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

Abstract

The reactions between ground-state (Formula presented.) (X (Formula presented.)) and (Formula presented.) forming (Formula presented.) and (Formula presented.) were investigated in the range of collision energies (Formula presented.) between (Formula presented.) and 10 K using a merged-beam approach. The reaction rates measured experimentally are compared to those obtained for the reaction between (Formula presented.) and (Formula presented.) forming (Formula presented.) 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 (Formula presented.) and the quadrupole of para (Formula presented.) and ortho (Formula presented.) molecules in the J = 1 rotational level. The enhancement of the reaction with (Formula presented.) was found to be significantly less than that of the reaction with (Formula presented.), reflecting the relative population of the J = 1 rotational level of (Formula presented.) (75%) and (Formula presented.) (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 (Formula presented.) and (Formula presented.) and forming (Formula presented.) and D ((Formula presented.)) near (Formula presented.) 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.