Selected ion flow drift tube studies of the kinetics of the reactions of Si +( 2P) with C 2H 2 and C 6H 6

Abstract

The reaction rate coefficients, k, for the reactions of ground state Si +( 2P) with C 2H 2 and C 6H 6 have been measured as a function of reactant ion/reactant neutral centre-of-mass kinetic energy, KE CM, in a selected-ion flow drift tube (SIFDT) apparatus, operated with helium at a temperature 298 ± 2 K. The values k of the studied reactions have very pronounced negative energy dependences; k decreases by about one order of magnitude when KE CM increases from near thermal values to about 2 eV. The results are interpreted in terms of a simple model assuming that the reactions proceed via the formation of long-lived complexes. These intermediate complexes decompose back to reactants or forward to products, the unimolecular decomposition rate coefficients for these reactions being k −1 and k 2 respectively. It is found that a power law of the form k −1/ k 2 = constant ( KE CM) m closely describes each reaction. The negative energy dependence of the reaction rate coefficient of the reaction of Si +( 2P) with C 2H 2 is rationalized in terms of the negative entropy change in the reaction. The dependence of the reaction rate coefficient of the reaction of Ar + with C 6H 6 on KE CM is also reported.