Ion mobilities and collision frequencies in gaseous CH3Cl, HCl, HBr, H2S, NO, and SF6: effects of polarity of the gas molecules

Abstract

Mobilities of positive and negative ions at 296 K have been measured at pressures in the range 50 to 1200 Torr. Least-means quares analysis of the pressure dependences gave the following mobilities normalized to a density of 2.69 × 1019 molecules cm−3: CH3Clμ+ = 0.39, μ− = 0.37; HCl 0.64, 0.59; HBr 0.48, 0.49; H2S 0.59, 0.60; NO 0.97, 1.53; SF6 μ− = 0.49 cm2 V−1 s−1. Except in one instance the ions exhibited a close to inverse dependence of mobility on gas density. The identifies of the ions are discussed in the light of high pressure mass spectrometric data from the literature. The dependence of the collision frequencies of the ions on μD/α1/2, the ratio of dipole moment to (polarizability)1/2 for the neutral gas molecules, was very similar to that observed previously by the ion cyclotron resonance technique at much lower pressures. The magnitudes of the collision frequencies at unit concentration are in reasonable agreement with predictions based on the ion–molecule collision rate theory of Barker and Ridge, but the dependence on μD/α1/2 is lower and closer to that predicted by ADO theory.