Ion trap studies of association processes in collisions of CH3(+) and CD3(+) with n-H2, p-H2, D2, and He at 80 K

Formation of stable CH5(+) and CD5(+) has been studied in a liquid nitrogen cooled radio-frequency (RF) ion trap. Ternary rate coefficients, k3, have been measured for He, H2, and D2 as the stabilizing collision partner. The results for He are in satisfying agreement with SIFT data. The experiments have been performed at very low densities permitting information to be extracted on the radiative association process. Because both k3 and the radiative rate coefficient k(r) are observed under the same experimental conditions, comparison leads to a reliable upper limit of the radiative stabilizing rate of the intermediate CH5(+) collision complex. The results are significantly smaller than the values currently discussed in the literature, but they are in accord with infrared relaxation rates measured directly for highly excited CH5(+) ions. Using two different mixtures of p-H2 and o-H2 at 80 K, the influence of the H2 rotational energy was determined. The derived ratio is four times smaller than predicted from theoretical work.