C3Πu State Research Articles

Streamer-induced kinetics of excited states in pure N2: II. Formation of N2(B 3Πg,v=0–21 ) through analysis of emission produced by the first positive system

Vibrational distributions of electronically excited states of N2 obtained through dipole-allowed radiative transitions provide an important tool to study the kinetics of non-equilibrium plasmas under various discharge conditions. In this work, we report, for the first time, streamer-induced visible/near-infra-red emission spectra developing during the first hundred nanoseconds after the initiation of the discharge. Emission through the first positive system of N2 was acquired in 500–1100 nm range, which allows a complete analysis of the N2(B 3Πg,v = 0–21) vibrational manifold. The investigated evolution of the vibrational distribution of the N2(B 3Πg , v = 0–21) state at the centre of the gap corresponds to the transition of the streamer head and the subsequent decay of the streamer channel. We show that the vibrational distribution characterising streamer head is determined by Franck–Condon factors, while during streamer relaxation, it is influenced by the complex interaction between triplet excited states of N2. Additionally, the observed N2(B 3Πg , v = 13–21) vibrational levels are likely produced by the interaction of high vibrational levels of N2(W 3Δu , B’ 3Σu− , B 3Πg) with N2(C3Πu) state. We also provide a detailed kinetic scheme for modelling vibrationally-resolved N2(B 3Πg ) state and compare model results with experimental outcomes.

Analysis of nonequilibrium atomic and molecular nitrogen radiation in pure N2 shockwaves

This paper presents an analysis of data previously reported from a test series in the Electric Arc Shock Tube Facility for incident shocks composed of pure nitrogen. The present work focuses on the lowest velocity measured in the test (approximately 7 km/s) where both molecular and atomic radiation are significant. Analysis of the spectral data with the NEQAIR radiative transport code obtains post-shock trends in temperature and number densities of the states of N2 and atomic N. The results suggest that chemical, and possibly thermal, equilibrium has not been obtained within the distance/time measured (about 5 cm/80 µs). In the nonequilibrium region of the shock, differences between rotational temperatures of different species are observed, as well as differences between rotational and vibrational temperatures. Population of rotational states above the dissociation limit is observed to occur over a distance of several cm. Finally, these data suggest that predissociation strongly affects the C3Πu state of N2, leading to a lower measured radiation than predicted. An effective average pre-dissociation rate is shown to produce reasonable agreement under quasi-steady state approximation.

Spatial distribution of the fluorescence induced by femtosecond laser filamentation in ambient air

Laser intensity has a great influence on the excitation, ionization and other dynamical processes of atoms and molecules. During the femtosecond laser filamentation, the different values of intensity along the propagation direction will have a great influence on the spatial distribution of the products generated by these processes, and that of the fluorescence emitted by these products. In this paper, by measuring the spatial distribution of fluorescence induced by femtosecond laser filament in air, we find that along the propagation direction, the fluorescence signal from N2 appears in front of that from N2+. Based on the experimental observations, we discuss the mechanism for the formation of N2 (C3Πu), and arrive at the conclusion that in the case of shorter focal length, the intersystem crossing process plays a main part in the formation of the C3Πu state of N2.

Emission of Molecular Nitrogen upon Electron Bombardment of Pyrolytic Aerogel SiO2 and Aluminum

It is found that the emission intensity of the 2+ band system of the N2 molecule (C3Πu → B3Πg transition) under electron bombardment of pyrolytic aerogel SiO2 in air at atmospheric pressure is 40–70% higher than that in the case of bombardment of aluminum under the same conditions. The dependences of the emission intensity at 337 nm (0-0 vibrational transition in the 2+ band system of the N2 molecule) on the target temperature in the range of 30–60°C are different for the aluminum and SiO2 targets, which indicates different excitation mechanisms of the N2(C3Πu) state. It is suggested that when the SiO2 target is bombarded, the N2(C3Πu) state is excited not only by direct collisions with the electrons of the primary beam and secondary electrons, but also by the energy transfer from SiO2 excitons and conduction electrons to N2 molecules upon their contact with the SiO2 surface.

