A new technique for measuring negative ion mobilities at atmospheric pressure

The electrode system used for determination of negative ion mobilities consists of three electrodes: two parallel plane electrodes (C 1 and C 2 ) and a point electrode (A) perpendicular to C 1 and C 2 . The central part of C 1 is a wire mesh. The gap between C 1 and C 2 is an ion drift space and, that between C 1 and A is used as a negative ion detector (detecting gap). C 1 is earthed, C 2 is kept at negative and A at positive potential. The potential across the detecting gap is kept at that corresponding to its Geiger counter region. When pulsed UV irradiation is directed at the mesh, perpendicular to it, photoelectrons are released simultaneously from the mesh and C 2 and cause two burst pulses to appear in the detecting gap. As the time interval between the two pulses is equal to the time of flight of negative ions in the drift space, their mobilities can easily be determined. Advantages of this technique are: the determination can be done easily and quickly, and the device is of very simple construction and may be used even for high pressures. It is not capable of the determination of positive ion mobilities. Results obtained for negative ion mobilities for zero field at atmospheric pressure for dry air, humid air, O 2 and SF 6 are: 2.25, 1.9, 2.3 and 0.57 cm 2 V -1 s -1 respectively.