Formation of Negative Ions in Gases by Secondary Collision Processes

Experiments on the formation of negative ions by electron bombardment of hydrogen, oxygen, and water vapor at pressures of 0–4 mm are described. The only ion observed in hydrogen is H⁻, formed by dissociative attachment. Pressure dependence of the O₂⁻ intensity shows that a secondary collision is involved in its formation. Data on the formation of O⁻ and O₂⁻ at low electron energies indicate that the secondary collision leading to the formation of O₂⁻ does not involve an O⁻ ion. The formation of O₂⁻ probably involves a stabilizing collision with a molecule of gas after the initial formation of the ion by electron attachment. From the behavior of H⁻, O⁻, and OH⁻ intensities with H₂O pressure, it is shown that the secondary process for the formation of OH⁻ in water vapor is probably $${\mathrm{H}}^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{OH}}^{-}+{\mathrm{H}}_2$$ . The cross section for this reaction is more than 10⁻¹⁷ cm².