Dissociative Attachment of Thermal Electrons to N2O and Subsequent Electron Detachment

The attachment of electrons to nitrous oxide has been studied, following pulse ionization, by a microwave conductivity technique. For pure N₂O, and N₂O diluted with N₂, extrapolation of the experimentally observed attachment rate constant to zero pressure gives an upper limit of ${\text{1 × 10}}^{\text{−15}}\hspace{0.17em}{\mathrm{cm}}^3\hspace{0.17em}{\mathrm{molecule}}^{\text{−1}}·{\sec }^{\text{−1}}$ for the rate constant for two‐body attachment at 298°K. When C₂H₆ or C₂H₈ are used as diluents a higher two‐body rate constant of ${\text{6× 10}}^{\text{−15}}\hspace{0.17em}{\mathrm{cm}}^3\hspace{0.17em}{\mathrm{molecule}}^{\text{−1}}·{\sec }^{\text{−1}}$ is found which is believed to represent the rate constant of the dissociative attachment reaction $e^{-}+{\mathrm{N}}_2\mathrm{O}\to {\mathrm{N}}_2+{\mathrm{O}}^{-}$ . The much lower value for pure N₂O or ${\mathrm{N}}_2\mathrm{O}–{\mathrm{N}}_2$ mixtures is explained by the regeneration of electrons, via the reactions ${\mathrm{O}}^{-}+{\mathrm{N}}_2\mathrm{O}\to \mathrm{NO}+{\mathrm{NO}}^{-}$ and ${\mathrm{NO}}^{-}+{\mathrm{N}}_2\mathrm{O}\to \mathrm{NO}+{\mathrm{N}}_2\mathrm{O}{\mathrm{+e}}^{-}$ , which largely nullifies the primary attachment step. The presence of a hydrocarbon prevents electron regeneration by irreversibly reacting with O⁻ and allows observation of the uncomplicated rate constant for dissociative attachment. The rate constant for dissociative attachment increases markedly with increasing temperature and in the range studied, 278 to 355°K, has an activation energy of $\text{0.42± 0.04\hspace{0.17em}}\mathrm{eV}$ . Confirming evidence for the proposed mechanism is obtained from the effect, on the attachment rate constant, of H₂ added to ${\mathrm{C}}_2{\mathrm{H}}_6–{\mathrm{N}}_2\mathrm{O}$ mixtures. The H₂ reacts with O⁻ by the associative detachment reaction ${\mathrm{O}}^{-}+{\mathrm{H}}_2\to {\mathrm{H}}_2\mathrm{O}{\mathrm{+e}}^{-}$ to nullify the primary attachment step and to reduce the rate constant by an appropriate amount. In the steady‐state radiolysis of N₂O, the yields of the products N₂ and NO are found to increase markedly with decreasing pressure below 50 torr. These increases in yields may be quantitatively related to the chain reaction resulting from dissociative attachment followed by electron regeneration.