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

Area of Science:

• Plasma Physics
• Electrochemistry
• Materials Science

Background:

• High voltage applied to aqueous solutions generates discharge plasmas.
• In wide gaps (> several mm), the plasma separates into anode (+) and cathode (-) regions.
• Properties of these separated plasmas, crucial for reactions, remain largely uncharacterized.

Purpose of the Study:

• To develop a time- and space-resolved emission spectrometer for optical diagnostics.
• To separately analyze the properties of anode (+) and cathode (-) plasmas in wide-gap in-solution discharges.
• To understand the distinct characteristics and potential applications of each plasma.

Main Methods:

• Development of a specialized discharge cell with a time- and space-resolved emission spectrometer.
• Optical diagnostics to measure electron number density and temporal emission characteristics.
• Analysis of plasma properties based on emission spectra and temporal decay.

Main Results:

• The cathode plasma (-) showed significantly higher electron number density than the anode plasma (+).
• Cathode plasma emissions persisted for approximately 0.5 μs after voltage decay, attributed to abundant free electrons.
• Hydroxyl radicals were effectively generated in the anode plasma via cation-electron collisions.

Conclusions:

• Wide-gap in-solution discharge simultaneously produces two plasmas with distinct properties.
• Differences in electron density are linked to cathode material ionization energies.
• The study enables selection of the appropriate plasma for specific chemical reactions.