SIFDT STUDIES OF THE REACTIONS OF C+, CH+ AND CH2+ WITH HCL AND CO2, AND CH3+ WITH HCL1

The rate coefficients k have been determined as a function of reactant ion/reactant molecule centre-of-mass energy E(r) for the reactions of C+, CH+ and CH2+ with both HCl and CO2 in a selected ion flow drift t ube (SIFDT) apparatus. In these experiments, the carrier gas was pure helium at a temperature of 300 K. For low values of E(r), the k values are close to the respective collisional rate coefficients k(c) but th e k decreases with increasing E(r). These results are interpreted in t erms of a simple model by which the reactions are considered to procee d via the formation of long-lived complexes which decompose back to th e reactants or forward to products, the unimolecular decomposition rat e coefficients for these reactions being k(-1) and k(2) respectively. It is found that a power law relation of the form k(-1)/k(2). = const x E(r)(m) closely describes each reaction. An attempt is made to ratio nalise the values of the indices m obtained for each reaction in terms of the number of rotational degrees of freedom in the separated react ants, and then some support is apparently evident for the theoretical indication that CO2 is a ''bent'' molecule. The k for the slower react ion of CH3+ with HCl was measured in the SIFDT in both helium and argo n carrier gas, and the results compared with previous results obtained in a VT-SIFT apparatus under truly thermalised conditions. The combin ed results of these studies are used to indicate which form of energy in the reactants (internal or translational) influences the rate at wh ich the reaction proceeds.