Associative Ionization Reaction Research Articles

Zero-dimensional analysis of the effect of water vapor on reducing electrons in the plasma sheath

In this study, an air-water vapor ionization reaction model is developed within the quantum-kinetic (Q–K) model of the direct simulation Monte Carlo method to investigate the detailed mechanism of how water vapor reduces electrons. The zero-dimensional simulations of a typical non-equilibrium flow field downstream of a normal shock are designed, where the electron number density decreases by two orders of magnitude due to water vapor. We conclude that the introduction of water vapor reduces the mole fractions of oxygen atoms and nitrogen atoms through five pairs of reactions and enhances the reverse nitric oxide associative ionization reaction, leading to electron consumption. The phenomena and corresponding mechanisms under varying mole fractions of water vapor, air temperatures, and water vapor temperatures are investigated. Based on the mechanisms, we propose that the addition of hydrogen ions could improve the water's mitigation effect, which is then proven to be able to reduce the electron number density by another two orders of magnitude, not only at high air temperatures but also at lower air temperatures or lower mass injection rates.

Sensitivity Analysis of Ionization in Two-Temperature Models of Hypersonic Air Flows

Plasma generation in hypersonic flows is analyzed using a two-temperature model of nonequilibrium air. The uncertainties in electron number density predictions are assessed for flow scenarios that correspond to both strongly shocked and strongly expanded flows, and the dependencies of the calculated uncertainties on individual input parameters are quantified. Ionization levels behind 5 and 7 km/s normal shocks are found to be most sensitive to the associative ionization reactions producing O2+ and NO+ in the region of peak electron number density, with nitric oxide kinetics dominating the uncertainty downstream. The higher levels of ionization behind a 9 km/s shock are found to strongly depend on the electron impact ionization of atomic nitrogen as well as the charge exchange between N2+ and N. Recombining flow scenarios depend on many of the same processes that influence the shocked flows, with the notable addition of the reassociation reaction O++N2↔NO++N, which is responsible for large uncertainties in electron number density in net recombining flows. The results provide valuable insight into the typical magnitude of uncertainty associated with plasma formation predictions in hypersonic flows and identify the parameters that should be targeted in efforts to reduce those uncertainties.

The associative ionization of N(2P) + O(3P).

The rate constant of the associative ionization reaction N(2P) + O(3P) → NO+ + e- was measured using a flow tube apparatus. A flowing afterglow source was used to produce an ion/electron plasma containing a mixture of ions, including N2+, N3+, and N4+. Dissociative recombination of these species produced a population of nitrogen atoms, including N(2P). Charged species were rejected from the flow tube using an electrostatic grid, subsequent to which oxygen atoms were introduced, produced either using a discharge of helium and oxygen or via the titration of nitrogen atoms with NO. Only the title reaction can produce the NO+ observed after the introduction of O atoms. The resulting rate constant (8 ± 5 ×10-11cm3s-1) is larger than previously reported N(2P) + O disappearance rate constants (∼2 × 10-11cm3s-1). The possible errors in this or previous experiments are discussed. It is concluded that the N(2P) + O(3P) reaction proceeds almost entirely by associative ionization, with quenching to the 2D or 4S states as only minor processes.

A Simple High-Flux Switchable VUV Lamp Based on an Electrodeless Fluorescent Lamp for SPI/PAI Mass Spectrometry.

Single-photon ionization (SPI) is a unique soft ionization technique for organic analysis. A convenient high-flux vacuum ultraviolet (VUV) light source is a key precondition for wide application of SPI techniques. In this study, we present a novel VUV lamp by simply modifying an ordinary electrodeless fluorescent lamp. By replacing the glass bulb with a stainless steel bulb and introducing 5% Kr/He (v/v) as the excitation gas, an excellent VUV photon flux over 4.0 × 1014 photons s-1 was obtained. Due to its rapid glow characteristics, the VUV lamp can be switched on and off instantly as required by detection, ensuring the stability and service life of the lamp. To demonstrate the performance of the new lamp, the switchable VUV lamp was coupled with an SPI-mass spectrometer, which could be changed to photoinduced associative ionization (PAI) mode by doping gaseous CH2Cl2 to initiate an associative ionization reaction. Two types of volatile organic compounds sensitive to SPI and PAI, typically benzene series and oxygenated organics, respectively, were selected as samples. The instrument exhibited a high detection sensitivity for the tested compounds. With a measurement time of 11 s, the 3σ limits of detection ranged from 0.33 to 0.75 pptv in SPI mode and from 0.03 to 0.12 pptv in PAI mode. This study provides an extremely simple method to assemble a VUV lamp with many merits, e.g., portability, robustness, durability, low cost, and high flux. The VUV lamp may contribute to the development of SPI-related highly sensitive detection technologies.

