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Role played by the N2(A3Σu+) metastable in stationary N2 and N2-O2 discharges

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.

ELECTRON AND HEAVY-PARTICLE KINETICS IN THE LOW PRESSURE OXYGEN POSITIVE COLUMN

A kinetic model for the low-pressure oxygen positive column is presented and discussed. The model is based on the electron Boltzmann equation and the rate balance equations for the dominant heavy-particle species, which are solved simultaneously in order to take into account the coupling between the electron and the heavy-particle kinetics. The effects of vibrationally excited molecules, dissociated atoms and metastable states on the electron kinetics are analysed in detail. The predicted populations of O2(X3 Sigma ), O2(a1 Delta ), O(3P), and O- are shown to agree satisfactorily with previously reported measurements. A combination of this kinetic model with the continuity and transport equations for the charged species e, O-, and O2+ is shown to provide characteristics for the maintenance field that agree reasonably well with experiment.

A time-dependent analysis of the nitrogen afterglow in and - Ar microwave discharges

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.

Self-consistent kinetic model of the short-lived afterglow in flowing nitrogen

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 N2(A 3 � + ) metastable, measured at approximately the same position in Sadeghi et al (2001 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 N2(B 3 � g) and N + (B 2 � + u ) and metastable N2(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 N2(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 N2(A 3 � + ) metastables and N( 4 S) atoms, and with the emission intensities of 1 + and 1 − system bands of N2.

Active species downstream of an Ar-O2 surface-wave microwave discharge for biomedicine, surface treatment and nanostructuring

Self-consistent theoretical models have been developed in order to investigate the early and remote flowing afterglows of a surface-wave Ar?O2 microwave discharge generated at 2.45?GHz in a 0.5?cm diameter tube at pressures between 1 and 12?mbar. The early afterglow that occurs downstream of the discharge fills up the tube that connects the discharge region with the large-volume processing reactor, where the late afterglow develops. The models provide the time-dependent density profiles of different species along the afterglow and their 3D spatial distribution in the processing reactor. Systematic calculations are performed for all mixture compositions from pure Ar to pure O2 at different pressures.It is shown that the Ar+, and can survive up to 1?10?ms in the early afterglow depending on the mixture composition and pressure. In low O2 content mixtures the ion densities can increase in the early afterglow, depending on the operating conditions, as a result of Penning ionization involving the Ar(4s) states and forming Ar+, followed by charge transfer. In pure Ar the UV emitting resonant state atoms remain up to 0.1?ms in the afterglow, but with O2 addition their lifetime becomes considerably shorter. The oxygen species important for many applications, such as O(3P) atoms and O2(a) metastable molecules, survive up to 100?ms, thus are the main components of the late afterglow. It is shown that the O2 molecules are strongly dissociated in the discharge, dissociation being more efficient in high Ar content mixtures. However, the dissociation degree decreases to a few per cent in the early afterglow in about 10?ms. In the case of O2(a) molecules, yields above the threshold yield for the iodine laser operation are obtained at 12?mbar for afterglow times of up to 10?ms. In the large-volume reactor it has been found that at low pressure the density of O(3P) atoms decreases by about one order of magnitude towards the walls, while that of O2(a) changes about 20%, although with pressure the density decreases become more pronounced. Very similar density distributions are found at different mixture compositions for O(3P) atoms, while the quasi-homogeneous O2(a) distribution found in high Ar content mixtures progressively turns into a more inhomogeneous one with O2 addition.

Effects of electron-electron collisions on the characteristics of DC and microwave discharges in argon at low pressures

The authors present a systematic investigation of the effects caused by electron-electron collisions on the electron kinetics in Ar under the action of DC and microwave fields. The analysis is based on solutions to the homogeneous electron Boltzmann equation using the effective field approximation, i.e. assuming that the electron energy distribution function is time independent. The calculated electron transport parameters and ionization coefficient are used together with a generalized charged particle balance equation to derive the characteristics for the maintenance field and the mean absorbed power per electron for steady-state DC and microwave discharges in Ar. Results are provided for the cases of (i) single-step ionization; (ii) multi-step ionization, via the metastable and the resonance levels of Ar.

Non-equilibrium kinetics in N2 discharges and post-discharges: a full picture by modelling and impact on the applications

The main concerns associated with the establishment of a self-consistent model for N2 discharges and post-discharges at low pressures (typically p ~ 1 Torr), as well as in mixtures of this gas with O2 and CH4 are analysed and discussed. The focus is given on the coupling of the various kinetics involved: electrons, vibrational molecules N2 , dissociated atoms N(4S), ionic species, and various atomic and molecular electronic states. The impact of N2–O2 and N2–CH4 systems on the applications is briefly summarized by reviewing the essential kinetics. The difficulty in incorporating a self-consistent model for the surface kinetics is also discussed and a state-of-the-art approach for wall reactions is presented.

Electron and metastable kinetics in the nitrogen afterglow

This work presents a theoretical study of the time-relaxation of both the electron energy distribution function and the populations of the different species of interest in the nitrogen afterglow of a ω/2π = 433 MHz discharge at p = 3.3 Torr, in a tube with radius R = 1.9 cm. It is shown that collisions of highly excited N2(X 1 � + g ,v 35) molecules with N( 4 S) atoms may be in the origin of the observed pronounced maxima for the concentrations of various species, including electrons, occurring downstream from the discharge after a dark zone. Slow electrons remain for very long times in the post-discharge (at least up to t ∼ 10 −3 –10 −2 s), and the strong coupling between the electron and metastable kinetics is clearly pointed out.

Nitrogen pink afterglow: the mystery continues

This work extends our previous analysis of the nitrogen pink afterglow, by comparing our model predictions with the recently reported measurements of metastable N( 2 P) atoms and N2(a 1 g) molecules. It is shown that both species reveal the presence of a characteristic maximum on their populations, occurring downstream from the discharge after an initial stage of decrease. Such behavior is a consequence of the V-V pumping-up mechanism taking place during the relaxation in the afterglow, which is followed by V-E transfers that create locally N2(A 3 +) and N2(a 0 1 u ) metastables. The model predictions significantly overestimate the density of the N2(a 1 g) state, revealing a problem in the description of the singlet kinetics. As singlet N2(a 0 1 u ) metastables play a crucial role in nitrogen ionization, the new results imply that the ionization mechanisms in the afterglow may have to be reviewed.

O2(a Δ1g) production in flowing Ar–O2 surface-wave microwave discharges: Possible use for oxygen-iodine laser excitation

Herein we present the calculations conducted on an Ar–O2 surface-wave microwave discharge and its afterglow, and show that this system can be effectively used for the oxygen-iodine laser excitation. It is demonstrated that at pressures higher than 10 mbar O2(a) yields higher than the threshold yield required for positive gain can be achieved along the afterglow. Additionally, the density of O(P3) atoms, which can quench the I(P21/2) excited state, can be tuned to the desired level.