Динамика излучения наносекундного поверхностного скользящего разряда в потоке с ударной волной

The spatio-temporal distribution of the radiation of the surface sliding discharge with a duration of ~ 300 ns in stationary air at pressures of 2–200 Torr and under the interaction with plane shock waves at Mach numbers 2.8-3.3 was experimentally studied. The dynamics of the discharge radiation based on the processing of streak images and 9-frame images of the glow, the spectra of the radiation, and the discharge current are analyzed. It is shown that the dependence of the radiation intensity of the discharge interacting with the shock wave correlates with the simulated time dependence of the population of the C3Πu state of nitrogen under shock compression of the plasma region.

Positive streamers in air of varying density: experiments on the scaling of the excitation density

Streamers are rapidly extending ionized finger-like structures that dominate the initial breakdown of large gas volumes in the presence of a sufficiently strong electric field. Their macroscopic parameters are described by simple scaling relations, where the densities of electrons and of excited molecules in the active streamer front scale as the square of the density of the neutral gas. In this work we estimate the absolute density of nitrogen molecules, excited to the C3Πu state that emit photons in the 2P–N2 band, by radiometrically calibrated short exposure intensified imaging. We test several pressures (100, 200 and 400 mbar) in artificial air at room temperature. Our results provide a first confirmation for the scaling of the density of excited species with the gas density. The method proposed here is particularly suitable to characterize the excitation densities in sprite streamers in the atmosphere.

The density profiles of N2([formula omitted]) and N2([formula omitted]) in the Pink Afterglow of the DC nitrogen flowing discharge

We have studied by optical emission spectroscopy (OES) the afterglow of a nitrogen DC flowing discharge in such experimental conditions that the Pink Afterglow (PA) was present in the post-discharge. The spectra were recorded in the range of 360–435nm. The emissions of the second positive system of the nitrogen molecules concerning the transitions N2(C3Πu, v)→N2(B3Πg, v′) with Δv=−2 and the emissions of the first negative system concerning the transitions N2+(B2Σu+,v)→N2+(X2Σg+,v′) with Δv=−1 were utilized to furnish the relative density of the N2(C3Πu, 0⩽v⩽4) and the N2+(B2Σu+,0⩽v⩽6) populations from that we have estimated the total density of the N2(C3Πu) and N2+(B2Σu+) electronic states. A kinetic numerical model was constructed for modeling the nitrogen post-discharge. The density profiles of the electronic states N2(C3Πu) and N2+(B2Σu+) obtained experimentally were fitted by the calculated density profiles furnished by the model. In this sense, we have calibrated the numerical model that will be employed in the calculation of the coefficients applied in the estimation of the N2(A3Σu+) and N2(a′Σu-) density profiles. We have developed a method of combined application of the OES and numerical modeling which permits the estimation of the N2(A3Σu+) and N2(a′Σu-) density profiles from the measured density of the N2(C3Πu) state corrected by the calculated coefficients. The estimated density profiles are compared to the calculated ones generated by the kinetic model. The N2(A3Σu+) densities along the afterglow obtained by our method are compared to those ones measured by intracavity laser absorption spectroscopy (ICLAS) (Sadeghi et al., 2001 [14]). The N2(a′Σu-) density profile in the PA, based on experimental data, is estimated for the first time here. The N2(A3Σu+) density found in the maximum of the Pink Afterglow is 2.0×1011cm−3 and the density of the N2(a′Σu-) state is 1.3×1011cm−3. The discharge operated at 30mA electric current, 500Pa gas pressure and 500sccm flow rate.

