Vibrational Kinetics Research Articles

Transport properties of high Mach number hypersonic air plasmas

Abstract The collisional and transport properties of hypersonic plasmas have been simulated by an air plasma chemistry model, which includes collisions with electrons, ions and neutral species, elastic scattering, vibrational relaxation and a transport model. Species and thermal fluxes at the surface of the vehicle have been investigated as a function of post-shock gas temperature and species sticking coefficient for a fixed altitude of 50 km. At temperatures below about 5,000 K, the leading species are vibrationally excited nitrogen molecules, while at higher temperatures the molecular nitrogen and atomic oxygen dominate. Large fluxes of oxygen atoms have been observed in the simulations, on the order of 10^20 cm^-2s^ -1, which may lead to high rates of surface oxidation. Another subject of this study is temperature equilibration. For electrons, equilibration takes tens of microseconds, while the vibrational temperature equilibration time vary widely; from a few microseconds to hundreds of microseconds, depending on the gas temperature. Finally, the adiabatic parameter  was calculated for post-shock gas temperatures 5,000 K and 10,000 K and total gas density 1.2×10^17 cm^(-3). For post-shock gas temperature 5,000 K, only a slight reduction of gamma from the ideal gas value of ~1.4 is observed, while for 10,000 K it drops to ~1.26, which has a considerable impact on plasma properties. It was further established that the vibrational kinetics plays a leading role. While a single excited vibrational level of the N2 molecule captures the salient properties of the plasma, a detailed vibration kinetics is required for high-fidelity simulations.

Zero-dimensional simulations of DC ns-pulsed plasma jet in N2 at near atmospheric pressure: validation of the vibrational kinetics

Abstract A Direct Current (DC) nanosecond (ns)-pulsed plasma jet fed with N2 at near atmospheric pressure (20 000 Pa) is studied using a transient zero-dimensional (0-D) model coupled with a two-term Boltzmann equation solver. Good agreement is observed between the simulated and measured plasma properties: including the electron density and the ratios of the N2(v = 1, 2, 3, 4) densities to the gas density. A variety of theoretical approaches are considered to determine the Vibrational-Vibrational (V-V) and Vibrational-Translational (V-T) rate coefficients. For the V-V kinetics, the simple form of an Harmonic Oscillator (sfHO), the Schwartz–Slawsky–Herzfeld (SSH) and the Forced Harmonic Oscillator (FHO) approaches are used. The SSH approach used in this study is an improved version based on the pure SSH approach. For the V-T kinetics, the sfHO, a fit function of the Semi-Classical (ffSC) calculations and a fit function of the Quasi-Classical Trajectory (ffQCT) calculations are used. The influence of these different approaches on the calculated temporal evolution profiles of the vibrational distribution functions (VDFs) within one pulse modulation cycle is revealed. It is observed that the use of these different approaches does not strongly affect the densities of the low vibrational levels (v < 5), whereas larger influences on the higher level densities are found. In addition, it is found that the simulated evolution of the VDFs are sensitive to the probabilities of the neutral wall reaction N2(v) + wall → N2(v − 1) in the range of 1 × 10−2 to 1 × 10−4. A further analysis of the wall quenching probabilities of N2(0 < v < 58) is of importance for a more accurate prediction of the VDFs.

H− production in hydrogen DC glow discharge

The H− ion dynamics in the positive column of H2 DC glow discharge was studied by the laser photodetachment technique in a wide range of pressure, 0.1–3 Torr, and current, 1–30 mA, which cover a range of E/N from ∼40 Td up to ∼170 Td. Using a partial modulation of the discharge current, it is shown that the H−concentration follows H atom dynamics due to a fast detachment reaction with the atoms; the higher the H density, the lower the H–/n e ratio. The dynamics of H atom density during discharge modulation was measured by time-resolved actinometry on Ar atoms, while H2 vibrational temperature was estimated by comparing measured and simulated H2 VUV absorption spectra. The analysis of the experimental dependencies of H− and H/H2 on the discharge parameters allowed estimating the effective rate constant of H− production in the discharge as a function of the reduced electric field. For this discharge model, self-consistent state-to-state vibrational kinetics as well as H2 highly excited electronic states were developed. The main processes that contribute to H− production and loss are discussed in detail. Dissociative attachment to vibrationally excited H2(v) molecules is the main channel of H – production but occurs via the excitation of the well-known low-energy ( ϵ th ≈ 3 eV) shape resonance of H2 −(X2Σu +) only at low E/N. At high E/N, the H– production mostly occurs via the excitation of high-energy H2 − states, such as H2 –(B2Σg +, A2Σg +, C2Πu) and Feshbach resonances similar to H2 −(2Σg +) Rydberg state.

