Microwave Discharge In N2 Research Articles

Relaxation of heavy species and gas temperature in the afterglow of a N2 microwave discharge

In this paper we present a self-consistent kinetic model to study the temporal variation of the gas temperature in the afterglow of a 440 Pa microwave nitrogen discharge operating at 433 MHz in a 3.8 cm diameter tube. The initial conditions in the afterglow are determined by a kinetic model that solves the electron Boltzmann equation coupled to the gas thermal balance equation and a system of rate-balance equations for N2 (X 1 ∑ g + , v ) molecules, electronically excited states of N2 , ground and excited states of atomic nitrogen and the main positive ions. Once the initial concentrations of the heavy species and gas temperature are known, their relaxation in the afterglow is obtained from the solutions to the corresponding time-dependent equations. Modelling predictions are found to be in good agreement with previously measured values for the concentrations of N(4 S) atoms and N2 (A 3 ∑u + ) molecules, and the radially averaged gas temperature Tg along the afterglow of a microwave discharge in N2 under the same working conditions. It is shown that gas heating in the afterglow comes essentially from the energy transfer involving non-resonant vibration–vibration (V–V) collisions between vibrationally excited nitrogen molecules, as well as from energy exchanges in vibration–translation (V–T) on N2 –N collisions.

Kinetic study on non-thermal volumetric plasma decay in the early afterglow of air discharge generated by a short pulse microwave or laser

This paper reports a kinetic study on non-thermal plasma decay in the early afterglow of air discharge generated by short pulse microwave or laser. A global self-consistent model is based on the particle balance of complex plasma chemistry, electron energy equation, and gas thermal balance equation. Electron-ion Coulomb collision is included in the steady state Boltzmann equation solver to accurately describe the electron mobility and other transport coefficients. The model is used to simulate the afterglow of microsecond to nanosecond pulse microwave discharge in N2, O2, and air, as well as femtosecond laser filament discharge in dry and humid air. The simulated results for electron density decay are in quantitative agreement with the available measured ones. The evolution of plasma decay under an external electric field is also investigated, and the effect of gas heating is considered. The underlying mechanism of plasma density decay is unveiled through the above kinetic modeling.

Time-resolved gas temperature evolution in pulsed Ar–N2 microwave discharge

Temporal evolution of the gas temperature (Tg) in a pulsed microwave surfaguide discharge is studied by measuring the N2 rotational temperature. We found that at high power applied per pulse, gas temperature grows linearly, and saturates after about 150 μs. This effect is absent at low power values, or at short pulse durations. Observed Tg behavior correlates with time-resolved measurements of the N2 vibrational temperature, as well as with N emission lines. Consequently, Tg time behavior was related to N atoms production in plasma. Using obtained Tg growth rates, the effective power used for plasma heating is determined.

Determination of ro-vibrational excitations of N2(B, v′) and N2(C, v′) states in N2 microwave discharges using visible and IR spectroscopy

UV–visible and infrared (IR) emission spectroscopy measurements were performed in a N2 microwave discharge at pressures between 0.1 and 3 Torr and powers of 200 and 300 W. Although emission spectroscopy in the IR region has rarely been investigated, this technique has nevertheless provided numerous key data. The plasma temperature as a function of pressure was measured using the rotational transitions of N2(B, 2–A, 0) in the N2 discharge. In addition, the vibrational distribution of the N2(B, v′ = 0–3) states was measured by means of IR emission spectroscopy. These experiments confirmed that the excitation of these states occurred primarily through electron collisions on the N2(A) metastable molecules.

Role of the wall reactor material on the species density distributions in an N2–O2 post-discharge for plasma sterilization

The species density distributions in a large post-discharge reactor placed downstream from a flowing microwave discharge in N2–O2 are calculated using a three-dimensional hydrodynamic model. The effects of surface losses of N(4S) and O(3P) atoms on the density distributions of different species in the reactor are investigated for three different wall materials: (i) Pyrex, (ii) aluminium and (iii) stainless steel. The effects produced by considering different surface loss probabilities corresponding to each one of these materials, as well as by assuming the production of NO from the wall, are evaluated and discussed. The simulation is conducted for the case of a 65 × 25 × 25 cm3 post-discharge reactor fed from a 2450 MHz discharge, at 2 Torr and 2 × 103 sccm flow rate, in an N2–xO2 mixture composition, with x = 0.5–7%.

