Plasma-liquid interactions: a review and roadmap

Plasma-liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas

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 Hα and Hβ 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

Electronic quenching of OH(A) by water in atmospheric pressure plasmas and its influence on the gas temperature determination by OH(A–X) emission

In this paper it is shown that electronic quenching of OH(A) by water prevents thermalization of the rotational population distribution of OH(A). This means that the observed ro-vibrational OH(A–X) emission band is (at least partially) an image of the formation process and is determined not only by the gas temperature. The formation of negative ions and clusters for larger water concentrations can contribute to the non-equilibrium. The above is demonstrated in RF excited atmospheric pressure glow discharges in He–water mixtures in a parallel metal plate reactor by optical emission spectroscopy. For this particular case a significant overpopulation of high rotational states appears around 1000 ppm H2O in He. The smallest temperature parameter of a non-Boltzmann (two-temperature) distribution fitted to the experimental spectrum of OH(A–X) gives a good representation of the gas temperature. Only the rotational states with the smallest rotational numbers (J 7) are thermalized and representative for the gas temperature