A Perspective on the Investigation of Spectroscopy and Kinetics of Complex Molecular Systems with Semiclassical Approaches.

In this Perspective we show that semiclassical methods provide a rigorous hierarchical way to study the vibrational spectroscopy and kinetics of complex molecular systems. The time averaged approach to spectroscopy and the semiclassical transition state theory for kinetics, which have been first adopted and then further developed in our group, provide accurate quantum results on rigorous physical grounds and can be applied even when dealing with a large number of degrees of freedom. In spectroscopy, the multiple coherent, divide-and-conquer, and adiabatically switched semiclassical approaches have practically permitted overcoming issues related to the convergence of results. In this Perspective we demonstrate the possibility of studying the semiclassical vibrational spectroscopy of a molecule adsorbed on an anatase (101) surface, a system made of 51 atoms. In kinetics, the semiclassical transition state theory is able to account for anharmonicity and the coupling between the reactive and bound modes. Our group has developed this technique for practical applications involving the study of phenomena like kinetic isotope effect, heavy atom tunneling, and elusive conformer lifetimes. Here, we show that our multidimensional anharmonic quantum approach is able to tackle on-the-fly the thermal kinetic rate constant of a 135 degree-of-freedom system. Overall, semiclassical methods open up the possibility to describe at the quantum mechanical level systems characterized by hundreds of degrees of freedom leading to the accurate spectroscopic and kinetic description of biomolecules and complex molecular systems.

Kinetic modelling of non-equilibrium plasma enhanced catalytic ammonia decomposition

It is important for the usage of ammonia as energy carrier to achieve ammonia decomposition at low temperature. In the present study, a reactor model of plasma catalytic ammonia decomposition over Fe catalyst at atmospheric pressure is developed in the aspect of plasma and chemical synergies kinetics. The plasma catalytic model accounted for multiple physical mechanisms in electronic and vibrational plasma kinetics and detailed ammonia decomposition mechanisms on the catalyst surface. In addition, a novel view that identify the surface chemisorption enhancement by plasma attributed to charge transfer via EMSI (electronic metal-support interaction) effect is highlighted. The result shows that the synergetic effect of plasma and thermal-catalyst is much greater than that of plasma or catalyst alone on ammonia decomposition, especially at low temperature. It is revealed that the surface desorption of nitrogen is still the rate-limiting step. Moreover, the numerical results also indicate that Langmuir-Hinshelwood reaction via 2 N(s)=>N2 is more likely to dominate the surface desorption of nitrogen in the presence of plasma rather than Eley-Rideal reaction via N + N(s)=>N2. At low electric field, there are infrequent free radicals (NH2, NH and N) and few vibrational ammonia (NH3(v)) generated by the plasma, thus considering that the adsorbed ammonia only undergoes a three-step dehydrogenation reaction. As a consequence, the rate of surface nitrogen desorption with plasma is greater than that with catalyst alone since plasma promotes the chemisorption process of ammonia. This work provides new fundamental insights into the synergistic effect of plasma and catalyst, as well as the direction for the ammonia decomposition catalyst design in plasma.

Modeling Study of Chemical Kinetics and Vibrational Excitation in a Volumetric DBD in Humid Air at Atmospheric Pressure

