The reaction rate coefficients, k, for the reactions of ground state Si+(2P) with C2H2 and C6H6 have been measured as a function of reactant ion/reactant neutral ntre-of-mass kinetic energy, KECM, 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 KECM 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 k2 respectively. It is found that a power law of the form k−1/k2 = constant (KECM)m closely describes each reaction. The negative energy dependence of the reaction rate coefficient of the reaction of Si+(2P) with C2H2 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 C6H6 on KECM is also reported.
Keywords:Association reaction; Bimolecular reaction; Ion/molecule reaction; Reaction complex; Reaction rate coefficient; Selected-ion flow drift tube
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