Equilibrium and Nonequilibrium Features in a Warm He+H 2 O Microwave Plasma at Atmospheric Pressure

A microwave surface-wave discharge at atmospheric pressure was generated using a He/H 2 O mixture and studied by optical emission spectroscopy. The main plasma parameters were determined based on the analysis of the spectra when partial thermodynamic equilibrium could be verified. The analysis showed that the system can be classified as a warm plasma with an average gas temperature of $1.7\times 10^{3}$ K, an axially averaged electron density of $2.4\times 10^{13}$ cm $^{-3}$ , and a semiempirically estimated average electron temperature of $15\times 10^{3}$ K. The experimental evidence and theoretical arguments suggest that the main source of ionization in this plasma is the electronic impact ionization of water molecules. An abnormal intensity in the O I [3p ${\,}^{3}\text{P}_{0,1,2}] \to [3{\mathrm{ s}}{\,}^{3}\text{S}^{\circ }_{1}$ ] transition was observed, revealing an overpopulation of the 3p $^{3}\text{P}_{0,1,2}$ states. This observation—which may be explored to obtain stimulated emission—was explained as an effect of the reabsorption of photons generated in the plasma by the H I Ly $_{\beta }$ emission by oxygen atoms (O $\text{I}{\,}^{3}\text{P}_{\mathrm {\mathbf {2}}} \to 3\text{d}{\,}^{3}{\rm D {^{\circ }}}_{1,2,3}$ transition), an effect generally known as photoexcitation by accidental resonance.