Flowing afterglow studies of organic positive ions generated by penning ionization using metastable argon atoms

Penning ionization of organic molecules has been used as a convenient, clean ion source for flowing afterglow studies of organic positive ion/molecule reactions. Ionization reactions of 36 compounds were investigated, including alkanes, alkenes, ketones, ethers, chlorides and silanes. In many cases the parent ion is produced solely, but a few compounds fragment to give a daughter ion as the only ion. Some of these ions were then studied in reactions with their parent neutral molecules and in hetero-ion/molecule reactions. The combination of mild ionization and rapid thermalization in helium buffer gas circumvents complications from excited reactants. The reactions between ethylene radical ion and ethylene were examined in detail and the results are compared with both low- and high-pressure results in the literature. Rate constants and branching ratios were measured and mechanistic implications are discussed. It was determined that the loss of metastable argon by diffusion and destruction at the walls is considerably slower than the comparable loss of ions and this must be taken into account in kinetic measurements. Other reactions, including proton transfer equilibria, were also studied briefly.