A zero-dimensionl model is developed to study the chemical kinetics of a volumetric dielectric barrier discharge (DBD) reactor operating with humid air at atmospheric pressure. This work focuses on the relation between molecular vibrational excitation, the plasma reactor input power and the number densities of several species that are known to play an important role in biomedical applications (e.g. O3,NO,NO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{O}_{3},\ extrm{NO, NO}_{2}$$\\end{document}, ...). A preliminary study is carried out to observe the influence of water molecules on the electron energy distribution function for different values of water concentration and reduced electric field. A simplified approach is then adopted to quantify the contribution of vibrationally-excited O2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{O}_{2}$$\\end{document} molecules to NO\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{NO}$$\\end{document} formation. The results obtained using our detailed model suggest that for the physical conditions considered in this work O2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{O}_{2}$$\\end{document} vibrational kinetics can be neglected without compromising the overall accuracy of the simulation. Finally, a reaction set is coupled with an equivalent circuit model to simulate the E-I characteristic of a typical DBD reactor. Different simulations were carried out considering different values of the average plasma input power densities. A particular focus was given to the influence of the Zeldovich mechanism on O3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{O}_{3}$$\\end{document} and NOX\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{NO}_\ extrm{X}$$\\end{document} production performing simulations where this reaction is not considered. The obtained results are shown and the role of vibrationally excited N2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{N}_{2}$$\\end{document} molecules is discussed. The simulation results indicate also that N2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{N}_{2}$$\\end{document} vibrational excitation, and more precisely the Zeldovich mechanism, has a larger effect on O3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{O}_{3}$$\\end{document} and NOX\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{NO}_\ extrm{X}$$\\end{document} production at intermediate input power levels.

Microcontact printing fabrication of diamond MEMS cantilevers on silicon substrate

Diamond shows great potentials as an ultimate material candidate for application in microelectromechanical systems (MEMS). In this work, we report microcontact printing (μCP) transfer as a facile technique to fabricate diamond MEMS cantilever arrays. We started from the Deryaguin–Landau–Verwey–Overbeek (DLVO) principles and formulated a colloidal “ink” that contains nanodiamonds (NDs) seeds well dispersed in polymethyl methacrylate/acetone. The NDs seeds were then transfer printed onto a silicon (100) substrate by using a flexible polydimethylsiloxane (PDMS) stamp with predefined pattern. The densely packed NDs pattern (∼109 cm−2) enables facile and selected-area growth of diamond with microwave plasma chemical vapor deposition (MPCVD) technique. A diamond film with minimal graphite contaminants and low residual stresses was obtained after growth at 1118 K for 3 h. The diamond cantilevers were released from the substrate by wet chemical etching using KOH (40 % wt.) and isopropyl alcohol (10 % vol.). An examination of the vibrational kinetics indicates that the diamond film has a high Young's modulus (∼795 GPa) and the cantilevers exhibit consistently distributed resonant frequencies and high quality factors in ambient air. The method opens the door for large-scale, cost-effective production of uniform diamond MEMS devices for sensor and quantum applications.

Regression models for calculating state-to-state coefficients of the rate of vibrational energy exchanges

The paper proposes an effective algorithm for solving problems of nonequilibrium gas dynamics taking into account detailed state-to-state vibrational kinetics. One of the problems of traditional methods is their high computational complexity, which requires a lot of time and memory. The work explored the possibilities of using relaxation rate prediction to improve the performance of numerical simulations of nonequilibrium oxygen flows instead of direct calculations. For this purpose, an approach based on nonlinear regression analysis was used, which made it possible to obtain computationally efficient approximation formulas for the energy exchange rate coefficients in the model of a forced harmonic oscillator, taking into account free rotations (FHO-FR), to significantly increase the calculation speed while maintaining accuracy, and to construct an optimized model FHO-FR-reg. Using the obtained regression formulas, numerical modeling was carried out, which made it possible to validate the model for the problem of oxygen flow behind an incident and reflected shock wave. A comparison between the forced harmonic oscillator (FHO) and FHO-FR models is not possible due to the high computational complexity of the second model. With the advent of a common approximation model, it became possible to compare simulation results for these models. Numerical calculations have shown that the FHO-FR-reg model gives values of gas-dynamic parameters close to the FHO model. The developed regression models make it possible to speed up the solution of the problem of modeling oxygen relaxation several times compared to other models of similar accuracy.

Modelling Syngas/Air Laser-Induced Ignition with the State-to-State Vibrational Kinetics Taken into Consideration

Modelling Syngas/Air Laser-Induced Ignition with the State-to-State Vibrational Kinetics Taken into Consideration

Numerical model of a gaseous inductive discharge in oxygen, taking into account the complete scheme of the vibrational kinetics of O2 molecules

In this work, the two-dimensional model of inductively coupled plasma discharge in oxygen was developed. The model includes hydrodynamic equations in drift-diffusion approximation for describing the kinetics of charged (electrons and ions) and neutral plasma particles, Maxwell’s equations for electromagnetic fields, and equations for the temperature and neutral gas flow as well as a detailed scheme of plasma-chemical reactions. The model was tested against theoretical and experimental data in a simple cylindrical chamber. The electron density and temperature distributions were obtained over a wide range of powers deposited in the plasma (100–500 W) and compared with literature data. Also, the complete scheme of the vibrational kinetics of O2 molecules was included in the model. The vibrational distribution function was calculated in low-pressure inductive discharge as well as spatial distributions of plasma parameters (density and temperature of electrons, excited components, neutral gas temperature, etc.) and flows of charged and active neutral particles at the reactor walls.