Amplified spontaneous C3Πu–B3Πg emission and rotational and vibrational state distributions in C3Πu state of N2 in femtosecond laser induced filament in air

A femtosecond laser-induced filament in air was investigated by detecting the C3Πu–B3Πg (0,0) fluorescence of N2. The intensity of the backward fluorescence increased exponentially as a function of the filament length, showing the amplification of the spontaneous emission. The vibrational and rotational temperatures of N2 in the C state determined by spectroscopic analyses were found to take respectively almost the same values of 2800(200) and 450(100)K in the wide laser intensity range between 0.5 and 6mJ/pulse, which can be regarded as evidence of the clamping of the laser field intensity in the filament.

Vibrational distribution of nitrogen molecules in the C3Πu state in a near-surface microwave plasma in nitrogen at pressures of 1–5 Torr

The radiation of the second positive nitrogen system has been used to study the spatial dependence of the vibrational distribution of nitrogen molecules in the C3Πu state in the near-surface plasma layer of an electrode microwave discharge in nitrogen at pressures of 1–5 Torr. It has been shown that the vibrational distribution changes at a scale of 100 μm. It has been concluded that this state is populated owing to the electron impact from the ground state. The possibility of using the local approximation for the electron energy distribution function to explain the experimental results has been analyzed.

Effect of the dc field on the near-surface plasma of a highly nonuniform microwave discharge

The effect of the dc electric field on the near-surface plasma of an electrode microwave discharge at pressures of 1–5 Torr was studied by the emission spectroscopy method. It is shown that the dc field weakly affects the vibrational distribution of nitrogen molecules in the C3Πu state, but changes the structure of the near-surface plasma (shifting the intensity maxima of the emission bands) and the strength of the microwave field near the electrode surface. It is also found that the ratio between the intensities of bands of different sequences of the second positive system of nitrogen radiated from the same state depends on the position along the discharge axis.

The Influence of Small Hydrogen Admixtures up to 5 % to a Low Pressure Nonuniform Microwave Discharge in Nitrogen

We present the results of spectroscopic and 2D modeling study of the influence of hydrogen addition to non-uniform nitrogen plasma of electrode microwave discharge. The axial intensity distributions of the Hα line and N2 (2+: C3Πu → B3Πg, 1+: B3Πg → A3Σ u + ) and N2 + (1−: B2Σ u + → X2Σ g + ) molecular bands are recorded for different incident microwave power input and hydrogen content. The corona model is used to determine electric field strength in nitrogen discharge from the intensity ratio of 2+ and 1− system of nitrogen bands. By means of 2D modeling spatial distributions of nitrogen molecules in C3Πu state, microwave field strength, electron density, concentrations of N2 +, N4 + are determined in nitrogen and in nitrogen–hydrogen mixtures. The concentration of N2H+ ions in nitrogen–hydrogen mixtures is determined also. The general conclusion of 2D modeling is in agreement with experimental results and shows that the influence of hydrogen addition to discharge is related to the fast conversion reactions of nitrogen ions (N2 +, N4 +) to N2H+ ion. These ion conversions lead to the change of ion plasma components transport properties, to the modification of microwave field strength in plasma, and consequently, to the alternation of all plasma parameters.

Optical emission characteristics of medium- to high-pressure N2 dielectric barrier discharge plasmas during surface modification of polymers

The authors measured the band spectra (first and second positive systems) of the nitrogen molecule by optical emission spectroscopy with an aim to understand the mechanism of surface processing by medium- to high-pressure dielectric barrier discharge (DBD) plasmas. The experimentally measured and calculated spectra were compared to determine the vibrational and rotational temperatures of the N2 (C3Πu) state in the generated plasmas. The authors generated the N2 DBD plasmas at a driving frequency of 1–7 kHz and a discharge pressure of 20–105 Pa for the surface modification of a polyethylene terephthalate (PET) sample. It was found that the vibrational temperature was greatly affected by the N2 pressure while the rotational temperature remained constant in the N2 pressure range of 20–105 Pa. The emission intensity of N2 first positive system (B3Π → A3Σ) rapidly decreased at an increasing N2 pressure due to the collisional relaxation process of the B3Π state with N2 molecules. The N2+(B2Σu+→X2Σg+) radiative transition was observed in the low-pressure DBD plasmas, which was attributed to the direct electron impact ionization of N2 molecules. The surface characterizations of treated PET samples by contact angle measurement and atomic force microscopy indicate that the low-pressure N2 DBD plasma is an effective method for the surface modification of polymers. Analysis indicates the plasma characteristics such as electron temperature and ion energy are mainly dependent on the N2 pressure, which turn to determine the surface properties of treated PET samples.

