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.

The front cover artwork is provided by Dr. Johannes Deiglmayr (ETH Zurich). The image shows the setup developed to study the fundamental ion–neutral reaction H2++H2→H3++H at collision energies encountered in outer space and the measured energy dependence of the cross section. Read the full text of the article at 10.1002/cphc.201600828.

We present measurements of the product-channel branching ratios of the reactions (i) HD + + HD forming H 2 D + + D (38.1(30)%) and HD 2 + + H (61.9(30)%), (ii) HD + + D 2 forming HD 2 + + D (61.4(35)%) and D 3 + + H (38.6(35)%), and (iii) D 2 + + HD forming HD 2 + + D (60.5(20)%) and D 3 + + H (39.5(20)%) at collision energies E coll near zero, i.e., below k B × 1 K. These branching ratios are compared with branching ratios predicted using three simple models: a combinatorial model (M1), a model (M2) describing the reactions as H-, H + -, D-, and D + -transfer processes, and a statistical model (M3) that relates the reaction rate coefficients to the translational and rovibrational state densities of the H n D 3– n + + H/D ( n = 0, 1, 2 or 3) product channels. The experimental data are incompatible with the predictions of models M1 and M2 and reveal that the branching ratios exhibit clear correlations with the product state densities.

The Front Cover picture shows the setup developed to study the fundamental ion–neutral reaction H2++H2→H3++H at collision energies encountered in outer space and the measured energy dependence of the cross section. More information can be found in the Full Paper by F. Merkt and co‐workers on page 3596 in Issue 22, 2016 (DOI: 10.1002/cphc.201600828).

Abstract. A high-power Raman lidar system has been installed at the high-altitude research station Schneefernerhaus (Garmisch-Partenkirchen, Germany) at 2675 m a.s.l., at the side of an existing wide-range differential-absorption lidar (DIAL). An industrial XeCl laser was modified for linearly polarized single-line operation at an average power of about 180 W. This high power and a 1.5-m-diameter receiver allow us to extend the operating range for water-vapour sounding to 20 km within one hour, at an uncertainty level of the mixing ratio of 1 to 2 ppm. This was achieved for a vertical resolution varied between just 0.2 and 0.6 km in the stratosphere and could be improved for stronger smoothing. The lidar was successfully validated with a balloon-borne cryogenic frost-point hygrometer (CFH). In addition, temperature measurements to altitudes around 87 km were demonstrated for one hour of signal averaging. The system has been calibrated with the DIAL, the CFH and radiosondes.