Electronic Spectroscopy of SmO in the 645 to 670 nm Range.

The associative ionization reaction Sm + O → SmO+ + e- is being investigated as an electron source that could transiently modify high-altitude electron densities via Sm vapor release. Electronic spectra have been obtained from tests where sounding rockets released Sm vapor, but the interpretation of these results has been hampered by the limited laboratory spectral data available for both SmO and SmO+. The present study extends the spectroscopic characterization of SmO in the 645-670 nm range, where the field data show the most prominent molecular emission features. Rotationally resolved excitation spectra, dispersed laser-induced fluorescence spectra, and fluorescence decay lifetimes are reported. The results are consistent with the assignment of a subset of the red-region bands to configurational transitions of the form Sm2+(4f56s)O2- ↔ Sm2+(4f55d)O2-. Analysis of the excited state hyperfine structure supports this configurational description.

An ultrasensitive SPI/PAI ion source based on a high-flux VUV lamp and its applications for the online mass spectrometric detection of sub-pptv sulfur ethers

Single-photon ionization mass spectrometry (SPI-MS) is an attractive analytical technique for the online detection of volatile organic compounds; however, the low photon flux of the vacuum ultraviolet (VUV) lamp commonly used in the SPI ion source and the corresponding low detection sensitivity remain a constraint to its wide field applications. In this study, a new VUV lamp-based SPI ion source was developed. By increasing the discharging volume and optimizing the configuration of the lens and ionizer, the photon flux of the VUV lamp and the sensitivity of the ion source were significantly improved. The VUV lamp output was 2.8 × 1015 photons s−1, which was focused into the small ionization zone, and the ion intensity of gaseous benzene under SPI achieved 1.7 × 104 counts per second per pptv (cps pptv−1). This ion source can also function in photoinduced associative ionization (PAI) mode by introducing gaseous CH2Cl2 to initiate an associative ionization reaction. Because of the high efficiency of the ion source, the amount of doped CH2Cl2 needed for PAI was greatly reduced (∼2.5% of that used in previous experiments). PAI exhibited an outstanding protonation effect on monosulfur ethers (CnH2n+1S). The signal intensities of the protonated molecular ions (MH+) were 46–128 times higher than those of the molecular ions (M+) produced by SPI. Since monosulfur ethers generally have lower SPI cross-sections than polysulfur ethers (CnH2n+1S2 or CnH2n+1S3), the PAI and SPI modes were selected for the online measurement of a series of mono- and polysulfur ethers, respectively. The obtained detection sensitivity of the mono- and polysulfur ethers reached 476.5 ± 1.72–2835.1 ± 99.5 and 47.9 ± 0.4–105.1 ± 2.3 counts pptv−1, respectively, in 10 s of sampling time. The corresponding 3σ limits of detection (LODs) were 0.12–0.71 and 0.06–0.14 pptv, respectively. This study provides advances in the development of a high-flux VUV lamp and a highly efficient SPI/PAI ion source, as well as an ultrasensitive analytical method for detecting sulfur ethers.

On the pulsed–pulseless mode transition of negative DC corona in atmospheric nitrogen

Pulsed mode as a common phenomenon appears in many kinds of DC corona discharge, whose characteristics can be affected by some specific factors. In this paper, an important research field of pulsed mode, pulsed–pulseless mode transition, is investigated in needle–plate electrodes in nitrogen at atmospheric pressure, and we discuss the effect of external circuit, gas temperature, and associative ionization on mode transition by experiment and simulation. The external circuit coupling with plasma can make the pulseless mode be achieved when there is a balance of charge between loss by discharge and gain by source before discharge quenches. The time-averaged gas temperature remains at 700 K which is regardless of source voltage and discharge mode, so gas heating is not a critical factor for mode transition. We investigate the effect of the associative ionization involving metastable particles by comparing the results with and without associative ionization reactions in the simulation; we find that the associative ionization is vital to determine the cathode voltage, discharge current, and the concentrative shape of discharge in the pulseless mode. Finally, we compare the pulsed–pulseless mode transition in nitrogen and air to clarify the effect of specific factors that depend on electronegativity of gas.