Kinetic processes initiated by a nanosecond high-current discharge in hot air

Results from numerical investigations of kinetic processes initiated by a pulsed nanosecond discharge in hot (T0 ≥ 1000 K) air at atmospheric pressure are presented. The calculated results on the dynamics of the electron density, the population of the N2(B3Πg) and N2(C3Πu) states, and the atomic oxygen density in the axial discharge region agree with experiment. The method for determining the gas temperature by measuring the rotational structure of the transitions N2(C3Πu, ν) → N2(B3Πg, ν′) of the 2+ nitrogen system is analyzed. It is shown that, in relatively weak reduced electric fields typical of secondary discharge pulses, the electron impact excitation of the N2(C3Πu) state from the ground state N2(X1Σg+) can be accompanied by its additional step population from the N2(B3Πg), N2(a′Σu−), and other electronic states. This effect substantially influences the rotational distribution of nitrogen molecules in the N2(C3Πu, ν) state; moreover, the temperature determined from this distribution can be substantially higher than the true gas temperature.

Spatiotemporal Distribution of Nitrogen Rotational Temperature during Pulsed Discharge in Air

We have investigated the gas temperature during pulsed discharge. The gas temperature is one of the most important parameters for application of pulsed discharge in atmospheric gases. At atmospheric pressure gases, the gas temperature can be assumed equal to its rotational temperature. In the present work, the rotational temperature of the nitrogen molecules in a coaxial discharge reactor was obtained by synthesizing measured and calculated spectra. The emission spectrum was obtained from the second positive system of the nitrogen molecule due to the transition from the C3Πu state to the B3Πg state. As the result, rapidly increasing of the temperature (approximately 60 K) was observed at the boundary between the streamer and the glow-like discharge phases. In this paper, the spatial and the temporal dependences of the rotational temperature during pulsed discharge are reported.

Spatiotemporal Distribution of Nitrogen Rotational Temperature during Pulsed Discharge in Air

We have investigated the gas temperature during pulsed discharge. The gas temperature is one of the most important parameters for application of pulsed discharge in atmospheric gases. At atmospheric pressure gases, the gas temperature can be assumed equal to its rotational temperature. In the present work, the rotational temperature of the nitrogen molecules in a coaxial discharge reactor was obtained by synthesizing measured and calculated spectra. The emission spectrum was obtained from the second positive system of the nitrogen molecule due to the transition from the C3Πu state to the B3Πg state. As the result, rapidly increasing of the temperature (approximately 60 K) was observed at the boundary between the streamer and the glow-like discharge phases. In this paper, the spatial and the temporal dependences of the rotational temperature during pulsed discharge are reported.

Nonuniform microwave discharge in a nitrogen-hydrogen mixture

The method of optical emission spectroscopy is used to investigate the influence of additions of hydrogen on the emission characteristics of a highly nonuniform electrode microwave discharge in nitrogen at a pressure of 1 torr. It is demonstrated that the pattern of the influence made by hydrogen addition (zero to 50% with respect to flow rate of hydrogen) on the intensity of emission bands of nitrogen is different in different parts of the discharge. It is further observed that the influence of hydrogen on the vibrational distribution of nitrogen molecules in the C3Πu state is different C3?uin different parts of the discharge. Analysis is made of various processes involving hydrogen, which affect the emission of nitrogen bands. An inference is made that ion conversion is an important mechanism of such influence. One-dimensional simulation is performed in view of these processes, and it is demonstrated that the experimentally observed effects may be associated with this mechanism.

