ION MOBILITY MEASUREMENTS FOR O-2(-K() IN HELIUM GAS AT 4.35)

The mobility of O2+ in He has been measured using a liquid helium cool ed, selected ion drift tube at 4.35 K. Tbe dependence of mobility on e ffective temperature, for O2+, is similar to that measured previously for rare gas ions moving in He. The mobility reaches a maximum at betw een 500 K and 600 K where it is in reasonable agreement with the exist ing room temperature measurements. Below 70 K there is a region in whi ch mobility is constant, but lower than the polarization limit, and be low 10 K, mobility decreases sharply. Although clustering reactions oc cur at low effective temperature, it is not certain that they are resp onsible for these phenomena, and although a quantum mechanical orbitin g resonance is expected to occur at low collision energies, there are no theoretical predictions of its importance for the O2+-He system.