Emission considering self-absorption of OH to simultaneously obtain the OH density and gas temperature: validation, non-equilibrium effects and limitations

The measurement of absolute densities of ubiquitous OH radicals and gas temperatures in water containing plasmas has recently drawn a lot of attention. In this paper, we extend the self-absorption model introduced in Du et al 2016 Plasma Sources Sci. Technol. 25 04LT02 with a description of the excited state by a superposition of two Boltzmann distributions to take the non-thermal rotational distribution of the excited state into account. This technique is applied to a diffuse He + H₂O RF discharge and it is shown that in addition to the determination of the ground state OH density and rotational temperature, the properties of the excited state OH( A ) can also be simultaneously determined. A model of the steady-state distribution of the hot and cold group density of OH( A ) is able to describe the dependence of the rotational population distributions of the excited state as a function of the water concentration. The method is also applied to a filamentary Ar + H₂O DBD. While the non-homogeneous nature of the DBD leads to complications, the production of OH( A ) by multiple production mechanisms leading to a complex nascent rotational population distribution causes the fitting procedure of the emission spectrum (with self-absorption) to break down. (paper)