Analysis of Associative Ionization Rates for Hypersonic Flows

Three of the key associative ionization reactions that lead to plasma formation around hypersonic vehicles are studied in detail. For production of , , and , cross sections derived from experimental measurements for the interactions of atoms in low-lying electronic states are compared to cross sections derived from the overall rates widely used to model these processes. The experimentally derived cross sections are then used to develop rate coefficients for each of the atomic interactions, including those in electronically excited states. Monte Carlo simulation is employed to determine the postreaction partitioning of energy across the products. The key finding of this study is that associative ionization for all three systems proceeds primarily through interactions involving at least one of the atoms lying in the first or second electronically excited state. Nonequilibrium between the translational and electronic modes of the atoms will therefore lead to a variation in the associative ionization rates. This conclusion indicates a need for a more comprehensive approach to modeling plasma generation in hypersonic flows that resolves the lowest lying electronic states of the atoms that participate in associative ionization.

The role of associative ionization reactions in the memory effect of atmospheric pressure Townsend discharges in N2 with a small O2 addition

In this work, we investigate the role played by associative ionization reactions in the memory effect of atmospheric pressure Townsend dielectric barrier discharges working in N2 with a small addition of O2. Systematic laser-induced fluorescence measurements are carried out in order to provide the absolute densities of N and O atoms and NO molecules for different oxygen concentrations. The use of these data in a dedicated model allows us, in particular, to estimate the rate of the reaction suspected of playing a significant role. According to the obtained results, this reaction is found responsible for a significant enhancement in the production of seed electrons between two successive discharges at 25 ppm of O2, in good agreement with experimental observations. It confirms the importance of this associative ionization reaction in these experimental conditions.

Modelling of an Atmospheric–Pressure Air Glow Discharge Operating in High–Gas Temperature Regimes: The Role of the Associative Ionization Reactions Involving Excited Atoms

A model of a stationary glow-type discharge in atmospheric-pressure air operated in high-gas-temperature regimes (1000 K < Tg < 6000 K), with a focus on the role of associative ionization reactions involving N(2D,2P)-excited atoms, is developed. Thermal dissociation of vibrationally excited nitrogen molecules, as well as electronic excitation from all the vibrational levels of the nitrogen molecules, is also accounted for. The calculations show that the near-threshold associative ionization reaction, N(2D) + O(3P) → NO+ + e, is the major ionization mechanism in air at 2500 K < Tg < 4500 K while the ionization of NO molecules by electron impact is the dominant mechanism at lower gas temperatures and the high-threshold associative ionization reaction involving ground-state atoms dominates at higher temperatures. The exoergic associative ionization reaction, N(2P) + O(3P) → NO+ + e, also speeds up the ionization at the highest temperature values. The vibrational excitation of the gas significantly accelerates the production of N2(A3∑u+) molecules, which in turn increases the densities of excited N(2D,2P) atoms. Because the electron energy required for the excitation of the N2(A3∑u+) state from N2(X1∑g+, v) molecules (e.g., 6.2 eV for v = 0) is considerably lower than the ionization energy (9.27 eV) of the NO molecules, the reduced electric field begins to noticeably fall at Tg > 2500 K. The calculated plasma parameters agree with the available experimental data.

Glow Discharge in a High-Velocity Air Flow: The Role of the Associative Ionization Reactions Involving Excited Atoms.

A kinetic scheme for non-equilibrium regimes of atmospheric pressure air discharges is developed. A distinctive feature of this model is that it includes associative ionization with the participation of N(2D, 2P) atoms. The thermal dissociation of vibrationally excited nitrogen molecules and the electronic excitation from all the vibrational levels of the nitrogen molecules are also accounted for. The model is used to simulate the parameters of a glow discharge ignited in a fast longitudinal flow of preheated (T0 = 1800–2900 K) air. The results adequately describe the dependence of the electric field in the glow discharge on the initial gas temperature. For T0 = 1800 K, a substantial acceleration in the ionization kinetics of the discharge is found at current densities larger than 3 A/cm2, mainly due to the N(2P) + O(3P) → NO+ + e process; being the N(2P) atoms produced via quenching of N2(A3∑u+) molecules by N(4S) atoms. Correspondingly, the reduced electric field noticeably falls because the electron energy (6.2 eV) required for the excitation of the N2(A3∑u+) state is considerably lower than the ionization energy (9.27 eV) of the NO molecules. For higher values of T0, the associative ionization N(2D) + O(3P) → NO+ + e process (with a low–activation barrier of 0.38 eV) becomes also important in the production of charged particles. The N(2D) atoms being mainly produced via quenching of N2(A3∑u+) molecules by O(3P) atoms.

