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

Abstract

We report on an experimental and theoretical investigation of the He⁺ + N₂ 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 N₂ 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 N₂ beam and the relative yields of the ionic reaction products N⁺ and N₂⁺ 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 N₂ 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 N₂ 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 N₂ 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 N₂ (negative Q_zz value) and H₂ (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 on the collision axis). With decreasing value of |M|, 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.