The Mechanism of the Low-Frequency Electrodeless Discharge in Chlorine and the Influence of Irradiation

The current flowing through a 50 c/s, electrodeless discharge in a short cylindrical glass vessel with plane electrodes at the ends filled with chlorine at 5-50 mm. Hg was investigated with an oscillograph. It consists essentially of between one and fifty or more distinct pulses per half cycle, the number increasing with the voltage. At sufficiently large voltage these pulses cover the greater part of a half cycle including the instant of zero voltage. By irradiating the discharge with light from an incandescent lamp the average pulse height decreases, radiation below 4,800 A. being strongly active. By irradiating only the central portion of the vessel the pulse height does not change Irradiating one wall reduces the height of the pulses in every second half cycle, namely those pulses whose electron avalanches start from that wall Once the discharge starts, electric charges collect on the inner surface of the vessel. Each current pulse consists of a series of avalanches which develop between small areas of the inner glass walls. One such area may discharge within a half cycle in several steps. The time variation of the applied field, the wall-charge field and the field in the gas are discussed and a new mechanism is developed which appears to apply to discharges between solid dielectrics. At room temperatures, chlorine molecules form an adsorbed layer, several molecules thick, on the glass walls. Irradiation produces photo-dissociation of Cl 2 , the atoms being effective in capturing electrons in this layer where slow electrons and many-body collisions are likely. This reduction of the number of secondary electrons reduces the pulse height. Adsorbed molecular gas layers also explain observations at higher temperatures At higher voltages the interval between two successive pulses becomes so small that one pulse reduces the height of the following one. This is probably due to metastable Cl atoms and resonance radiation diffusing to the wall where they cause dissociation of molecules in the adsorbed layers, the chlorine atoms capturing electrons as before. It also explains why at high voltages the effect of irradiation becomes insignificant