Mobility of NOin helium gas at 77 and 4.3 K

The mobility of NO + in helium has been measured at 77 and 4.3 K using a very low temperature drift tube mass spectrometer. The dependence of mobility on the reduced electric field E / N at 77 K is similar to that of rare-gas ions in helium at 82 K. The mobility measured at 4.3 K reaches a maximum at an effective temperature around 450 K and a minimum around 15 K. Below 100 K the mobility is lower than the polarization limit, which is given by the long-range polarization interaction between the ion and the neutral atom or molecule. As possible reasons for this phenomenon, the clustering reaction, the rotational excitation by the collision and the attractive r −6 term in the interaction potential are discussed.