Positive ion mobilities and conversion rates in a helium afterglow

An X-band (3 cm) microwave cavity method has been used to measure the electron loss rate in the afterglow of a pulsed discharge in helium. Further purification of spectroscopically pure gas samples was undertaken to give more reliable results and afterglow periods up to 60 msec were investigated. The positive ion reduced mobility μ 0 (as deduced from a measurement of the ambipolar diffusion coefficient) was found to be constant at 16.7 ± 0.5 cm 2 v -1 sec -1 in the pressure range 4-83 mmHg and this is attributed to the molecular ion He 2 + . Measurements at pressures below 4 mmHg indicated the presence of both atomic and molecular ions and an extrapolation to zero pressure yielded a figure of 10.6 cm 2 v -1 sec -1 for the mobility of the atomic species He + . The frequency of conversion of atomic into molecular ions by three-body collisions was found to be 115 p 2 sec -1 in good agreement with theory.