Modeling and experimental study of molecular nitrogen dissociation in an Ar–N2 ICP discharge

Abstract

The dissociation of nitrogen molecules in an Ar–N2 inductively coupled plasma (ICP) discharge is studied both experimentally and theoretically. To measure the absolute N atom density and emission intensity of Ar and N2 excited levels, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy and optical emission spectroscopy are used. We observe an increase in N atom density with increasing pressure whereas the N atom density decreases for pressures higher than 100 mTorr in a pure nitrogen discharge. On adding argon to the mixture, we observe that the dissociation rate is enhanced when going from a pure nitrogen discharge to an argon mixed discharge. To calculate the plasma parameters, a global (volume-averaged) model is developed. The variation of the electron temperature and the particle densities are calculated by solving the particle and energy balance equations. The model calculations are compared with the measurement results and the production and loss rates of each species are described under each discharge condition. From the model calculation, the dissociation of N2 molecules in the Ar–N2 mixed discharge occurs mainly by electron impact dissociation at low pressures, while at high pressures the dissociative recombination is enhanced by charge transfer between Ar+ and N2(X) as well as metastable–metastable pooling dissociation due to the high density. In addition, the surface sticking coefficient of nitrogen atoms in a planar ICP discharge (including glass and stainless steel walls) is deduced from TALIF measurements and is estimated to be 0.02 under our set-up conditions.