Small gas-phase dianions of Zn₃O₄^2-, Zn₄O₅^2-, CuZn₂O₄ ^2-, Si₂GeO₆^2-, Ti₂O ₅^2- and Ti₃O₇^2-

We have searched for new species of small oxygen-containing gas-phase dianions produced in a secondary ion mass spectrometer by Cs⁺ ion bombardment of solid samples with simultaneous exposure of their surfaces to O₂ gas. The targets were a pure zinc metal foil, a copper-contaminated zinc-based coin, two silicon-germanium samples (Si _1-xGe_x (with x = 6.5% or 27%)) and a piece of titanium metal. The novel dianions Zn₃O₄^2-, Zn ₄O₅^2-, CuZn₂O₄ ^2-, Si₂GeO₆^2-, Ti₂O ₅^2- and Ti₃O₇^2- have been observed at half-integer m/z values in the negative ion mass spectra. The heptamer dianions Zn₃O₄^2- and Ti ₂O₅^2- have been unambiguously identified by their isotopic abundances. Their flight times through the mass spectrometer are ∼20 μs and ∼17 μs, respectively. The geometrical structures of the two heptamer dianions Ti₂O₅^2-, and Zn ₃O₄^2- are investigated using ab initio methods, and the identified isomers are compared to those of the novel Ge ₂O₅^2- and the known Si₂O ₅^2- and Be₃O₄^2- dianions.