Time dependent optical emission spectroscopy of sub-microsecond pulsed plasmas in air with water cathode

This paper reports an experimental study of sub-microsecond pulsed discharges between a metal pin electrode and a tap water cathode in atmospheric pressure air. The dynamics of excited molecular and atomic species produced in the discharge is investigated by time resolved optical emission spectroscopy. The initial diffuse plasma constricts around 300 ns after the start of the voltage pulse. This constriction is correlated with an increase in gas temperature from 1000 to 5000 K and a strong increase in emission from O (777 nm), H-alpha and NH(A-X). The formation of OH(A-X) is discussed in the framework of OH (and thus H2O2 production) in plasmas in and in contact with liquids. It is argued that electron dissociative recombination of the water ion plays an important role in the production of OH(A) and that the relative intensity of the OH(A) emission may not provide a good estimate of the OH concentration without correction for electronic quenching of OH(A).