SELECTED-ION FLOW DRIFT-TUBE STUDIES OF THE REACTIONS OF S-4) WITH CH4, C2H2, C2H4, AND C3H8((S)

The reaction rate coefficients, k, and the product distributions for t he reactions of ground-state S+(S-4) with small hydrocarbon molecules CH4, C2H2, C2H4, and C3H8 have been measured as a function of reactant ion/ reactant neutral average center-of-mass kinetic energy, KE(CM), in a selected ion flow drift tube (SIFDT) apparatus. The measurements have been performed at a temperature 298 +/- 2 K with helium as a buff er gas. The values k of the studied reactions have pronounced negative energy dependencies. For some reactions k decreases by about one orde r of magnitude when KE(CM) is increasing from near thermal values to a few electronvolts. This is interpreted in terms of a simple model ass uming the reactions to proceed via the formation of long-lived complex es. The lifetimes of these intermediate complexes against decompositio n back to reactants and forward to products (or corresponding unimolec ular rate coefficients k(-1) and k(2)) govern the overall rate of the reactions. It is found from the kinetics of the studied reactions that a power law of the form k(-1)/k(2) = const(KE(CM))(m), where m is a c onstant parameter, can be used to describe the energy dependencies of the overall reaction rate coefficients of studied reactions.