Mobilities of Polyatomic Ions

Experimental data on the mobilities of the polyatomic ions SO2F−, SO2F2−, SF5−, SF6−, F−(SF6), and SO2F2–(SO2) in an He carrier gas were compared with the predictions of the mobility theories of Langevin and of Mason and Schamp. As a consequence of the low polarizability of He and the large size of the ions, these mobilities were found to be dominated by the interaction of the ion-neutral repulsive cores rather than the conventional point-charge-induced-dipole interaction. In fact, these data were reasonably well reproduced by simply a hard-sphere collision model, completely neglecting any polarization force. Differences in the ion sizes were found to be manifested in readily measured differences in the mobilities. Experimental mobilities of SF5− and SF6− in O2, CH4, and SF6 gases were also compared with theory. These gases all have larger polarizabilities than He, and a fit of the mobilities required a 12–6–4 interaction potential with a relatively large r−6 term.