Spectroscopic study of an atmospheric pressure dc glow discharge with a water electrode in atomic and molecular gases

The positive column of dc excited atmospheric pressure glow discharges in a metal pin-water electrode system is investigated in air, N₂, He, Ar, N₂O, CO₂ and He-N₂ mixtures. The electric field and the plasma temperatures in the positive column are measured and the effect of the filling gas on the optical emission is examined. Estimates of the electron temperature and density and the observed emission properties indicate that the formation processes of the excited dissociation fragments of H ₂O are due to recombination processes rather than direct electron excitation. Deviation from a Boltzmann rotational population distribution of OH(A) is observed in all gases due to the formation process of OH(A). In the case of He the non-Boltzmann behaviour is more pronounced which can be caused by the lower gas temperature as a higher gas temperature leads to more thermalization of the rotational population distribution of OH(A). The results in this paper suggest that vibrational energy transfer contributes to the non-Boltzmann distribution of OH(A). The temperature corresponding to the small rotational numbers of OH(A) can be used as an estimate of the gas temperature.