The high degree of vibrational excitation of O 2 ground state molecules recently observed in inductively coupled plasma discharges is investigated experimentally in more detail and interpreted using a detailed self-consistent 0D global kinetic model for oxygen plasmas. Additional experimental results are presented and used to validate the model. The vibrational kinetics considers vibrational levels up to v=41 and accounts for electron impact excitation and de-excitation (e-V), vibration-to-translation relaxation (V-T) in collisions with O 2 molecules and O atoms, vibration-to-vibration energy exchanges (V-V), excitation of electronically excited states, dissociative electron attachment, and electron impact dissociation. Measurements were performed at pressures of 10-80 mTorr (1.33 and 10.67 Pa) and radio frequency (13.56 MHz) powers up to 500 W. The simulation results are compared with the absolute densities in each O 2 vibrational level obtained by high sensitivity absorption spectroscopy measurements of the Schumann-Runge bands for O 2 (X, v=4-18), O( 3 P) atom density measurements by two-photon absorption laser induced fluorescence (TALIF) calibrated against Xe, and laser photodetachment measurements of the O − negative ions. The highly excited O 2 (X, v) distribution exhibits a shape similar to a Treanor-Gordiets distribution, but its origin lies in electron impact e-V collisions and not in V-V up-pumping, in contrast to what happens in all other molecular gases known to date. The relaxation of vibrational quanta is mainly due to V-T energy-transfer collisions with O atoms and to electron impact dissociation of vibrationally excited molecules, e+O 2 (X, v)→O( 3 P)+O( 3 P).

Optical emission spectroscopy, ranging from visible to near infrared, is used to determine densities and rotational temperatures of N 2 (B 3 g ) and N 2 (C 3 u ) states in a nitrogen-argon (0-95% Ar) discharge, under moderate pressures . The plasma is sustained by a helical cavity with an excitation frequency of 27 MHz and power fixed to 28 W. Firstly, in the case of a pure N 2 discharge, the two states turn out to have a similar rotational temperature, which approximates the gas temperature reasonably well. With a gradual increase in the Ar concentration up to 95%, the rotational temperature of N 2 (C 3 u ) roughly doubles while that of N 2 (B 3 g ) stays unchanged at 430 ± 50 K regardless of the gas composition. Secondly, as observed, the densities of the N 2 (B 3 g ) and N 2 (C 3 u ) states increase with increasing Ar percentage in the gas mixture. The increase in the emission intensity values is less marked for positions corresponding to both ends of the cavity. In fact, the difference in the emission level between the power input and helix middle positions is reduced, revealing that the total discharge is more uniform along the cavity for large argon concentrations. The experimental results show a strong dependence of temperatures and densities on the Ar amount in the gas mixture. A kinetic model is developed to explain this phenomenon, which is then used in modelling density evolutions versus relative abundance of Ar and versus the position along the cavity axis. The model indicates the importance of the role of electron and metastable species in the above-described discharge.

Optical emission spectroscopy in vacuum ultraviolet and UV spectral ranges is applied to study densities, and vibrational and rotational temperatures of the N 2 molecule in a nitrogen-argon (0-95% Ar) plasma sustained at a pressure of 400 Pa by a helical cavity supplied with a power of 28 W and an excitation frequency of 27 MHz. The spatial investigation of all emission systems from UV to NIR shows a minimum situated in the middle of the helical structure and two maxima located at the positions where the RF power is transmitted to the gas and at the end of the helix. The minimum was deepest for emission of the first positive (1 + ) nitrogen system. This hollow shaped density profile due to the presence of a non-linear phenomenon in the discharge is constant whatever the gas composition. The emissions related to Lyman-Birge-Hopfield and the second positive (2 + ) systems of molecular nitrogen, and N( 2 P) atoms, are analyzed versus the Ar percentage. Additionally, the NO(A 2 + → X 2 ) and OH(A 2 + → X 2 ) emission systems coming from impurities are investigated. All the densities of the considered species increase with Ar amount. The rotational and vibrational temperatures of the emitter species are determined through the comparison between experimental and simulated spectra. In the case of a N 2 discharge, all the rotational temperatures deduced through the nitrogen emission systems are equal and can be assimilated to the gas temperature. With the increase in the Ar amount, only the rotational temperature obtained from the 1 + system is close to the gas temperature. The rotational and vibrational temperatures related to the NO(A 2 + ) species are constant whatever the gas composition. The vibrational distribution function of N 2 (a 1 g ) state presents a Boltzmann law with a vibrational temperature in the range 5600-8000 K (±1000 K) for the N 2 -x% Ar mixture with x < 75%. When the Ar percentage increases above this limit, we observe strong deviations from the Boltzmann law and no temperature can be deduced. Some kinetic considerations, where the nitrogen and argon metastables play an important role, are discussed to explain the strong dependence of the temperatures and density species toward the Ar amount in the gas mixture.