Rotational temperatures in RF plasma of Ar-H2O mixtures at low and moderated pressure

An Ar-H2O discharge with Ar content from 0 to 95 % at 67 and 266 Pa have been studied by optical emission spectroscopy. The plasma was excited at 27 MHz by a helical cavity with a power set at 30 W. The rotational temperatures of N2(C3Πu) and OH(A2 Σ + ) emitters were determined through spectral simulations. For added Ar percentages smaller than 95 %, the rotational temperature of N2(C3Πu) state was shown to be constant and equal to the gas temperature with a value of 450 ± 50 K. The effect of energy transfer from argon metastables is probably weak considering the results on the rotational temperature of N2(C3Πu) at 95 % Ar / 5 % H2O. The experimental OH spectra could be fitted only with a two temperatures model; T1= 450 K for both pressures and T2= 15000 K with a contribution of 22 and 32 % for 266 and 67 Pa, respectively, as result of the excitation mechanisms involving electron impact collision with water.