Microwave electrode discharge in nitrogen: Structure and characteristics of the electrode region

The parameters of the electrode region of an electrode microwave discharge in nitrogen are studied by emission spectroscopy. The radial and axial distributions of the intensities of the bands of the second (N2(C3Πu → B3Πg)) and first (N2(B3Πg → A3Σu+)) positive systems of molecular nitrogen and the first negative system of nitrogen ions (N2+ (B2Σu+ → X2Σg+)), the radial profiles of the electric field E and the electron density Ne, and the absolute populations of the vibrational levels vC = 0–4 of the C3Πu excited state of N2 and the vibrational level vBi = 0 of the B2Σu+ excited state of a molecular nitrogen ion are determined. The population temperature of the first vibrational level TV of the ground electronic state X1Σg+ of N2 and the excitation temperature TC of the C3Πu state in the electrode region of the discharge are measured. The radius of the spherical region and the spatially integrated plasma emission spectra are studied as functions of the incident microwave power and gas pressure. A method for determining the electron density and the microwave field strength from the plasma emission characteristics is described in detail.

Diagnostics of a nonequilibrium nitrogen plasma from the emission spectra of the second positive system of N2

A method is proposed for determining the electron density Ne and the electric field E in the non-equilibrium nitrogen plasma of a low-pressure discharge from the spectra of the second positive system of N2. The method is based on measuring the specific energy deposition in the plasma and the distribution of nitrogen molecules over the vibrational levels of the C3Πu state, as well as on modeling this distribution for a given energy deposition. The fitting parameters of the model are the values of Ne and E. A kinetic model of the processes governing the steady-state density of the C3Πu nitrogen molecules is developed. The testing of this method showed it to be quite reliable. The method is of particular interest for diagnosing electrodeless discharges and provides detailed information on the processes occurring in the discharge plasma. Preliminary data are obtained on the plasma parameters in a cavity microwave discharge and an electrode microwave discharge. In particular, it is found that the electric field in an electrode microwave discharge in nitrogen is lower than that in a hydrogen discharge. This effect is shown to be produced by stepwise and associative processes with the participation of excited particles in nitrogen.

Gas kinetic studies using a table-top set-up with electron beam excitation: quenching of molecular nitrogen emission by water vapour

A table-top set-up for studying gas kinetic processes in dense gases via time resolved optical spectroscopy is presented. The set-up uses low energy electron beams for gas excitation. A thin silicon nitride membrane separating the electron source and the gas cell allows to use electron energies as low as 10–15 keV. The electron source is operated in a fast pulsing mode with a pulse width of 5 ns and repetition rates up to 30 kHz. Light emission from the target gas sample is studied and time resolved photon counting is used to measure decay times and collisional rate constants for specific excited states. The experimental concept is applied to measure the rate constants for quenching of molecular nitrogen in the C 3Πu state (vibrational levels 0 and 1) by water vapour. Quenching rate constants of (7.1 ±0.7) × 10-10 s-1 cm3 and (6.7 ±0.7) × 10-10 s-1 cm3 were obtained for the C 3Πu (v = 0) and C 3Πu (v = 1) vibrational levels, respectively.

Optical diagnostics of the hollow needle to plate electrical discharge in air at atmospheric pressure

Vibrational and rotational excitation of the N2(C3Πu) state in a DC hollow needle to plate electrical discharge at atmospheric pressure was studied by means of multichannel emission spectrometry. Vibrational distributions and rotational temperatures of the N2(C3Πu) electronic state were obtained for a negative polarity of the needle as a function of mean energy density dissipated in the discharge reactor. Vibrational excitation of the N2(C3Πu) apparently decreases with increasing energy density and the shape of vibrational distributions reflects predominant processes responsible for the N2(C3Πu) state population, i.e. electron impact excitation of N2(X1Σg+,ν) ground state and energy pooling mechanism by N2(A3Σu+) metastables. Rotational temperatures evolve in the range 1000 ÷ 1800 K and show linear increase with energy density dissipated in the discharge reactor.