Is it possible to deduce the ground state OH density from relative optical emission intensities of the OH(A ²Σ⁺-X ²Π_i) transition in atmospheric pressure non-equilibrium plasmas? - An analysis of self-absorption

The measurement of absolute densities of reactive species and radicals such as OH is of growing interest for many plasma applications. In this paper, we extend the use of a self-absorption model for atomic emission spectroscopy to molecular emission spectroscopy. The proposed analysis of self-absorbed molecular emission spectra is a simple and inexpensive method to determine OH(X) densities and rotational temperatures compared to laser induced fluorescence. We compare the recorded absolute OH density in a non-equilibrium diffuse atmospheric-pressure RF glow discharge by this method with broadband UV absorption considering a number of rotational lines with J′ 6.5, the detection limit of the line integrated OH(X) density with this method is of the order of 2 × 10¹⁹ m^-2. The accuracy of the density is sensitive to the rotational temperature of the OH(A) state and the non-equilibrium rotational population distribution.