Multipole-moment effects in ion-molecule reactions at low temperatures: part I - ion-dipole enhancement of the rate coefficients of the He + + NH 3 and He + + ND 3 reactions at collisional energies E coll / k B near 0 K.

The energy dependence of the rates of the reactions between He ⁺ and ammonia (NY ₃ , Y = {H,D}), forming NY ₂ ⁺ , Y and He as well as NY ⁺ , Y ₂ and He, and the corresponding product branching ratios have been measured at low collision energies E _coll between 0 and k _B ·40 K using a recently developed merged-beam technique [Allmendinger et al. , ChemPhysChem , 2016, 17 , 3596]. To avoid heating of the ions by stray electric fields, the reactions are observed within the large orbit of a highly excited Rydberg electron. A beam of He Rydberg atoms was merged with a supersonic beam of ammonia using a curved surface-electrode Rydberg-Stark deflector, which is also used for adjusting the final velocity of the He Rydberg atoms, and thus the collision energy. A collision-energy resolution of about 200 mK was reached at the lowest E _coll values. The reaction rate coefficients exhibit a sharp increase at collision energies below ∼ k _B ·5 K and pronounced deviations from Langevin-capture behaviour. The experimental results are interpreted in terms of an adiabatic capture model describing the rotational-state-dependent orientation of the ammonia molecules by the electric field of the He ⁺ atom. The model faithfully describes the experimental observations and enables the identification of three classes of | JKMp 〉 rotational states of the ammonia molecules showing different low-energy capture behaviour: (A) high-field-seeking states with | KM | ≥ 1 correlating to the lower component of the umbrella-motion tunnelling doublet at low fields. These states undergo a negative linear Stark shift, which leads to strongly enhanced rate coefficients; (B) high-field-seeking states subject to a quadratic Stark shift at low fields and which exhibit only weak rate enhancements; and (C) low-field-seeking states with | KM | ≥ 1. These states exhibit a positive Stark shift at low fields, which completely suppresses the reactions at low collision energies. Marked differences in the low-energy reactivity of NH ₃ and ND ₃ -the rate enhancements in ND ₃ are more pronounced than in NH ₃ -are quantitatively explained by the model. They result from the reduced magnitudes of the tunnelling splitting and rotational intervals in ND ₃ and the different occupations of the rotational levels in the supersonic beam caused by the different nuclear-spin statistical weights. Thermal capture rate constants are derived from the model for the temperature range between 0 and 10 K relevant for astrochemistry. Comparison of the calculated thermal capture rate coefficients with the absolute reaction rates measured above 27 K by Marquette et al. ( Chem. Phys. Lett. , 1985, 122 , 431) suggests that only 40% of the close collisions are reactive.