Znternarional Journal zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Mass Spectrometry and Zon Processes 129 (1993) 13 I- 143 0168-I 176/93/%06.000 1993 - Elsevier Science Publishers B.V. All rights reserved 131 SIFDT studies of the reactions of C+, CH+ and CH,f with HCl and C02, and CH; with HCl’ J. Gloslk , D. Smith, P. Spa&l, W. Freysinger, W. Lindinger* . t Institut fir Ionenphysik, UniversitCt Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria (Received 1 February 1993; accepted 1 April 1993) Abstract The rate coefficients k have been determined as a function of reactant ion/reactant molecule centre-of-mass energy E, for the reactions of C?, CH+ and CH: with both HCl and CO2 in a selected ion flow drift tube (SIFDT) apparatus. In these experiments, the carrier gas was pure helium at a temperature of 300 K. For low values of E,, the k values are close to the respective collisional rate coefficients k, but the k decreases with increasing E,. These results are interpreted in terms of a simple model by which the reactions are considered to proceed via the formation of long-lived complexes which decompose back to the reactants or forward to products, the unimolecular decomposition rate coefficients for these reactions being k_, and k2 respectively. It is found that a power law relation of the form k-,/k2 = const x Z$’ closely describes each reaction. An attempt is made to rational& the values of the indices m obtained for each reaction in terms of the number of rotational degrees of freedom in the separated reactants, and then some support is apparently evident for the theoretical indication that CO2 is a “bent” molecule. The k for the slower reaction of CH: with HCl was measured in the SIFDT in both helium and argon carrier gas, and the results compared with previous results obtained in a VT-SIFT apparatus under truly thermalised conditions. The combined results of these studies are used to indicate which form of energy in the reactants (internal or translational) influences the rate at which the reaction proceeds. Key words: Selected ion flow drift tube; Ion/molecule reactions; Reaction rate coefficients 1. Introduction A large number of ion/molecule reactions have been studied using fast flow tube methods (see the data compilation in ref. 1). Thus the rate coeffi- cients and product ions for many reactions have been determined at room temperature, and an appreciable number over a range of temperature, using both flowing afterglow (FA) and selected ion flow tube (SIFT) techniques, and over a range *Corresponding author. t Permanent address: Department of Electronics and Vacuum Physics, Mathematics and Physics Faculty, Charles University, 180 00 Prague, Czech Republic. ‘Dedicated to the memory of Professor Maximilian Pahl. of reactant ion/reactant molecule centre-of-mass collision energies using selected ion flow drift tube (SIFDT) techniques. For fast reactions, i.e. those that proceed at or near their collisional limit- ing rates, then it is commonly observed that the measured rate coefficients are sensibly tempera- ture invariant [2,3]. However, over the wider range of interaction energy available in SIFDT experiments there are many examples of reactions for which the rate coefficients actually increase above the so-called Langevin limiting value which is only appropriate to the thermal temperature regime. We discuss this topic later. In recent SIFDT studies in our laboratory we have studied a number of reactions for which the