Mobilities of Polyatomic Ions

Experimental data on the mobilities of the polyatomic ions ${\mathrm{SO}}_2{\mathrm{F}}^{-},{\mathrm{SO}}_2{\mathrm{F}}_2^{-},{\mathrm{SF}}_5^{-},{\mathrm{SF}}_6^{-},{\mathrm{F}}^{-}({\mathrm{SF}}_6)$ , and ${\mathrm{SO}}_2{\mathrm{F}}_2^{–}({\mathrm{SO}}_2)$ 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 ${\mathrm{SF}}_5^{-}$ and ${\mathrm{SF}}_6^{-}$ 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.