Studies of ion-molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion-molecule reactions at low temperatures and present, as example, a measurement of the H ion prepared in a single rovibrational state at collision energies in the range E col /k B = 5-60 K. To reach such low collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields.The first beam is a supersonic beam of pure ground-state H 2 molecules and the second is a supersonic beam of H 2 molecules excited to Rydberg-Stark states of principal quantum number n selected in the range 20-40. Initially, the two beams propagate along axes separated by an angle of 10• . To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg-Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state H 2 beam and by adapting the electric-potential functions to the electrodes of the deflector. The collision energy is varied down to below E col /k B = 10 K, i.e., below

The energy dependence of the rate coefficient of the H + 2 + H 2 → H + 3 + H reaction has been

The reactions between H2+ and HD forming H3+ + D as well as H2D+ + H were measured at collision energies between 0 and kB·30 K and a resolution of 75 mK and the H3+/H2D+ product branching ratio and the product kinetic-energy distribution were determined.

The reactions between ground-state H + 2 (X 2 + g (v = 0, J = 0)) and D 2 forming HD + 2 + H and H 2 D + + D were investigated in the range of collision energies E coll between E coll /k B = 0 and 10 K using a merged-beam approach. The reaction rates measured experimentally are compared to those obtained for the reaction between H + 2 and H 2 forming H + 3 + 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 H + 2 and the quadrupole of para D 2 and ortho H 2 molecules in the J = 1 rotational level. The enhancement of the reaction with D 2 was found to be significantly less than that of the reaction with H 2 , reflecting the relative population of the J = 1 rotational level of H 2 (75%) and D 2 (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 H + 2 and D 2 and forming H 2 D + and D (η) near E coll = 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.