Mechanism of thermal electron attachment in N2O and N2O–hydrocarbon mixtures in the gas phase

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Abstract
The attachment of thermal electrons to nitrous oxide at room temperature has been studied, following pulse radiolysis, by a microwave conductivity technique. For pure N2O at pressures from 10 to 300 torr, the results are explained by a combination of two‐body attachment followed by reactions leading to partial electron detachment, a two step three‐body process, and a process giving overall four‐body behavior. The results for mixtures of N2O with alkanes (C2H6, C3H8, n‐C4H10, iso‐C4H10, n‐C5H12, and neo‐C5H12) and butenes (1‐, 2‐cis‐, 2‐trans‐, and iso) are also explained in the same way, but with no electron detachment. Common values of 5×10−15 cm3/molecule sec for the two‐body rate constant and 4.6×10−33 cm6/molecule2 sec for the three‐body rate constant (with N2O as the third body) explain the data. The three‐body rate constants increase with molecular complexity (6×10−34 cm6/molecule2 sec for C2H6 to 1.55×10−31 cm6/molecule2 sec for neo‐C5H12). The four‐body rate constants range from ∼10−53 to ∼10−51 cm9/molecule3 sec. The branched compounds such as neopentane and isobutene have higher three‐body rate constants than the linear isomers. The attachment rates of mixtures of those compounds with the higher three‐body rate constants appear to saturate as pressures increase. From the results for N2O‐neo‐C5H12 mixtures a value of (5.8±0.6) ×10−13 cm3/molecule sec has been determined for the rate constant of the initial two‐body electron capture by N2O to form a short‐lived N2O−. The autoionization lifetime of N2O− is estimated to be 1.8×10−10 sec or greater. The problem of excess nitrogen in N2O–hydrocarbon radiolysis is discussed in relation to these results.