Optical emission spectroscopy as a diagnostic for plasmas in liquids: Opportunities and pitfalls

In this contribution, optical emission spectroscopy is evaluated and thoroughly analysed as a diagnostic to characterize plasmas in and in contact with liquids. One of the specific properties of plasmas in and in contact with liquids is the strong emission of OH(A-X) and of hydrogen lines. As an example a 600 ns pulsed dc excited discharge in Ar, He and O₂ bubbles in water is investigated by time resolved optical emission spectroscopy. It is shown that the production processes of excited species and the plasma kinetics strongly influence the emission spectrum. This complicates the interpretation of the spectra but provides the opportunity to derive production mechanisms from the time resolved emission. The importance of recombination processes compared with direct electron excitation processes in the production of excited states of the water fragments in plasmas with high electron densities is shown. The OH(A-X) emission spectrum illustrates that even in these highly collisional atmospheric pressure discharges the rotational population distribution deviates from equilibrium. A two-temperature fit of the OH rotational population distribution leads to realistic gas temperatures for the temperature parameter corresponding to small rotational numbers. The H_α and H _β lines are fitted with two component profiles corresponding to two different electron densities. The obtained electron density is in the range 10²¹-10²³ m^-3. Possible complications in the interpretation of obtained temperatures and electron densities are discussed.