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