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

An unexpected rotational distribution of the ground electronic state OH(X 2 Π) 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 2 Σ + ) are known, the presented results show that even the ground electronic state OH(X 2 Π) 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.