Thermal electron attachment to O 2 in the presence of various compounds as studied by a microwave cavity technique combined with pulse radiolysis

Abstract

Attachment of thermal electrons to O 2 in the presence of various compounds such as rate gases (He, Ne, Ar, Kr and Xe), hydrocarbons (CH 4, C 2H 6, C 3H 8, n-C 4H 10, n-C 5H 12, neo-C 5H 12, n-C 6H 14, and C 2H 4), and alcohols (CH 3OH and C 2H 5OH) has been investigated using a microwave cavity technique combined with X-ray pulse radiolysis. It has been found that in almost all mixtures the electron attachment obeys the two-stage Block-Bradbury mechanism. In some mixtures the attachment rates exhibit a trend of saturation with increase of gas-pressures. The rate constant of initial electron capture which can be regarded as a limit of that saturation has been found to be (4.8 ± 0.6) × 10 −11 cm 3/s in the presence of propane, n-pentane, neopentane and n-hexane, while in the presence of rate gases the values of this rate constant have relatively large errors due to long extrapolations. Three-body rate constants have been obtained for various third bodies used; in rare gases they are very small (ranging rom (2.3 ± 0.3) × 10 −32 cm 6/s for Ne to (8.5 ± 0.5) × 10 −32 cm 6/5 for Xe), while in hydrocarbons they increase rapidly with molecular (from ~3 × 10 −31 cm 6/5 for CH 4 to 8 × 10 −30 cm 6/s for n-C 5H 12, etc.), and in alcohols they are somewhat larger than the others. The difference in these values is ascribed to the difference of efficiency of collisional stabilization of vibrationally excited molecular ion O − 2 *. This stabilization process can be classified mainly into two cases depending on whether the vibrational relaxation is effected by VT transitions or VV transitions. Conventional theories have failed to explain the magnitudes of VT transition probabilities obtained here. The lifetime of O − 2 * (υ′ = 4) has been estimated to be (1.0 ± 0.3) × 10 −10 s.