OH A2Σ Research Articles

Ignition and early stage combustion of H2–O2 mixture upon the photodissociation of O2 molecules by UV laser radiation: Experimental and numerical study

The ignition and combustion of H2O2 mixture upon the photodissociation of O2 molecules by the resonance laser radiation with a wavelength of 193 nm are studied both experimentally and computationally. The experimental test bench equipped with CARS and fluorescent diagnostic techniques was created and applied for the measurement of the induction time and velocity of combustion front during laser-induced ignition of the H2O2 mixture in the model chamber. The complementary experiments on measuring the temperature and recording the emission of OH(X2Π) and OH(A2Σ+) radicals indicate that it is possible to ignite the H2O2 mixture with ϕ= 1–3 and P0 = 1–3 atm at a rather low temperature ∼700 K, which is below the autoignition temperature, under the action of focused laser radiation (λ= 193 nm) with the energy in the pulse of ∼25–150 mJ. And the induction time is rather small and varies in the range of 8–50 µs depending on mixture parameters and laser pulse energy. The analysis of ignition and combustion processes in the H2O2 mixture under the action of resonance laser radiation is carried out on basis of two-dimensional numerical simulation with the use of the detailed kinetic mechanism of H2 oxidation supplemented by the set of reactions describing the formation of electronically excited radicals OH(A2Σ+). A special methodology for the calculation of the radiation absorption and determination of oxygen atoms concentration along the laser beam path was applied. The elaborated model allows us to describe the experimentally measured data for the induction time, the velocity of the combustion front, the space-time distribution of the OH radicals in the ground and excited states, and the temperature evolution in the focal region of the initiating laser radiation.

Multibubble Sonochemistry and Sonoluminescence at 100 kHz: The Missing Link between Low- and High-Frequency Ultrasound.

Ultrasonic frequency is one of the most important parameters that decides the characteristics of acoustic cavitation. Low- (16-50 kHz) and high- (≥200 kHz) frequency ultrasounds present opposite physical and chemical behaviors and have been extensively studied, yet frequencies in between are poorly characterized. In this study, acoustic cavitation at the intermediate ultrasonic frequency of 100 kHz is compared with that at 20 kHz and at 362 kHz by different experimental investigations: sonochemical yield (H2O2), images of sonochemiluminescence and sonoluminescence, as well as sonoluminescence spectra in aqueous media saturated with Ar or Ar/(20 vol %)O2. The chemical activity (H2O2 yield) of cavitation bubbles at 100 kHz presents a transitional behavior between low and high frequencies. The active cavitation zone distributes in the whole sonicated volume, similarly to high-frequency ultrasound and much further than at 20 kHz. The spectral shape of 100 kHz spectra is similar to that at 20 kHz. On the contrary, 100 kHz ultrasound provides the dissociation of O2 and N2 molecules inside the bubble, which is more typical for high-frequency ultrasound. This faculty is explained by the more extreme conditions reached at collapse compared with 20 kHz. Rovibronic temperatures of OH (A2Σ+) excited radicals derived from spectroscopic simulations confirm this interpretation.

Hydroxyl Radical Fluorescence and Quantum Yield Following Lyman-α Photoexcitation of Water Vapor in a Room Temperature Cell and Cooled in a Supersonic Expansion.

Photoexcitation of water by Lyman-α (121.6 nm) induces a dissociation reaction that produces OH(A2Σ+) + H. Despite this reaction being part of numerous studies, a combined understanding of the product and fluorescence yields is still lacking. Here, the rotational and vibrational distributions of OH(A) are determined from dispersed fluorescence following photoexcitation of both room-temperature and jet-cooled water vapor, for the first time in the same experiment. This work compares new data of state-resolved fluorescence with literature molecular branching ratios and brings previous studies into agreement through careful consideration of OH(A) fluorescent and predissociation lifetimes and confirms a fluorescent quantum yield of 8%. Comparison of the room-temperature and jet-cooled OH(A) populations indicate the temperature of H2O prior to excitation has subtle effects on the OH(A) population distribution, such as altering the rotational distribution in the ν' = 0 population and affecting the population in the ν' = 1 state. These results indicate jet-cooled water vapor may have a 1% higher fluorescence quantum yield compared to room-temperature water vapor.

