Investigation of the helium afterglow I. Mass spectrometric observations

Mass spectrometric observations of helium afterglows by ion sampling from the walls of a large discharge vessel are described. It is shown that in very pure cataphoretically-cleaned helium in the pressure range 0·3 to 2 torr the only important processes occurring in the late afterglow are ambipolar diffusion of atomic and molecular ions and electrons, and conversion of atomic to molecular ions by three-body collisions with neutral helium atoms. The atomic and molecular ion mobilities corresponding to the observed diffusion rates are respectively 11·2 ± 0·4 cm 2 v -1 s -1 and 15·9 ± 0·4 cm 2 v -1 s -1 , in acceptable agreement with several previous mobility measurements, and the value obtained for the three-body conversion coefficient, 85 ± 3 torr -2 s -1 (pressure reduced to 0 °C) is in good agreement with a recent similar measurement by other workers in a higher pressure range. The wall temperature of the discharge vessel during these measurements was 295 °K. Measurements carried out in helium samples which were not subjected to cataphoretic purification demonstrated the serious effect of small amounts of impurity on the decay rate of the molecular helium ion.