SIFDT studies of some CH +3 ternary association reactions

Abstract

Following our previous detailed studies of CH + 3 association reactions in a VT-SIFT under truly thermal conditions, we have studied some of the same reactions under non-thermal conditions in a variable-temperature selected ion flow drift tube (VT-SIFDT). The ternary association rate coefficients, k 3, for the reactions of CH + 3 with N 2 and CO in helium carrier gas (and hence with helium atoms as the stabilizing third bodies) at temperatures of variously 80 and 300 K are observed to vary according to the power law k 3 ≈ E −1.5 c, where E c is the CH + 3/He centre-of-mass energy. For the CH + 3 association reaction with H 2, k 3 varies as k 3 ≈ E −1.5 r, where E r is the CH + 3/H 2 centre-of-mass energy. These experimental results are in accordance with a simple theoretical model we have constructed based on the theoretical ideas of Bates and of Herbst, which describe the temperature dependence of k 3 in terms of the number of rotational degrees of freedom in the separated reactants. For the CH + 3 +CO reaction at the higher values of E c and at a carrier gas temperature of 300 K, a more rapid decrease in k 3 occurs than is predicted by our simple model and this is attributed to the onset of vibrational excitation of the CH + 3 ions. This latter phenomenon is expected to be more important for polyatomic ions, which possess low-energy vibrational states and some experimental evidence is presented in support of this.