Vibrational Kinetics of NO and N2 in the Earth's Middle Atmosphere During GLE69 on January 20, 2005

AbstractThe mechanisms of the production of vibrationally excited NO and N2 molecules at the altitudes of the middle atmosphere of the Earth during high‐energetic proton precipitation on 20 January 2005 are considered. The study of vibrational populations N2(X1Σg+,v′ > 0) during high‐energetic proton precipitation has shown different principal mechanisms in the N2(X1Σg+,v′ > 0) excitation. First, the excitation by secondary electrons is principal for vibrational levels v′ = 1−10. Second, it is obtained that intramolecular electron energy transfer process in N2(A3Σu+)+N2 collisions dominates in vibrational excitation of high vibrational levels v′ = 20−30. It is shown that the chemical reaction of metastable atomic nitrogen with molecular oxygen is the main production mechanism of vibrationally excited NO(X2Π,v > 0) and of the radiation of 5.3 and 2.7 μm infrared emissions at these altitudes. The calculated intensities of the 5.3 μm emission are compared with experimental data from SABER instrument on TIMED spacecraft received at the time of the proton precipitation. The role of VV′‐processes in the radiation of 5.3 μm infrared emission is discussed.

Oxygen atom and ozone kinetics in the afterglow of a pulse-modulated DC discharge in pure O2: an experimental and modelling study of surface mechanisms and ozone vibrational kinetics

The chemical kinetics of oxygen atoms and ozone molecules were investigated in a fully-modulated DC discharge in pure oxygen gas in a borosilicate glass tube, using cavity ringdown spectroscopy (CRDS) of the optically forbidden O(3P2)→O(1D2) absorption at 630 nm. Measurements were made over a range of tube temperatures (10 °C and 50 °C) gas pressures (0.5–4 Torr) and discharge current (10–40 mA). The discharge current was square-wave modulated (on for 0.2 s and off for 1 s), allowing the build-up to steady-state and the decay in the afterglow to be studied. This paper focusses on the afterglow period. The O atom density decays non-exponentially in the afterglow, indicating a surface loss probability dependent on incident active particle fluxes. The oxygen atom absorption peak lies on a time-varying absorption continuum due (in the afterglow) to the Chappuis bands of ozone. The ozone density passes through a maximum a few 100 ms into the afterglow, then decays slowly. An existing time-resolved self-consistent 1D radial model of O2 positive column discharges was modified to interpret the new results. The ozone behaviour in the afterglow can only be modelled by the inclusion of: (1) surface production of O3 from the reaction of O2 molecules with adsorbed O atoms, (2) reactions of vibrationally-excited ozone with O atoms and with O2(a1Δg) molecules, and (3) surface loss of ozone with a probability of around 10−5.

State-specific slip boundary conditions in non-equilibrium gas flows: Theoretical models and their assessment

In this study, we develop and assess a new approach to modeling slip boundary conditions in gas mixtures with coupled state-to-state vibrational-chemical kinetics and surface physical and chemical processes: adsorption, desorption, vibrational energy transitions, and chemical reactions. Expressions for velocity slip, temperature jump, and mass fluxes of species are derived on the basis of the advanced kinetic boundary condition taking into account gain and loss of particles in surface processes; theoretical expressions for the mass fluxes obtained in the frame of various approaches are compared. The developed model is implemented to the fluid-dynamic solver for modeling dynamics and state-to-state air kinetics in the boundary layer near stagnation point. Several test cases corresponding to a various degree of gas rarefaction are considered. Recombination probabilities and effective reaction rates are calculated and compared to recent molecular-dynamic simulations; the proposed model yields the best agreement for the recombination rate coefficient. It is shown that temperature jump significantly affects fluid-dynamic parameters and surface heat flux; the role of heterogeneous reactions on the silica surface is weaker. In the surface heating, there is a competition between these two effects: whereas the temperature jump reduces the wall heat flux, surface reactions cause its increase, but to a lesser extent. It is concluded that the model proposed in this study describes self-consistently detailed vibrational kinetics, rarefaction effects, and surface reactions and can be applied both in continuum and slip flow regimes.

