The mobilities of ions and cluster ions drifting in polar gases

Abstract

The mobility of ions drifting in polar gases is explored both theoretically and experimentally. New experimental results are presented for (i) NO⁺ ions drifting in H₂O (the reduced zero-field mobility K⁽⁰⁾₀ is 0.66 ± 0.07 cm² V⁻¹ s⁻¹), (ii) H₃O⁺(H₂O)₃ ions drifting in H₂O K⁽⁰⁾₀=0.43 ± 0.06 cm₂ V⁻¹ s⁻¹) and (iii) NO⁺(CH₃COCH₃)_n ions (n=2,3) drifting in CH₃COCH₃ K⁽⁰⁾₀=0.041 ± 0.010 cm² V⁻¹ s⁻¹ for n=2 and K⁽⁰⁾₀=0.050 ± 0.015 cm² V⁻¹ s⁻¹ for n=3). A number of theoretical models for ion mobilities in polar gases are described. The models are compared with the available experimental data and a reasonable agreement is obtained. For larger cluster ions the measured mobilities are considerably smaller than the calculated values. Some possible reasons for the discrepancies are discussed, including momentum transfer outside the capture cross section, dipole–dipole interactions, ligand exchange, inelastic collisions and the validity of Blanc’s law.