Mechanism of thermal electron attachment in N2O--CO2 mixtures in the gas phase

The attachment of thermal electrons to nitrous oxide in N2O--CO2 mixtures has been studied at room temperature in the pressure range 5--120 torr. Ionization was by pulse radiolysis and the electron concentration was measured as a function of time by microwave conductivity. Addition of even less than 0.1% CO2 to N2O causes a marked increase in attachment rate. However, this enhancement soon saturates in that further additions of CO2 have less and less effect. Experiments with ternary mixtures including C2H6 showed a further enhancement which was much larger than the additive effects of CO2 and C2H6 alone. These observations can be explained by a two step three-body process producing vibrationally excited N2O-* if the rate constant for stabilization of N2O-* by CO2 is 4 x 10-30 cm6/molecule2xsec. The decrease in effectiveness with increased CO2 pressure is interpreted as the collisional ionization of a complex ion, [N2OxCO2]-*. The nonadditive effect of hydrocarbon results from the rapid reactive destruction of such complexes by collision with the hydrocarbon. A detailed quantitative treatment of the proposed mechanism was successful in explaining most features of the data. In a limited set of experiments, allene : N2O mixtures were found to behave much like CO2--N2O