Influence of water vapor and negative ions on self-organized luminous pattern formation in an atmospheric-pressure dc glow discharge

An atmospheric-pressure dc glow discharge with a liquid anode sometimes forms a self-organized luminous pattern on the liquid anode surface. However, the mechanism of this interesting phenomenon has not been understood yet. In general, the self-organized pattern is described mathematically by ''the reaction-diffusion system'', which is simultaneous partial differential equations with two variables.We expect that the two variables in the formation of the plasma self-organized pattern are the densities of electrons and negative ions. This is because the presence of molecular oxygen or water vapor is the key to the pattern formation. In this work, we examined the densities of OH radicals and negative ions using laser-induced fluorescence spectroscopy and laser photodetachment, respectively. We observed higher densities of OH and water vapor when the self-organized luminous pattern was formed on the liquid anode surface. The density of water vapor was estimated from the collisonal quenching frequency of OH(A²Σ⁺). In addition, the negative ion density was also higher when the self-organized luminous pattern was formed.