OPTOGALVANIC STUDY OF A NEGATIVE-ION PLASMA IN A HOLLOW-CATHODE DISCHARGE IN OXYGEN

The properties of a negative ion plasma in a hollow cathode discharge have been investigated in oxygen using optogalvanic and probe methods. In the former method, the laser photodetachment technique is applied, and data which support the existence of negative ions are obtained by using cw and pulsed lasers. Appreciable amounts of negative ions are generated between 40 and 150 Pa and at relatively lower discharge currents. It is found that O- or O3- are more dominant at medium currents while O2- is more dominant at lower currents. At high currents, negative ions disappear. The electron energy distribution is depleted in the low-energy region below 1 eV. At higher energies, the energy distribution is almost Maxwellian with an electron temperature of 2.5-3.0 eV. The electron density ranges from 10(9) to 10(10) cm-3 and the ratio of the negative to positive ion density is 0.2-0.6 depending on the pressure and the current.