Electron detachment from negative ions of sulphur hexafluoride-swarm experiments

The electron detachment coefficient delta /N was measured as a function of E/N in sulphur hexafluoride. Negative ions of SF 6 were generated in a parallel-plate test cell by electron attachment near a flash-illuminated cathode. A small applied electric field pulled the thin layer of ions into the volume. After a preset time, a step function high-voltage pulse was applied which led to electron detachment and multiplication of the detached electrons. At constant E/N, the collected charge was measured as a function of the electrode spacing. Analysis of these curves with literature data of the ionization coefficient, alpha , and the attachment coefficient, eta , yielded values of delta . Data were obtained for the following SF 6 pressures: 2.5, 5, 10 and 20 mbar. E/N ranged from 380 to 770 Td.