The kinetic modeling of low-pressure (p ∼ 1−10 torr) stationary nitrogen discharges and the corresponding afterglows is reviewed. It is shown that a good description of the overall behavior of nitrogen plasmas requires a deep understanding of the coupling between different kinetics. The central role is played by ground-state vibrationally excited molecules, N2(X 1 Σ + g , v), which have a strong influence on the shape of the electron energy distribution function, on the creation and destruction of electronically excited states, on the gas heating, dissociation and on afterglow emissions. N2(X 1 Σ + g , v) molecules are actually the hinge ensuring a strong link between the various kinetics. The noticeable task done by electronically excited metastable molecules, in particular N2(A 3 Σ + u) and N2(a 1 Σ − u), is also pointed out. Besides contributing to the same phenomena as vibrationally excited molecules, these electronic metastable states play also a categorical role in ionization. Furthermore, vibrationally excited molecules in high v levels are in the origin of the peaks observed in the flowing afterglow for the concentrations of several species, such as N2(A 3 Σ + g), N2(B 3 Πg), N + 2 (B 2 Σ + u) and electrons, which occur downstream from the discharge after a dark zone as a consequence of the V-V up-pumping mechanism.

The role played by the N2(A3Σu+) metastable on the overall kinetics of N2 and N2-O2 stationary discharges is illustrated by using a kinetic model based on the self-consistent solutions to the Boltzmann equation coupled to the rate balance equations for the vibrationally and electronically excited molecules, atoms and charged particles, in which the sustaining electric field is self-consistently determined. It is shown that together with the vibrational distribution of N2(X1Σg+,v) molecules, the metastable state N2(A3Σu+) plays a central role in the whole problem, since some important aspects of these discharges, such as ionization, gas phase chemistry and gas heating are associated with different processes involving the N2(A3Σu+) state.

Abstract.A zero-dimensional kinetic model has been developed to investigate the coupled electron and heavy-particle kinetics in Ar-O 2 surface-wave microwave discharges generated in long cylindrical tubes, such as those launched with a surfatron or a surfaguide. The model has been validated by comparing the calculated electron temperature and species densities with experimental data available in the literature for different discharge conditions. Systematic studies have been carried out for a surfacewave discharge generated with 2.45 GHz field frequency in a 1 cm diameter quartz tube in Ar-O 2 mixture at 0.5-3 Torr pressures, which are typical conditions found in different applications. The calculations have been performed for the critical electron density n e =3.74×10 11 cm −3 . It has been found that the sustaining electric field decreases with Ar percentage in the mixture, while the electron kinetic temperature exhibits a minimum at about 80%Ar. The charged and neutral species densities have been calculated for different mixture compositions, from pure O 2 to pure Ar, and their creation and destruction processes have been identified. The O 2 dissociation degree increases with Ar addition into O 2 and dissociation degrees as high as 60% can be achieved. Furthermore, it has been demonstrated that the dissociation degree increases with the discharge tube radius, while decreases with the atomic surface recombination of O-atoms. The density of O − negative ions is very high in the plasma, the electronegativity of the discharge can be higher than 1, depending on the discharge conditions. §

This paper presents a theoretical analysis of the nitrogen afterglow induced by a microwave discharge in and - Ar. The initial conditions at the beginning of the afterglow are obtained by solving the electron Boltzmann equation, under the effective field approximation, coupled to the rate-balance equations for the ) levels, the electronically excited states of , the atoms and the main positive ions. The electric field for the maintenance of the discharge is self-consistently determined. Once the concentrations of heavy species in the discharge have been obtained, the relaxation in the afterglow of the above system of equations is investigated. It is shown that, as a result of the mechanisms leading to associative ionization by collisions between the electronic metastable species and , associated with the near-resonant V - E energy-exchange reaction , the characteristic emission of the system of can occur in the afterglow of a microwave discharge at p = 2 Torr after a time s. However, in the case of - Ar mixtures the state arises only for higher pressures and longer residence times (such as - s in a - 50% Ar mixture at p = 10 Torr). The predicted dependences on the pressure and gas-mixture composition of the temporal evolutions of [] and [] concentrations are shown to be in qualitative agreement with reported spectroscopic measurements.

A detailed kinetic model for the flowing nitrogen microwave discharge and post-discharge is developed with the aim of gaining a deeper understanding into the processes responsible for the formation of the short-lived afterglow of nitrogen and for the enhancement of the concentration of N 2 (A 3 + u ) metastable, measured at approximately the same position in Sadeghi et al J. Phys. D: Appl. Phys. 34 1779. The present work shows that the peaks observed in the afterglow, for the density of molecules in radiative N 2 (B 3 g ) and N + 2 (B 2 + u ) and metastable N 2 (A 3 + u ) states, can be explained as a result of a pumping-up phenomenon into the vibrational ladder produced by near-resonant V-V energy-exchange collisions, involving vibrationally excited molecules N 2 (X 1 + g , v) in levels as high as v ∼ 35. The present predictions are shown to be in good agreement with the measured concentrations for N 2 (A 3 + u ) metastables and N( 4 S) atoms, and with the emission intensities of 1 + and 1 − system bands of N 2 .