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The energy dependence of the rates of the reactions between He$^+$ andammonia (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 atlow collision energies between 0 and $k_{\mathrm{B}}\cdot40$ K using a recentlydeveloped merged-beam technique [Allmendinger {\it et al.}, ChemPhysChem {\bf17}, 3596 (2016)]. To avoid heating of the ions by stray electric fields, thereactions are observed within the large orbit of a highly excited Rydbergelectron. A beam of He Rydberg atoms was merged with a supersonic beam ofammonia using a curved surface-electrode Rydberg-Stark deflector, which wasalso used for adjusting the final velocity of the He Rydberg atoms, and thusthe collision energy ($E_{\rm coll}$). A collision-energy resolution of about200 mK was reached at the lowest $E_{\rm coll}$ values. The reaction ratecoefficients exhibit a sharp increase at collision energies below $\simk_{\mathrm{B}}\cdot5$ K and pronounced deviations from Langevin-capturebehaviour. The experimental results are interpreted in terms of an adiabaticcapture model describing the rotational-state-dependent orientation of theammonia molecules by the electric field of the He$^+$ atom. The modelfaithfully describes the experimental observations. The enhancement of thereaction yields of both reactions observed at the lowest collision energies isattributed to high-field-seeking states which experience linear Stark shifts atlow electric fields. Thermal capture rate constants are derived from the modelfor the temperature range between 0 and 10~K relevant for astrochemistry.Comparison of the calculated thermal capture rate coefficients with theabsolute reaction rates measured above 27 K by Marquette {\it et al.} (Chem.Phys. Lett., 1985, {\bf 122}, 431) suggests that only 40\% of the closecollisions are reactive.

physical chemistry chemical physics/pccp. physical chemistry chemical physics

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 collision energies near $0$ K