Pressure dependence of the atmospheric electrolyte cathode glow discharge spectrum

The intensity of the spectral lines emitted by the electrolyte cathode glow discharge plasma was investigated as a function of the air pressure at different discharge currents and pH values of the cathode solution. The observed increase in the metal line intensity is explained by three-body collisional recombination of positive metal ions in the cathode dark space (M+ + e + e --> M + e) followed by diffusion of neutral metal atoms into the negative glow, where electron impact excitation of the neutral atoms takes place. On the basis of these two processes, the dependence of the intensity on pressure was calculated by an approximating equation, giving an excellent agreement with the experimental results in the pressure range 530-1200 mbar. Furthermore, the model predicts the appearance of an intensity maximum as a function of the air pressure. This maximum was observed in preliminary experiments in the pressure range 1200-2300 mbar.