Measured and calculated mobilities of cluster ions drifting in helium and in nitrogen

Abstract

New experimental results are presented for the mobilities of (1) the H₃O⁺(H₂O)₃ cluster ion drifting in He (the reduced zero-field mobility K₀⁽⁰⁾ is 11.5 ± 0.4 cm² V⁻¹ s⁻¹, (2) NO⁺ (CH₃COCH₃)_n cluster ions drifting in He (K₀⁽⁰⁾ = 7.5 ± 0.4 cm² V⁻¹ s⁻¹ for n = 2, K₀⁽⁰⁾ = 7.3 ± 0.5 cm² V⁻¹ s⁻¹ for n = 3) and (3) the NO⁺(CH₃CN)₂ cluster ion drifting in N₂ (K₀⁽⁰⁾) = 1.70 ± 0.06 cm³ V⁻¹ s⁻¹). These results together with other recently reported cluster ion mobilities in He are quantitatively explained using the hard-sphere collision model by von Helden et al. (J. Phys. Chem. 97 (1993) 8182–8192). The role of ion-induced dipole interactions is investigated and shown to be minor for the mobilities of all but the smallest ions in He. The existing data on cluster-ion mobilities in N₂, including the result here, are systematized. As a result of the stronger polarizability of N₂, the hard-sphere collision model does not agree with the data, and a better explanation is obtained using a simple model that includes ion-induced dipole interactions. The size of the cluster has only a minor effect on the mobility in N₂. The remaining discrepancies between the measured and calculated results are attributed to ion-quadrupole or dipole induced-dipole interactions.