Validation of non-equilibrium kinetics in CO2–N2 plasmas

This work explores the effect of N2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent kinetic model, previously validated for pure CO2 and CO2–O2 discharges, is further extended by adding the kinetics of N2. The vibrational kinetics considered include levels up to v = 10 for CO, v= 59 for N2 and up to v 1= 2 and v 2= v 3= 5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and account for electron-impact excitation and de-excitation (e–V), vibration-to-translation (V–T) and vibration-to-vibration energy exchange (V–V) processes. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge operating in pure CO2 and in CO2–N2 mixtures, at pressures in the range 0.6–4 Torr (80.00–533.33 Pa) and a current of 50 mA. The experimental results show a higher vibrational temperature of the different modes of CO2 and CO and an increased dissociation fraction of CO2, that can reach values as high as 70%, when N2 is added to the plasma. On the one hand, the simulations suggest that the former effect is the result of the CO2–N2 and CO–N2 V–V transfers and the reduction of quenching due to the decrease of atomic oxygen concentration; on the other hand, the dilution of CO2 and dissociation products, CO and O2, reduces the importance of back reactions and contributes to the higher CO2 dissociation fraction with increased N2 content in the mixture, while the N2(B3Πg) electronically excited state further enhances the CO2 dissociation.

Low temperature state-to-state vibrational kinetics of O + N2(v) and N + O2(v) collisions

Starting from an equilibrium initial condition at 3000 K, the non-equilibrium vibrational kinetics of a 500 K atmospheric pressure N2/N/O2/O/NO mixture is investigated with a zero-dimensional time-dependent numerical solver. Attention is devoted to the O + N2(v) and N + O2(v) collisions, thanks to new QCT state-to-state rates from the literature. The Zeldovich and the dissociation–recombination reactions are considered; for the N + O2(v) collisions the inelastic vT processes are considered too. Interpolation formulae of the corresponding rates are proposed.The mixture behaviour depends on the combined effects of the different processes, however under the conditions adopted the forward second Zeldovich reaction switches on the kinetics towards a new equilibrium.The aim of the present study is to open the door to more complex applications of plasmas in the fields of the plasma medicine, the fertilizers production and, more in general, of the nitrogen fixation.

Influence of Filler Concentration on the Mechanical Properties of a Polymer Composite

The results of a study of carbon fiber-filled samples with an epoxy amine-cured matrix are presented. Independent quasi-periodic oscillations of the density and Young's modulus were recorded for the first time, against the backdrop of a monotonic increase in the strength of the samples. The probable causes of the appearance of the vibrational kinetics of the physico-mechanical parameters of the composite are determined depending on the content of the filler in the reaction volume of constant magnitude.

Investigation of nitrogen fixation in low-pressure microwave plasma via rotational–vibrational NO and N2 kinetics

A pulsed microwave surfaguide-type discharge used for nitrogen fixation in N2–O2 gas mixtures is characterized by optical emission spectroscopy. Results show that both rotational and vibrational temperatures are elevated in the active zone near the waveguide, decaying along the discharge tube in both upstream and downstream. The characteristic length of optical emission from NO(A-X) transition gets contracted when pressure increases, specifically at P≥2 Torr. The degree of vibrational non-equilibrium (defined as the ratio between vibrational and rotational temperatures) is decreased by a factor of two when pressure changes from 0.6 to 10 Torr. Non-equilibrium likely disappears as the discharge pressure rises, resulting in a gas temperature elevation. A correlation between gas residence time, pulse duration, and characteristic times for different energy transfer channels is discussed. The rotational–vibrational dynamics differs for NO and N2 during the pulse. Both species lose vibrational excitation at the beginning of the pulse, whereas N2 gets re-excited again during the second half of the pulse, which may occur as a result of an efficient pumping-up effect through the vibrational–vibrational energy transfer. At the same time, vibrational relaxation of NO takes place primarily due to a strong vibrational–translational exchange via NO–O2 and NO–O collisions.

