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When high voltage is applied to a gap between electrodes in aqueous solution, a discharge plasma is generated that connects the edges of the facing electrodes. When the width of the gap between the electrodes is extended to more than several millimeters, the plasma is separated into two and they are localized at the vicinities of the edges of the anode (+plasma) and cathode (-plasma). Although they are expected to supply characteristic reaction fields, the properties of the plasmas, such as electron number density and temperatures of transient species, have not yet been clarified. In the present study, a time- and space-resolved emission spectrometer with a discharge cell was developed for optical diagnostics of the +plasma and -plasma separately. The electron number density for the +plasma was obviously lower than that for the -plasma. The difference in the electron number density should result from the difference in the ionization energy of the cathode materials. From the temporal evolutions of the emissions from the components, the emissions from the -plasma were sustained for about 0.5 μs after the decay of the applied voltage, probably due to the large number of free electrons in the -plasma. It is also clarified that hydroxyl radicals are effectively generated by the collisions between cations deriving from water and low-energy free electrons in the +plasma. The wide-gap in-solution discharge supplies two plasmas simultaneously with different properties. For plasma reactions, one plasma with suitable properties can be selected.

Time-and Space-resolved Optical Diagnostics for Discharge Plasmas Separately Formed in Aqueous Solution