Chemical effects of low energy electron impact on hydrocarbons in the gas phase. I. Neopentane. [''Simulated'' radiolysis of neopentane; 3. 5-15. 0 eV electrons]

The chemical effects induced by impact of low energy electrons of gaseous neopentane were investigated. An original set-up for the irradiation of a flowing gas at low pressure (10/sup -2/Torr) with 3.5--15.0 eV electrons was used. Electron beam energy definition and current intensity were +- 0.6 eV and 7--15 ..mu..A, respectively. Analysis of the products was performed by gas chromatography. The yields of the various products were followed vs. incident electron energy. Specific features observed in the appearance curves indicate that some products arise from the dissociation of molecules excited in either a triplet state at 8.0--8.5 eV or singlet states at 9.2 and 9.6 eV, while others are associated with superexcited states lying around 10.6 eV and with positive ions such as isobutene (AP = 10.4 eV), tert-butyl (AP = 10.6 eV), and ethyl (AP = 13.8 eV) ions. The electron impact excitation cross section shows a steep increase at the ionization potential, which is in contrast with photon impact results. A reaction scheme is proposed and the yields of primary species are evaluated from the experimental data. The observations relative to formation mechanism confirm to photolysis and radiolysis data, except for the process neo-C/sub 5/H/sub 12/* ..-->.. neo-C/submore » 5/H/sub 11/ + H, which is associated not only with the triplet state at 8.0--8.5 eV but also with superexcited states. The relative contribution of excited molecules and ions to product formation above the ionization potential was distinguished. However, absolute excitation and ionization functions were not derived because of saturation phenomena. Only the relative contribution of excitation and ionization, N/sub E//N/sub I/sup +//, and the radiation chemical decomposition yield of neopentane, G(-NP), were evaluated. At 13 eV electron energy, G(-NP) is close to the data obtained in conventional radiolysis. This emphasizes the important role played in radiation chemistry by secondary electrons of energy slightly above the ionization potential.« less