Multipole-moment effects in ion–molecule reactions at low temperatures: part I – ion-dipole enhancement of the rate coefficients of the He⁺ + NH₃ and He⁺ + ND₃ reactions at collisional energies E_coll/k_B near 0 K

Abstract

The energy dependence of the rates of the reactions between He+ and ammonia (NY3, Y = {H,D}), forming NY2+, Y and He as well as NY+, Y2 and He has been measured at low collision energies near 0 K.

Multipole-moment effects in ion–molecule reactions at low temperatures: part II – charge–quadrupole-interaction-induced suppression of the He+ + N2 reaction at collision energies below kB·10 K

“…We consider the total charge–quadrupole interaction potential for a molecule in a state which correlates adiabatically to the | JM 〉 state in zero electric field as: 12,24 V ( JM ) int ( R ) = V L ( R ) + Δ E JM ( R ).This interaction potential can then be used to calculate the collision-energy-dependent rate coefficients, k JM ( E coll ), as described in Article I. 17…”

“…The effect of the electric dipole moment of the neutral molecule on ion–molecule reactions is to increase the capture rates compared to the Langevin rates at low collision energies or temperatures. 8,10,17,26 The rate enhancements predicted theoretically depend strongly on the degree of sophistication of the treatment of the orientation of the molecular permanent dipole moment during the collision. 22 Calculations assuming full orientation are the simplest but greatly overestimate the rate enhancements at low temperatures or collision energies.…”

“…22 Calculations assuming full orientation are the simplest but greatly overestimate the rate enhancements at low temperatures or collision energies. The explicit consideration of the rotational-state-dependent orientation of the molecules in the field of the ion via the Stark effect in the realm of adiabatic capture models 11–14,26–29 typically leads to much less pronounced enhancements, except at the lowest collision energies or temperatures (typically below 1 K) 17,26 and for rotational states that are subject to a linear negative Stark shift at low fields. 17 It is only very recently that the first experimental studies of the rates of reactions between ions and polar molecules have been reported at collision energies below ≈ k B ·10 K, revealing the details of the rotational-state-specific rate enhancements at low collision energies for the reactions of He + with CH 3 F, 26 NH 3 and ND 3 .…”

“…This article is the second in a series of three articles exploring the effects of molecular electric multipole moments on the rates of ion–molecule reactions at low energies. The first and third articles in this series present studies of the reactions of He + with ammonia (NH 3 and ND 3 ) 17 (referred to as Article I below) and methane (CH 4 and CD 4 ) 18 and focus on the role of the interaction between the charge of the ion and the electric dipole and octupole moments of the neutral molecule, respectively. In the present article, the effect of the charge–quadrupole interaction is investigated with the example of the He + + N 2 reaction, for which we have measured the collision-energy dependence of the reaction rate coefficient in the range of collision energies between 0 and k B ·10 K at a resolution of about k B ·250 mK near E coll = 0.…”

“… 17 It is only very recently that the first experimental studies of the rates of reactions between ions and polar molecules have been reported at collision energies below ≈ k B ·10 K, revealing the details of the rotational-state-specific rate enhancements at low collision energies for the reactions of He + with CH 3 F, 26 NH 3 and ND 3 . 17 …”