Mobilities of Negative Ions in SF₆

Mass identified mobilities were measured at 300°K for negative ions formed by a pulsed spark discharge in SF₆. Zero field mobilities of the four ion species, SF₅⁻, SF₆⁻, SF₆⁻(SF₆), and SF₆⁻(SF₆)₂, were found to be 0.595 ± 0.007, 0.542 ± 0.007, 0.470 ± 0.010, and 0.420 ± 0.010 cm² V⁻¹·sec⁻¹, respectively. The SF₆⁻(SF₆)₂ ion was observed to be collisionally destroyed at $E / n$ values greater than 100 × 10⁻¹⁷ V·cm². The present data indicate that the zero field mobility of 0.570 cm² V⁻¹·sec⁻¹ previously reported by McAfee and Edelson and by McDaniel and McDowell may have referred to the unresolved SF₅⁻¹ and SF₆⁻ ions. All four mobilities measured in this experiment are significantly below the predictions of the Langevin mobility theory in its polarization limit.