Characteristics of atmospheric Ar/NH3 DBD and its comparison with He/N2 DBD

The discharge mode and photoelectric characteristics of Ar/NH3 DBD (dielectric barrier discharge) are studied to discuss the stability of the discharge under the influence of Penning ionization and attachment reaction. There are three discharge modes, including stable uniform glow discharge, unstable glow discharge and unstable columnar discharge. Discharge instability, including nonuniform discharge and discharge channel split, occurs under low (<0.10%) or high (>0.35%) volume fractions of NH3, which are understood to be caused by an insufficiently strong Penning effect or a strong attachment reaction, respectively. The attachment reaction of NH3 can also lead to a weaker emission intensity for Ar/NH3 DBD, in particular regarding the emission of OH(A2Σ+). Ar/NH3 and He/N2 DBDs are also compared. In He/N2 DBD, the emission intensity of OH(A2Σ+) changes less with increasing N2 volume fraction, which may be attributed to the lack of attachment reaction. Compared with Ar/NH3 DBD, the single discharge channel in He/N2 DBD is narrower, but the discharge area is wider, which should be induced by the higher Townsend ionization coefficient nonlinearity of He/N2 and the absence of attachment reaction in He/N2, respectively. In the end, a one-dimensional fluid model of Ar/NH3 DBD is built to verify the explanation of experimental results.

Influence of water content on the inactivation of P. digitatum spores using an air–water plasma jet

In order to investigate whether an air–water plasma jet is beneficial to improve the efficiency of inactivation, a series of experiments were done using a ring-needle plasma jet. The water content in the working gas (air) was accurately measured based on the Karl Fischer method. The effects of water on the production of OH (A2Σ+–X2Πi) and O (3p5P–3s5S) were also studied by optical emission spectroscopy. The results show that the water content is in the range of 2.53–9.58 mg l−1, depending on the gas/water mixture ratio. The production of OH (A2Σ+–X2Πi) rises with the increase of water content, whereas the O (3p5P–3s5S) shows a declining tendency with higher water content. The sterilization experiments indicate that this air–water plasma jet inactivates the P. digitatum spores very effectively and its efficiency rises with the increase of the water content. It is possible that OH (A2Σ+–X2Πi) is a more effective species in inactivation than O (3p5P–3s5S) and the water content benefit the spore germination inhibition through rising the OH (A2Σ+–X2Πi) production. The maximum of the inactivation efficacy is up to 93% when the applied voltage is −6.75 kV and the water content is 9.58 mg l−1.

CARS and Fluorescent study of ignition of H2/O2 mixtures upon photodissociation of O2 molecular

The research is devoted to gas mixtures ignition by UV laser radiation. The dissociation of O2 molecules by a pulse of excimer ArF laser radiation at 193-nm wavelength with formation of the chemically active oxygen atoms initiating chain reactions which cause ignition of H2/O2 mixture was employed. The experimental test bench was created with CARS and fluorescent techniques for experimental investigation of some peculiarities of mixture ignition and combustion caused by such photo-dissociation, at conditions typical for combustion chamber. Two-dimensional numerical modeling of combustion process in model combustion chamber, based on kinetic mechanism of H2 oxidation including atom O(1P) and radicals OH(A2Σ+), was performed.

An experimental study of OH(A2Σ+) + H2: Electronic quenching, rotational energy transfer, and collisional depolarization.

Zeeman quantum beat spectroscopy has been used to determine the thermal (300 K) rate constants for electronic quenching, rotational energy transfer, and collisional depolarization of OH(A2Σ+) by H2. Cross sections for both the collisional disorientation and collisional disalignment of the angular momentum in the OH(A2Σ+) radical are reported. The experimental results for OH(A2Σ+) + H2 are compared to previous work on the OH(A2Σ+) + He and Ar systems. Further comparisons are also made to the OH(A2Σ+) + Kr system, which has been shown to display significant non-adiabatic dynamics. The OH(A2Σ+) + H2 experimental data reveal that collisions that survive the electronic quenching process are highly depolarizing, reflecting the deep potential energy wells that exist on the excited electronic state surface.

