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

Abstract

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 between 0 and $k_{\mathrm{B}}\cdot 40$ K using a recently developed merged-beam technique [Allmendinger {\it et al.}, ChemPhysChem {\bf 17}, 3596 (2016)]. 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 was also used for adjusting the final velocity of the He Rydberg atoms, and thus the collision energy ($E_{\mathrm{coll}}$). A collision-energy resolution of about 200 mK was reached at the lowest $E_{\mathrm{coll}}$ values. The reaction rate coefficients exhibit a sharp increase at collision energies below $\sim k_{\mathrm{B}}\cdot 5$ 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. The enhancement of the reaction yields of both reactions observed at the lowest collision energies is attributed to high-field-seeking states which experience linear Stark shifts at low electric fields. 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 {\it et al.} (Chem. Phys. Lett., 1985, {\bf 122}, 431) suggests that only 40\% of the close collisions are reactive.

18 pages, 8 figures

References in corpus (4)

• Dynamics of gas phase Ne${}^{\ast }$ + NH${}_3$ and Ne${}^{\ast }$ + ND${}_3$ Penning ionization at low temperatures
• Observation of enhanced rate coefficients in the H${}_2^{+}$ + H${}_2$ $\to$ H${}_3^{+}$ + H reaction at low collision energies
• Ion-molecule reactions below 1~K: Observation of a strong enhancement of the reaction rate of the ion-dipole reaction He${}^{+}$+ CH${}_3$F
• Analysis of gaseous ammonia (NH${}_3$) absorption in the visible spectrum of Jupiter - Update

Cited by in corpus (2)

• Opposite effects of the rotational and translational energy on the rates of ion-molecule reactions near $0\text{K}$: the ${\text{D}}_2^{+}+{\text{NH}}_3$ and ${\text{D}}_2^{+}+{\text{ND}}_3$ reactions
• Observation of quantum capture in an ion-molecule reaction