Mobilities of He+, Ne+, Ar+, and Kr+ in He gas at 4.35 K

Mobilities of He+, Ne+, Ar+, and Kr+ have been measured in He gas at 4.35 K. An injected-ion drift tube which can be cooled by liquid helium was used. It has a Wien filter on the ion injection line and a quadrupole mass filter on the detection line so that mass identification is explicitly made. Ions are injected into the drift tube with 20 eV, and it is assured that the thermalization of ions is completed well before reaching the gate for mobility measurement. The correction for thermal transpiration in the pressure measurement was made by Takaishi–Sensui’s empirical formula. The reduced mobility K was measured against E/N, where E is the electric field strength and N is the gas number density. Then the E/N was converted to the effective temperature Teff by Wannier’s formula. The K(Teff) obtained are compared with the previous experimental and theoretical results, some of which are given in K0(Tg), where K0 is the zero field mobility and Tg is the gas temperature. The agreement between the present results and the previous experimental results is generally good at high temperature where they are available. Recommended numerical values are presented.

The present work confirms the findings in the preliminary report by Kojima et al. that the K of He+ in He has a maximum around E/N=5 Td which corresponds to Teff=18 K, and it decreases steeply below that temperature. An apprehension that the structure might be caused by clustering is discussed and denied. The maximum found in K of He+ in He is considered to be the structure predicted theoretically as a result of orbiting resonance scattering. The Ks obtained for Ne+, Ar+, and Kr+ have a typical shape of mobilities in heterogeneous gas. They have a maximum around Teff=500–1000 K and become more or less flat below 50 K. From the maximum position the well depths of the interaction potentials are estimated and compared with theoretical calculations. There is a shallow minimum of K between 50 and 100 K for Ar+ and Kr+. The Ks at the flat are a few percent larger than the polarization limit Kpol. Below Teff=6 K, Ks tend to decrease steeply. This steep decrease of K is also suggested to be an orbiting resonance scattering effect although no theoretical calculation is available.