Modelling of a post-discharge reactor used for plasma sterilization

A three-dimensional hydrodynamic model is developed to simulate a post-discharge reactor placed downstream from a flowing microwave discharge in N2–O2 used for plasma sterilization. The temperature distribution and the density distributions of NO(B 2Π) molecules and O(3P) atoms, which are known to play a central role in the sterilization process, are obtained in the reactor in the case of discharges at 915 and 2450 MHz, pressure range 1–8 Torr and N2–xO2 mixture composition, with x = 0.2–2%. Excluding the flow direction, sufficiently low temperatures ideal for sterilization have been found in most parts of the reactor. The highest NO(B) and O(3P) concentrations at the reactor entrance are achieved at the highest pressure values investigated here. However, these larger densities rapidly decrease within a few centimetres below the values obtained at lower pressure. On the contrary, at low pressure the density distributions of NO(B) and O(3P) are quasi-homogeneous in most of the horizontal planes. At 8 Torr the densities increase orders of magnitude in the reactor as the gas flow increases from 1 × 103 to 4 × 103 sccm, while at 2 Torr this increase does not reach even one order of magnitude. In agreement with the experiment, the densities of NO(B) and O(3P) have been found to increase at 2 Torr as the O2 percentage increases in the discharge gas mixture, whereas at 8 Torr the density of NO(B) decreases with O2 percentage and the O(3P) density presents only minor changes.

Effect of Vibrational Excitation on the Collisional Removal of Free Radicals by Atoms: OH(v=1) + N

The collisional removal of vibrationally excited OH(upsilon=1) by N(4S) atoms is investigated. The OH radical was prepared by 193 nm photolysis of H2O2, and N(4S) atoms were generated by a microwave discharge in N2 diluted in argon. The concentrations of OH(upsilon=0 and 1) were monitored by laser-induced fluorescence as a function of the time after the photolysis laser pulse. The N(4S) concentration was determined from the OH(upsilon=0) decay rate, using the known rate constant for the OH(upsilon=0) + N(4S) --> H + NO reaction. From comparison of the OH(upsilon=0 and 1) decay rates, the ratio of the rate constant k(upsilon=1)(OH-N) for removal of OH(upsilon=1) in collisions with N(4S) and the corresponding OH(upsilon=0) rate constant, k(upsilon=0)(OH-N) was determined to be 1.61 +/- 0.42, yielding k(upsilon=1)(OH-N) = (7.6 +/- 2.1) x 10(-11) cm3 molecule(-1) s(-1), where the quoted uncertainty (95% confidence limits) includes the uncertainty in k(upsilon=0)(OH-N). Thus, the collisional removal of OH(upsilon=1) by N(4S) atoms is found to be faster than for OH(upsilon=0).

Modelling of a N2–O2 flowing afterglow for plasma sterilization

A kinetic model for a flowing microwave discharge in N2–O2 at ω/(2π) = 2450 and 915 MHz, in the pressure range p = 1–10 Torr, is constructed with the purpose of studying the conditions that maximize the concentrations of NO(B 2Σ) molecules and O(3P) atoms, which are known to play a central role in the sterilization processes. The former are responsible for the emission of UV photons associated with the NOβ bands. The NO(B) concentration is found to pass through a maximum, at approximately 1–3% of O2 added to the mixture, which is in good agreement with the measured maximum of UV emission intensity, and with the shortest time required for the inactivation of spores. For such an O2 percentage, the NO(B) also remains in the afterglow, with only a small reduction, up to a few ∼100 ms. Furthermore, the NO(B) concentration peaks with increasing pressure, with the corresponding maximum shifted to lower O2 percentages, in agreement with the observations of UV intensity. The concentration of O(3P) atoms is practically unchanged along the afterglow, at least up to times as high as 100 ms.

