons has been measured at the heavy-ion storage ring ASTRID. Complete branching ratios for all the possible product channels have been determined at zero relative energy using an energy-sensitive detector masked by grids with known transmissions. In the dissociative recombination of H3O+, water molecules are produced with a probability of 33%, whereas the production of atomic oxygen is negligible. Atomic carbon is, on the other hand, produced with a branching ratio of 30% in the dissociative recombination of CH3+. For all three molecular ions, the three-particle breakup is a major process. Relative cross sections for dissociative recombination of H3O+ and for dissociative excitation of H3O+ have been measured for relative electron energies up to 40 eV. Implications for the modeling of the chemistry of interstellar molecular clouds are discussed.