Study of vibrational kinetics of CO2 and CO in CO2–O2 plasmas under non-equilibrium conditions

This work explores the effect of O2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent model, previously validated for pure CO2 discharges, is further extended by adding the vibrational kinetics of CO, including electron impact excitation and de-excitation (e-V), vibration-to-translation relaxation (V-T) and vibration-to-vibration energy exchange (V-V) processes. The vibrational kinetics considered include levels up to v = 10 for CO and up to v 1 = 2 and v 2 = v 3 = 5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and accounts for e-V, V-T in collisions between CO, CO2 and O2 molecules and O atoms and V-V processes involving all possible transfers involving CO2 and CO molecules. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge ignited in pure CO2 and CO2–O2, operating at pressures in the range 0.4–5 Torr (53.33–666.66 Pa). The experimental results show a lower vibrational temperature of the different modes of CO2 and a decreased dissociation fraction of CO2 when O2 is added to the plasma but an increase of the vibrational temperature of CO. On the one hand, the simulations suggest that the former effect is the result of the stronger V-T energy-transfer collisions with O atoms which leads to an increase of the relaxation of the CO2 vibrational modes. On the other hand, two main mechanisms contribute to the lower CO2 dissociation fraction with increased O2 content in the mixture: the back reaction, CO(a3Πr) + O2 → CO2 + O and the recombinative detachment O− + CO → e + CO2.

Vibrational kinetics in repetitively pulsed atmospheric pressure nitrogen discharges: average-power-dependent switching behaviour

Characterisation of the vibrational kinetics in nitrogen-based plasmas at atmospheric pressure is crucial for understanding the wider plasma chemistry, which is important for a variety of biomedical, agricultural and chemical processing applications. In this study, a 0-dimensional plasma chemical-kinetics model has been used to investigate vibrational kinetics in repetitively pulsed, atmospheric pressure plasmas operating in pure nitrogen, under application-relevant conditions (average plasma powers of 0.23–4.50 W, frequencies of 1–10 kHz, and peak pulse powers of 23–450 W). Simulations predict that vibrationally excited state production is dominated by electron-impact processes at lower average plasma powers. When the average plasma power increases beyond a certain limit, due to increased pulse frequency or peak pulse power, there is a switch in behaviour, and production of vibrationally excited states becomes dominated by vibrational energy transfer processes (vibration–vibration (V–V) and vibration–translation (V–T) reactions). At this point, the population of vibrational levels up to increases significantly, as a result of V–V reactions causing vibrational up-pumping. At average plasma powers close to where the switching behaviour occurs, there is potential to control the energy efficiency of vibrational state production, as small increases in energy deposition result in large increases in vibrational state densities. Subsequent pathways analysis reveals that energy in the vibrational states can also influence the wider reaction chemistry through vibrational–electronic (V–E) linking reactions (N + N N + N and N + N N + N), which result in increased Penning ionisation and an increased average electron density. Overall, this study investigates the potential for delineating the processes by which electronically and vibrationally excited species are produced in nitrogen plasmas. Therefore, potential routes by which nitrogen-containing plasma sources could be tailored, both in terms of chemical composition and energy efficiency, are highlighted.

Influence of oxygen on the ro-vibrational kinetics of a non-equilibrium discharge in CO2–O2 mixtures

Storing excess renewable energy in hydrocarbons produced from CO2 potentially solves the intermittency issue of renewable energy sources in a green manner. The required reduction of CO2 to CO can be efficiently accomplished with non-equilibrium plasma conversion. On an industrial scale, effects of impurities on the reduction must be taken into account. During this study, the effects of oxygen are considered, as the impurity O2 is both a product of the reduction reaction and abundant in air. In this paper, the influence of O2 addition on the ro-vibrational kinetics of a pulsed DC CO2 glow discharge at 2.5–6.0 Torr—serving as a model non-equilibrium system—is studied in situ with quantum cascade laser infrared absorption spectroscopy. The temporal evolution of the ro-vibrational temperatures is measured, as well as the conversion of CO2 to CO. Trends in the temperature evolutions when increasing the flow rate from 7.4 to 30.0 sccm, varying the pressure, and increasing the O2 admixture up to 90% in increments of 10% are utilised to determine the underlying kinetic processes. Our results show that any decrease in the conversion of CO2 to CO caused by increasing O2 addition cannot be attributed to an induced change in the vibrational kinetics, since the asymmetric stretch mode of CO2—which is associated with dissociation via vibrational excitation—is not quenched. Measured changes in the temporal temperature trends are explained by species-dependent intra- and intermolecular collisional energy transfer processes.