Role of long-lived N2(X1Σg+,v) molecules andN2(A3Σu+) and N2(a'1Σu-) states in the lightemissions of an N2 afterglow

The enhancement of N2(B3Πg) andN2+(B2Σu+) states, responsible by the emission bands forthe 1+ and 1- systems of N2, in the short-lived afterglow of aflowing N2 microwave discharge at ω/(2π) = 433 MHz is investigatedby modelling. It is shown that pronounced maxima for both emissions areobtained downstream from the discharge by assuming the formation ofN2(A3Σu+) and N2(a'1Σu-) states bycollisions of N2(X1Σg+,v) molecules in highly vibrationallevels with N(4S) atoms. Other channels that can also produce the emissionsare investigated as well, but the common factor in all of them is the roleplayed by the vibrationally excited N2(X,v) molecules in levels {>}35.During the relaxation process a pumping-up effect for the vibrational quantaat the higher vth levels occurs as a result of near-resonant V-Venergy-exchange collisions, that produces a rise in the observed peaks for theN2 electronic states in the flowing afterglow.

Kinetics of N2(A 3Σu+) molecules and ionization mechanisms in the afterglow of a flowing N2 microwave discharge

To gain an understanding of the processes responsible for theformation of the well-known short-lived afterglow (SLA) or pink afterglowof nitrogen, different diagnostic techniques are implemented in theafterglow of a 440 Pa microwave nitrogen discharge in a 3.8 cm diameterflow tube. Using the intracavity laser absorption spectroscopy technique,we measure the space-dependent absolute density of N2(A 3Σu+; v = 0) metastable molecules, as well as their rotationaltemperature, which in fact corresponds to the gas temperature, Tg. The density of N2(A 3Σu+) molecules is about5×1017 m-3 at the end of the discharge zone (z = 4 cm). Itthen continuously decays by almost two orders of magnitude to reach aminimum around z = 12 cm before monotonically increasing to a secondarymaximum of 5×1016 m-3 located around z = 19 cm. It thenslowly decays at longer distances. The space-dependentN2(B 3Πg) fluorescence intensity evolves in exactly thesame way. A simple kinetic model is developed and we conclude thatmetastable molecules are locally formed in the SLA and not carried to thisregion by the gas flow. Contributions to the creation ofN2(A 3Σu+) molecules from N-N atom recombination, aswell as from high vibrational levels of the ground-state N2(X, v)molecules are analysed. To account for the high density ofN2(A 3Σu+) molecules, despite the (2-3)×1021 m-3 density of N atoms in the afterglow, we proposethat N(2P) metastable atoms, resulting from the efficient quenching ofN2(A 3Σu+) molecules by N(4S) atoms, reactimmediately with vibrationally excited N2(X 1Σg+;v''⩾10) molecules to again form N2(A 3Σu+) molecules. The absolute electron density in the SLA was alsomeasured by microwave interferometry. Its axial dependence also shows apronounced minimum at z = 12 cm before reaching a maximum of6×1015 m-3 at z = 19 cm. The possible processes for thislocal ionization, in the absence of any electric field, could be binarycollisions of the electronically excited molecules, and/or of thevibrationally excited ground-state molecules in v''⩾30levels.

Relaxation of the electron energy distribution function in the afterglow of a N2 microwave discharge including space-charge field effects.

The relaxation of the electron energy distribution function (EEDF) in the post-discharge of an omega/(2pi)=2.45 GHz microwave discharge in N2 has been investigated by solving the time-dependent Boltzmann equation, including a term taking into account electron losses by diffusion under the presence of a space-charge field. It is shown that although the high-energy tail of the EEDF is rapidly depleted in times of 10(-7) s (p=2 and 10 Torr), the electron density n(e)(t), the electron transport parameters, and the rate coefficients for some processes induced by electron impact, with energy thresholds typically smaller than approximately 2-3 eV, such as, e.g., stepwise excitation of N2(B (3)Pi(g)) and N2(C (3)Pi(u)) states from N2(A (3)Sigma(+)(u)) metastables and excitation of N2(X (1)Sigma(+)(g),v) levels, are only slowly modified in the time interval t approximately 10(-7)-3 x 10(-4) s due to the large characteristic times for ambipolar diffusion. As a result of modifications in n(e)(t), the change from ambipolar to free diffusion regimes occurs abruptly at t approximately 3 x 10(-4) and approximately 10(-3) s for p=2 and 10 Torr, respectively.

