THE MOBILITY OF N-2(-K() AND CO+ IN HELIUM GAS AT 4.35)

The ion mobilities of N2+ and CO+ in He have been measured using a liq uid-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, whe re it is in reasonable agreement with the room temperature measurement s of other experimenters. Below 100 K there is a region in which the m obilities are constant, N2+ close to the polarization limit and CO+ so mewhat lower, possibly due to the occurrence of collisionally induced, rotational excitation. At very low effective temperature though, belo w 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.