Deviation of the rate of the reaction from Langevin behaviour below 1 K, branching ratios for the and product channels, and product-kinetic-energy distributions

The reactions between ground-state H2+ (X 2Σg+(v=0,J=0)) and D2 forming HD2++H and H2D++D were investigated in the range of collision energies Ecoll between Ecoll/kB=0 and 10 K using a merged-beam approach. The reaction rates measured experimentally are compared to those obtained for the reaction between H2+ and H2 forming H3++H under similar experimental conditions. Below 1 K, a clear enhancement of the reaction rate coefficient compared to the Langevin rate measured at higher collision energies was observed in both reaction systems. This enhancement is interpreted as originating from the interaction between the charge of H2+ and the quadrupole of para D2 and ortho H2 molecules in the J = 1 rotational level. The enhancement of the reaction with D2 was found to be significantly less than that of the reaction with H2, reflecting the relative population of the J = 1 rotational level of H2 (75%) and D2 (33%) in natural samples at low temperatures. Simulations of the experimental results based on the theoretical predictions of the reaction cross sections by Dashevskaya et al. [J. Chem. Phys. 145, 244315 (2016)] reveal agreement within the experimental uncertainties. The branching ratio η of the reaction involving H2+ and D2 and forming H2D+ and D (η=[H2D+][H2D+]+[HD2+]) near Ecoll=0 was determined to be 0.341(15). Time-of-flight measurements of the velocity distributions of the reaction products are compatible with an isotropic product emission with an average total kinetic energy of 0.45(5) eV for both channels, representing about 30% of the total energy released by the reaction.