Spatially Resolved Optical Emission and Modeling Studies of Microwave-Activated Hydrogen Plasmas Operating under Conditions Relevant for Diamond Chemical Vapor Deposition.

A microwave (MW) activated hydrogen plasma operating under conditions relevant to contemporary diamond chemical vapor deposition reactors has been investigated using a combination of experiment and self-consistent 2-D modeling. The experimental study returns spatially and wavelength resolved optical emission spectra of the d → a (Fulcher), G → B, and e → a emissions of molecular hydrogen and of the Balmer-α emission of atomic hydrogen as functions of pressure, applied MW power, and substrate diameter. The modeling contains specific blocks devoted to calculating (i) the MW electromagnetic fields (using Maxwell's equations) self-consistently with (ii) the plasma chemistry and electron kinetics, (iii) heat and species transfer, and (iv) gas-surface interactions. Comparing the experimental and model outputs allows characterization of the dominant plasma (and plasma emission) generation mechanisms, identifies important coupling reactions between hydrogen atoms and molecules (e.g., the quenching of H( n > 2) atoms and electronically excited H2 molecules (H2*) by the alternate ground-state species and H3+ ion formation by the associative ionization reaction of H( n = 2) atoms with H2), and illustrates how spatially resolved H2* (and Hα) emission measurements offer a detailed and sensitive probe of the hyperthermal component of the electron energy distribution function.

Reentry blackout prediction for atmospheric reentry demonstrator mission considering uncertainty in chemical reaction rate model

A numerical simulation model of plasma flows and electromagnetic waves around a vehicle was developed to predict a radio frequency blackout. Plasma flows in the shock layer and the wake region were calculated using a computational fluid dynamics technique with a three-dimensional model. A finite-catalytic wall condition known to affect plasma properties, such as the number density of electrons, was considered for accurate prediction. A parametric study was performed to investigate the effect of uncertainty in the chemical reaction rate model on evaluating a radio frequency blackout. The behavior of electromagnetic waves in plasma was investigated using a frequency-dependent finite-difference time-domain method. Numerical simulations of reentry blackout were performed for the Atmospheric Reentry Demonstrator mission at various altitudes. The plasma flows and the complex movement of electromagnetic waves around the Atmospheric Reentry Demonstrator vehicle were clarified. The predicted signal loss profile was then directly compared with the experimental flight data to validate the present models. The numerical results generally reproduced the trends over altitudes of the measured data. It is suggested that the present simulation model can be used to investigate the radio frequency blackout and signal loss of electromagnetic waves in the communication of a reentry vehicle. It was confirmed that high associative ionization reaction rates contribute to reducing the electron density in the wake region and radio frequency blackout. It is suggested that the accuracy of predicting the signal loss improved when considering the uncertainty in the chemical reaction model for associative ionizations.

Energy Dependent Stereodynamics of the Ne(^{3}P_{2})+Ar Reaction.

The stereodynamics of the Ne(^{3}P_{2})+Ar Penning and associative ionization reactions have been studied using a crossed molecular beam apparatus. The experiment uses a curved magnetic hexapole to polarize the Ne(^{3}P_{2}), which is then oriented with a shaped magnetic field in the region where it intersects with a beam of Ar(^{1}S). The ratios of Penning to associative ionization were recorded over a range of collision energies from 320 to 500 cm^{-1} and the data were used to obtain Ω state dependent reactivities for the two reaction channels. These reactivities were found to compare favorably to those predicted in the theoretical work of Brumer etal.

Expansion of the equilibrium constants for the temperature range of 300K to 20,000K

Chemical-kinetic parameters of the equilibrium constants to evaluate the reverse rate coefficients in the shock layer of a blunt body and the expanding flows are derived for the temperature range from 300 K to 20,000 K. The expanded equilibrium constants for the chemical reactions of the dissociation, ionization, associative ionization, and neutral and charge exchange reactions of the atmospheric species and carbon materials are proposed in the present work. In evaluating the equilibrium constants, the inter-nuclear potential energies of the molecular species are calculated by the analytical potential function of the Hulburt-Hirschfelder model, and the parameters of the analytical model are determined from the semi-classically calculated RKR potentials. The electronic states and energies of the atoms are calculated by the electronic energy grouping model, and the rovibrational states and energies of each electronic states of the molecules are evaluated by the WKB method. The expanded equilibrium constants for 31 types of the reactions are provided for the best curve-fit functions, and the recombination reaction rate coefficients evaluated from the present equilibrium constants are compared with existing measured values.

