The mobility of N₂⁺and CO⁺in helium gas at 4.35 K

The ion mobilities of N 2 + and CO + in He have been measured using a liquid-helium-cooled, selected ion drift tube at 4.35 K. Mobility reaches a maximum at an effective temperature of between 400 K and 600 K, where it is in reasonable agreement with the room temperature measurements of other experimenters. Below 100 K there is a region in which the mobilities are constant, N 2 + close to the polarization limit and CO + somewhat lower, possibly due to the occurrence of collisionally induced, rotational excitation. At very low effective temperature though, below 10 K, the mobilities decrease sharply, below the polarization limit. This may be further evidence for the existence of orbiting resonance such as that suggested previously by Kojima et al. for He + in He.