Intracavity laser absorption spectroscopy applied to measure theabsolute density and temperature of N2(A3Σ+u) metastablemolecules in a flowing N2 microwave discharge

The first absolute density measurement of N2(A 3Σ+u;v = 0) metastable molecules in the short-lived nitrogenafterglow of a 440 Pa microwave discharge is achieved by using a intracavitylaser absorption spectroscopy technique. The results obtained show that thistechnique is very well suited for the diagnostics of N2(A 3Σ+u) in the nitrogen afterglow. This is a first importantexperimental step with the aim of probing regions where the densities areexpected to be the most significant. The densities measured in the dischargezone and in the short-lived afterglow maximum emission zone are about5×1017 and 6×1016 molecule m-3, respectively.The gas temperature is also deduced both from the Doppler profile of thespectrally resolved rotational lines and from the rotational distribution inthe N2(A 3Σ+u;v = 0) state. These temperatures areabout 1000 K in the discharge zone and 530 K in the short-lived afterglowmaximum emission zone, respectively. The relatively high density of metastablemolecules in the short-lived afterglow maximum emission zone reveals theimportant role played by these species in the short-lived nitrogen afterglowand can give a new insight into the mechanisms involved in the so-callednitrogen short-lived afterglow emission. We also conclude that this highdensity of N2(A 3Σ+u) in the short-lived afterglowresults from an equilibrium between some local production mechanisms anddestruction by reaction with N atoms.

Effect of nitrogen concentration on plasma reactivity and diamond growth in a H2CH4N2 microwave discharge

The effect of nitrogen on the growth of diamond by microwave plasma-assisted chemical vapour deposition, when added in a H 2 CH 4 gas mixture was investigated by micro-Raman spectroscopy and by scanning electron microscopy. A clear improvement in the surface morphology and quality of the diamond films indicates the beneficial effect of adding nitrogen to the gas mixture. The concomitant study of plasma parameters and diamond films properties leads to a better understanding of the effect of nitrogen in the diamond growth process and to the determination of optimal concentrations of nitrogen and methane in the mixture. We report in this paper the effect of varying the concentration of nitrogen introduced in different H 2 CH 4 mixtures. The plasma reactivity is studied by optical emission spectroscopy during diamond film growth, and we focus our experiments on the variation of the emission line of the CH and CN radicals. These species can be considered as key species to explain reactions of the nitrogen in the gaseous phase and its interaction with the surface of the film in growth.

Determination of C-atom density downstream an Ar-CH4 rf plasma torch

Nitrogen, oxygen, and carbon atoms have been detected by optical emission spectroscopy in a common flowing post-discharge of an Ar-x CH4 rf plasma torch and an Ar-2%N2 microwave discharge. Gas temperature as determined from CN and N2 rotational svstems is 3200 ± 200 K in the rf torch and 700 ± 100 K in the post-discharge at a time 3 X10−2 s after the rf plasma torch. From the intensities of the N2 1st positive system, of NO, and of CN violet hands, the atom densities have been determined as 2 X 1016 cm3 for N, 1015 cm3 for O, and 6 X 1012 cm3 for C in the post-discharge at p=650 torr, with 103 of CH4 in the Ar torch.

Dual-frequency N2 and NH3 plasma modification of polyethylene and polyimide

Surfaces of polyethylene and polyimide films were treated in NH3 and N2 plasma using ‘‘dual-frequency’’ excitation: The samples were exposed to a microwave (2.45 GHz) glow discharge, while variable radio frequency (13.56 MHz)—induced negative direct current bias voltage values were simultaneously applied. The surface chemical structure was determined by high-resolution x-ray photoelectron spectroscopy. Up to 40 at. % of nitrogen was incorporated onto the sample surface by exposure to a microwave discharge in N2, while systematically lower N uptake was found in NH3 plasma. Nitrogen was found to be bonded predominantly in amine (C–N) groups by NH3 -type plasma, and in imine groups (C=N) by N2 plasma. Surface ‘‘damage’’ of polyimide, including opening of benzene rings and breaking of carbonyl groups, accompanied by the formation of increasing concentrations of chemically reactive receptor sites, has been demonstrated with increasing the energy of bombarding species.