Electron impact ionization/dissociation of hydrocarbon molecules relevant to the edge plasma

Total and partial cross-sections for electron impact ionization of various C1, C2 and C3 hydrocarbon molecules have been determined experimentally from a threshold up to 1000 eV. Here we will discuss as an illustrative example ionization and dissociation reactions of methane and acetylene. Using a deflection and retarding field method, the ion kinetic energy distributions of all product ions are measured. Extensive three-dimensional ion trajectory simulations of the ion source are performed to determine the ion loss resulting from high initial kinetic energy of ions. Discrimination factors are determined as a function of the initial ion kinetic energy. Multiplication of these factors with the measured ionization efficiency curves leads to accurate relative partial ionization cross-sections. In combination with the kinetic energy distributions measured as a function of the electron energy it is also possible to determine cross-sections that are differential with respect to the initial kinetic energy of the ion. Thereby different reaction pathways, such as Coulomb explosion and dissociative single ionization, that lead to a specific product ion, can be identified. Moreover, as a particular instructive example unimolecular dissociation processes of C3H52+ formed via electron impact ionization of the hydrocarbon molecule propane have been studied and will be presented here. Fragment ions are identified and the kinetic energy release distribution of the Coloumb explosion of C3H52+ is determined utilizing the mass analysed ion kinetic energy scan technique. In parallel, high level ab initio quantum chemical calculations of the structure and energetics of C3H52+ have been performed. (author)