Abstract
Measurements are made of the velocity distributions of He+, Ne+ and Ar+ ions drifting in their respective parent gas under the action of a uniform electric field at room temperature (297-300K). Considerable improvements have been implemented in the present apparatus in comparison with older systems. The drift tube is greatly enlarged and its pressure accurately controlled by a regulating servo-valve. A new ion optical system coupled with a wider quadrupole analyser head ensures a high collection efficiency of ions transmitted through the retarding potential grids. A novel in-drift obstacle and a 90 degrees off-axis detector effectively suppress interference from UV photons and metastable species. Experimental results for E/N ranging from 60 to 320 Td are compared with both the Wannier equation for the RMS ion velocity and the corresponding Maxwellian distribution. The typical experimental curve is found to possess only a single maximum with its peak displaced towards the low velocity end and an enhanced high velocity tail in comparison with the Maxwellian.