Associative ionization reaction N + O → NO+ + e– in slow collisions of atoms

The endothermic associative ionization reaction N(2D) + O(3P) → NO** → NO(1Σ+) +e- in slow collisions of the atoms has been considered in terms of the multichannel quantum defect theory. The dependences of the partial and total cross sections of the reaction on the energy of the colliding atoms in the range of 0–0.3 eV have been calculated. It has been shown that the cross sections have a pronounced resonance structure, which is formed as a result of the multichannel interaction of autoionization states of the intermediate Rydberg complex NO** with dissociative states. The temperature dependence of the reaction rate constant is presented. The results are compared with those of other calculations and available experimental data.

Erratum to: “Associative ionization reactions involving excited atoms in nitrogen plasma”

A model of kinetic processes in gas-discharge plasmas of pure nitrogen and its mixtures with nitrogen oxide and oxygen is presented. A distinctive feature of the model is that it includes associative ionization reactions involving N(2P) electronically excited atoms. Taking into account these processes allows one to explain both the anomalously slow decay of gas-discharge nitrogen plasma and the increase in the electron density in the region of the so-called pink afterglow in nitrogen. The possibility of substantially accelerating secondary ionization by adding NO molecules to a partially dissociated nitrogen is demonstrated. It is shown that such acceleration is caused by the associative ionization reaction N(2P) + O(3P) → e + NO+. The calculated results agree well with available experimental data.

Associative ionization reactions involving excited atoms in nitrogen plasma

Associative ionization reactions involving excited atoms in nitrogen plasma

Competitive processes in the associative ionization ofC−withC+,N+, andO+

Absolute integral cross sections have been measured for associative ionization reactions involving the ${\mathrm{C}}^{\ensuremath{-}}$ and ${\mathrm{C}}^{+}$, ${\mathrm{N}}^{+}$, and ${\mathrm{O}}^{+}$ reactants. These measurements, obtained using a merged-beam setup in the keV range, provide us with useful experimental information on the efficiency and mechanisms of molecular ion formation from ionic reactants. The relative magnitudes of the different cross sections are rationalized by considering the spin multiplicities of initial and final states, and the exothermicities of the detachment and transfer ionization channels. The very different production efficiencies of $\mathrm{C}{\mathrm{O}}^{+}$ ions via the ${\mathrm{O}}^{\ensuremath{-}}+{\mathrm{C}}^{+}$ and ${\mathrm{C}}^{\ensuremath{-}}+{\mathrm{O}}^{+}$ channels are explained by statistical and energetic considerations. The potential energy curves of CO and $\mathrm{C}{\mathrm{O}}^{+}$ have been calculated by quantum ab initio methods in order to characterize the reactive pathways leading to autoionization. Thermal rate coefficients are derived to serve the plasma physics community.

Modeling of associative ionization reactions in hypersonic rarefied flows

When vehicles reenter the Earth’s atmosphere from space, the hypersonic conditions are sufficiently energetic to generate ionizing reactions. The production of a thin plasma layer around a hypersonic vehicle can block radio waves sent to and from the vehicle, leading to communications blackout. For Earth entry from orbit, the maximum energy involved in molecular collisions requires only associative ionization of air-species to be considered. In the present study, the modeling of such reactions is considered in detail using the direct simulation Monte Carlo (DSMC) method. For typical Earth entry conditions, with a velocity near 8km∕s, it is shown that the average ionizing reaction probabilities are small. Special numerical techniques must therefore be used in the DSMC technique in order to numerically resolve these reactions. Additional simulation problems arise from the relatively small mass of the electrons in comparison to the other atoms and molecules in these flow fields. Artificially increasing the electron mass greatly increases computational efficiency, and the viability of this approach is investigated. Simulation results are presented for conditions corresponding to the RAM-C II hypersonic flight experiment that gathered measurements of electron number density. It is demonstrated that simulation results for electron number density in this energy regime are relatively insensitive to the mass of the electrons. Direct comparison of DSMC results with the RAM-C II measurements for electron number density shows excellent agreement. These satisfactory comparisons represent the first direct verification of the ability of the DSMC technique to successfully predict the weak plasma generated around a hypersonic vehicle.