${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ ions produced by dissociative attachment of electrons to ${\mathrm{H}}_2$O and ${\mathrm{D}}_2$O have been used as primary ions in the measurement of a cross section of (1.6 ± 0.3) × ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ for the negative-ion-molecule reaction ${\mathrm{H}}^{-}$ + ${\mathrm{H}}_2$O → ${\mathrm{OH}}^{-}$ + ${\mathrm{H}}_2$ at an ${\mathrm{H}}^{-}$ energy of 1.93 ± 0.45 eV and (5.8 ± 1.2) × ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ for the reaction ${\mathrm{D}}^{-}$ + ${\mathrm{D}}_2$O → ${\mathrm{OD}}^{-}$ + ${\mathrm{D}}_2$ at a ${\mathrm{D}}^{-}$ energy of 1.84 ± 0.38 eV. The method is quite generally applicable to the study of negative-ion reactions in the energy region from thermal energies to ∼3 eV.

• Received 12 July 1968

DOI:https://doi.org/10.1103/PhysRevLett.21.664