Mobility and formation kinetics of NH₄⁺(NH₃)_n cluster ions (n=0–3) in helium and helium/ammonia mixtures

Abstract

NH₄⁺(NH₃)_n (n = 0–3) cluster ions are produced in a field-free flow tube section of a Selected Ion Flow–Drift Tube (SIFDT) apparatus. Cluster ion mobilities are measured in mixtures of He and NH₃ and used to obtain the individual mobilities in helium and in ammonia by applying Blanc’s law to the mixtures. Mobilities of the cluster ions are also measured in pure helium by producing the ions in the ion source of a Flowing Afterglow, Selected Ion Flow–Drift Tube Apparatus (FA-SIFDT). The measurements in pure helium compare well with the mobilities in helium obtained by applying Blanc’s law to the mixtures. The zero field mobilities of the cluster ions in helium are 22.1 ± 0.4 cm² V⁻¹ s⁻¹ for NH₄⁺ 16.6 ± 0.4 cm² V⁻¹ s⁻¹ for NH₄⁺(NH₃), 12.2 ± 0.4 cm² V⁻¹ s⁻¹ for NH₄⁺ (NH₃)₂ and 12.1 ± 0.4 cm² V⁻¹ s⁻¹ for NH₄⁺ (NH₃)₃). The decrease with increasing size of the cluster can be explained in terms of the sizes of the geometric cross sections. The zero-field mobilities in NH₃ are 0.94 ± 0.35 cm² V⁻¹ s⁻¹ for NH₄⁺, 0.83 ± 0.22 cm² V⁻¹ s⁻¹ for NH₄⁺(NH₃), 0.50 ± 0.27 cm² V⁻¹ s⁻¹ for NH₄⁺(NH₃)₂ and 0.25 ± 0.20 cm² V⁻¹ s⁻¹ for NH₄⁺(NH₃)₃. The small values of the mobilities in these polar gas systems are understood in terms of the strong ion–dipole interactions. Calculated mobilities in NH₃ are obtained by computing the collision cross section with the ion–dipole interactions taken into account; the results compare well with the measurements for NH₄⁺ and NH₄⁺(NH₃). However, the measured mobilities of the larger cluster ions are smaller than the computed values. The discrepancies may be due to several factors including dipole–dipole interactions, ligand exchange reactions,formation of long-lived quasibound complexes, and efficient transfer of kinetic energy into internal energy of the cluster ion and the ammonia molecules.