This paper describes the use of Optical Emission Spectroscopy (OES) to measure electron densities and temperatures in non-equilibrium plasmas. The ways to interpret relative line-intensities of neutral argon atoms are evaluated based upon a collisional-radiative model including atomic collisional processes. A conversion from an excitation temperature determined from relative line intensities assuming a Boltzmann population distribution to the thermal electron temperature in the electron temperature range 1–4 eV and electron density range 10¹⁰–10¹² cm^–3 is given. Procedures to obtain electron temperature T_e and density N_e of non-equilibrium argon plasma by OES measurement with collisional radiative model. Graphical Abstract

We measured the band spectra (first and second positive systems) of the nitrogen molecule to examine the vibrational and rotational temperatures of the $C{}^3\Pi$ and $B{}^3\Pi$ states by optical emission spectroscopy. We compared the experimentally measured and the calculated spectra to determine those temperatures of the generated plasma. We generated a microwave discharge nitrogen plasma in a cylindrical quartz tube ($26\mathrm{mm}$ inside diameter) with a discharge pressure of $0.5–1.0\text{Torr}$ . The microwave frequency was $2.45\mathrm{GHz}$ and the output power was set at $600\mathrm{W}$ . It was found that $T_v\approx 0.5–0.7\mathrm{eV}$ and $T_r\approx 0.07–0.15\mathrm{eV}$ at $B{}^3\Pi$ ($v=7$ , 8, and 9), whereas $T_v\approx 0.65–0.9\mathrm{eV}$ and $T_r\approx 0.06–0.16\mathrm{eV}$ at $C{}^3\Pi$ ($v=0$ and 1). Both rotational temperatures obtained from first and second positive systems were in good agreement. We also compared the measured vibrational populations with theoretical calculations, in which vibrational distribution function at ${\mathrm{N}}_2$ $X$ and electron energy distribution function are calculated self-consistently.

Population densities of excited states of argon atoms are theoretically examined for ionizing argon plasma in a state of nonequilibrium under atmospheric pressure from the viewpoint of elementary processes with collisional radiative model. The dependence of excited state populations on the electron and gas temperatures is discussed. Two electron density regimes are found, which are distinguished by the population and depopulation mechanisms for the excited states in problem. When the electron impact excitation frequency for the population or depopulation is lower than the atomic impact one, the electron density of the plasma is considered as low to estimate the population and depopulation processes. Some remarkable characteristics of population and depopulation mechanisms are found for the low electron density atmospheric plasma, where thermal relaxation by atomic collisions becomes the predominant process within the group of close-energy states in the ionizing plasma of atmospheric pressure, and the excitation temperature is almost the same as the gas temperature. In addition to the collisional relaxation by argon atoms, electron impact excitation from the ground state is also an essential population mechanism. The ratios of population density of the levels pairs, between which exists a large energy gap, include information on the electron collisional kinetics. For high electron density, the effect of atomic collisional relaxation becomes weak. For this case, the excitation mechanism is explained as electron impact ladderlike excitation similar to low-pressure ionizing plasma, since the electron collision becomes the dominant process for the population and depopulation kinetics.

We generate a microwave discharge nitrogen plasma using a rectangular waveguide and a quartz tube (30 mm i.d.) inserted along the vector of the electric field, with an adjustable short-circuited plunger. The typical discharge conditions used are as follows: microwave frequency 2.45 GHz, input power 550 W, nitrogen (99.5%) gas flow rate 0.01–1.6 l/min, and discharge pressure p=0.5–10 Torr. Electron temperature T _e, and density n _e are measured using a double probe. Vibrational temperature T _v is obtained from the population density of the vibrational exited states of N₂ C ³Π_u by spectroscopic examination. Rotational temperature T _r is obtained from a comparison of the rovibronic spectra observed experimentally with the calculated ones. It is found that T _e≃0.5–2 eV, T _v≃0.45–1 eV, and T _r≃0.06–0.1 eV under the present pressure conditions, whereas n _e≃10¹¹–10¹² cm^-3 at a low discharge pressure, and n _e≃10⁸–10⁹ cm^-3 at a high discharge pressure. We discuss the measured vibrational populations by theoretical calculations in which the variation in their number density is expressed in terms of molecular processes in the nitrogen plasma.