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The reactions of He + with OCS and CO 2 have been studied at collision energies between ∼ k B ⋅ 200 mK and ∼ k B ⋅ 30 K by merging a beam of Rydberg He atoms with rotationally cold (∼3.5 K) seeded supersonic expansions containing either OCS or 13 CO 2 or a mixture of OCS (mole fraction 23.2%) and 13 CO 2 (76.8%). The observed product ions of the He + + 13 CO 2 and He + + OCS reactions are 13 CO + , and CS + and CO + , respectively. The He + + OCS capture rate coefficient increases by ∼75% with decreasing collision energy over the investigated range, whereas that of He + + 13 CO 2 decreases by ∼40%. The analysis of the experimental results using an adiabatic-channel capture model indicates that these opposite collision-energy dependences of the rate coefficients arise from the interaction between the charge of the ion and the electric multipole moments of OCS and 13 CO 2 . From the relative product-ion yields observed when using the mixture of OCS and 13 CO 2 , the He + + OCS collisions are inferred to be ∼20% more reactive than those between He + and 13 CO 2 . The comparison of the calculated thermal rate coefficients with earlier experiments suggests that about half of the He + + 13 CO 2 collisions are reactive.

We present experimental and theoretical studies of the He + + CH 4 and He + + CD 4 reactions at collision energies in the k B ·(0-10) K range. Helium atoms in a supersonic beam are excited to a low-field-seeking Rydberg-Stark state and merged with a supersonic beam of CH 4 or CD 4 using a curved surface-electrode deflector. The ion-molecule reactions are studied within the orbit of the helium Rydberg [He( n )] electron, which suppresses stray-electric-fields-induced heating and makes it possible to reach very low collision energies. The collision energy is varied by adjusting the velocity of the He( n ) atoms with the surface deflector, keeping the velocity of the methane beam constant. The reaction product ions (C(H/D) p + with p ∈ {1,2,3}) are collected in a time-of-flight mass spectrometer and monitored as a function of the collision energy. No significant energy-dependence of the total reaction yields of either reactions is observed. The measured relative reaction rate coefficient for the He + + CH 4 reaction is approximately twice higher than the one for the He + + CD 4 reaction. The CH + , CH 2 + and CH 3 + (CD + , CD 2 + and CD 3 + ) ions were detected in ratios 0.28(±0.04) : 1.00(±0.11) : 0.11(±0.04) [0.35(±0.07) : 1.00(±0.16):0.04+0.09-0.04]. We also present calculations of the capture rate coefficients for the two reactions, in which the interaction between the charge of the helium ion and the octupole moment of the methane molecule is included. The rotational-state-specific capture rate coefficients are calculated for states with J = (0-3) at collision energies below k B ·15 K. After averaging over the rotational states of methane populated at the rotational temperature of the supersonic beam, the calculations only predict extremely weak enhancements (in the order of ∼0.4%) of the rate coefficients compared to the Langevin rate constant k L over the collision-energy range considered.

We report on an experimental and theoretical investigation of the He + + N 2 reaction at collision energies in the range between 0 and k B ·10 K. The reaction is studied within the orbit of a highly excited Rydberg electron after merging a beam of He Rydberg atoms (He( n ), n is the principal quantum number), with a supersonic beam of ground-state N 2 molecules using a surface-electrode Rydberg-Stark decelerator and deflector. The collision energy E coll is varied by changing the velocity of the He( n ) atoms for a fixed velocity of the N 2 beam and the relative yields of the ionic reaction products N + and N 2 + are monitored in a time-of-flight mass spectrometer. We observe a reduction of the total reaction-product yield of ∼30% as E coll is reduced from ≈ k B ·10 K to zero. An adiabatic capture model is used to calculate the rotational-state-dependent interaction potentials experienced by the N 2 molecules in the electric field of the He + ion and the corresponding collision-energy-dependent capture rate coefficients. The total collision-energy-dependent capture rate coefficient is then determined by summing over the contributions of the N 2 rotational states populated at the 7.0 K rotational temperature of the supersonic beam. The measured and calculated rate coefficients are in good agreement, which enables us to attribute the observed reduction of the reaction rate at low collision energies to the negative quadrupole moment, Q zz , of the N 2 molecules. The effect of the sign of the quadrupole moment is illustrated by calculations of the rotational-state-dependent capture rate coefficients for ion-molecule reactions involving N 2 (negative Q zz value) and H 2 (positive Q zz value) for | J , M 〉 rotational states with J ≤ 5 ( M is the quantum number associated with the projection of the rotational angular momentum vector J⃑ on the collision axis). With decreasing value of | M |, J⃑ gradually aligns perpendicularly to the collision axis, leading to increasingly repulsive (attractive) interaction potentials for diatomic molecules with positive (negative) Q zz values and to reaction rate coefficients that decrease (increase) with decreasing collision energies.

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.