Unexpected rotational distribution of the ground electronic state OH(X 2Π) generated by plasma at the interface with liquid water

Abstract

An unexpected rotational distribution of the ground electronic state OH(X 2Π) was observed by laser-induced fluorescence in a surface barrier discharge in contact with a liquid water interface. Analysis assuming a Boltzmann distribution yielded an unrealistically high rotational temperature above 600 K in the post-discharge phase. This is interpreted as a quasi-Boltzmann rotational distribution that is not in equilibrium with the translational temperature of the background gas. While the peculiarities of rotational distribution of excited OH(A 2Σ+) are known, the presented results show that even the ground electronic state OH(X 2Π) does not always have to be a reliable indicator for spectroscopic thermometry in plasmas containing water. A series of similar observations challenges the usual approach in kinetic models, when the rotational distribution is assumed to follow the Boltzmann function with the temperature of surrounding gas.