Generation of reactive atomic species of positive pulsed corona discharges in wetted atmospheric flows of nitrogen and oxygen

The emission spectra of excited radicals (OH (A2Σ), O (3p5 P), Hα (3P)) and emissive species ( N2 (C3Πu)) produced by positive pulsed high-voltage needle-plane corona discharges in atmospheric N2 and O2 flows wetted with 10% H2O at 80 °C are used to investigate the relative concentrations of the produced radicals. The results indicate that the tendencies of the concentrations of radicals with discharge conditions are similar to each other due to their similar excitation processes by electron collision. The influence of oxygen flow mixed with the nitrogen flow on the emission intensities of O (3p5P → ), Hα (3P → 2S), ( 0-0), and N2 (C3Πu → B3Πg 1-0) is presented. When the flow rate of oxygen addition is varied from 0–30 ml min−1, the emission intensities of O (3p5P → ), Hα (3P → 2s), and ( 0-0) increase and reach a maximum. Then, if the oxygen flow rate increases further, the emission intensities tend to decrease. However, the intensity of N2 (C3Πu → B3Πg 1-0) decreases monotonously with the increasing oxygen flow, which indicates that the electron density decreases with the increasing oxygen flow. By the tendencies of the relative intensities to N2 (C3Πu → B3Πg 1-0), the concentrations of the total produced O, H, and are shown to increase with the oxygen flow. Based on the reactions for the production of H and O without and with the addition of O2, the analytic solutions for H and O production are derived in accordance with the experimental results.

Ignition and dynamics of nanosecond pulsed helium streamers over a water electrode

Applications of atmospheric pressure non-equilibrium plasma jets for a variety of biomedical applications have motivated fundamental studies of the gas-phase plasma in the presence of water. The initiation of guided streamers, excited with 164 ns, 5.2 kV pulses, as well as the dynamics of the streamer were investigated using a micro-hollow needle-to-plate configuration with and without water. Addition of water to the ground plate results in the enhanced ionization at the needle anode, earlier streamer formation, greater total energy deposition, and stronger emissions by excited states including N2, N2+, OH, and O. Importantly, spatiotemporally-resolved emission spectroscopy reveals that excitation transfer via collisions with heavy particles played an important role in the production of OH(A2Σ) in addition to direct electron impact.

The OES Diagnosis in Removal of HCHO by the Uniform Bipolar Nanosecond-Pulsed DBD Using Wire-Cylinder Electrode Configuration in Atmospheric N2

In this paper, the uniform bipolar nanosecondpulsed dielectric barrier discharge (NPDBD) with 15-ns rising time generating in a wire-cylinder reactor is employed to remove the gaseous formaldehyde in atmospheric pressure N 2 . The image, the waveforms of pulse voltage and discharge current, the optical emission spectra of the discharges, and the removal rate of formaldehyde are recorded and calculated, respectively. It is found that the emission intensity of OH (A 2 Σ → X 2 Π, 0-0) rises when increasing the input power density (PD). Furthermore, with the increase of input PD and the emission intensity of OH (A 2 Σ → X 2 Π, 0-0), the removal rate of formaldehyde rises correspondingly. The results also indicate that when the O 2 concentration rises within 0%-5%, the removal rate of formaldehyde increases, however, when the O 2 concentration rises within 5%-20%, the formaldehyde removal rate decreases correspondingly. In addition, at the same conditions, the DBD synergistically removing formaldehyde with Al 2 O 3 and TiO 2 could increase the removal rate by 10%-30% compared with the discharge only.

Characterization of atmospheric pressure H2O/O2 gliding arc plasma for the production of OH and O radicals

Atmospheric pressure H2O/O2 gliding arc plasma is generated by a 88 Hz, 6 kV AC power supply. The properties of the produced plasma are investigated by optical emission spectroscopy. The relative intensity, rotational, vibrational, excitation temperatures and electron density are studied as a function of applied voltage, electrode spacing, and oxygen flow rate. The rotational and vibrational temperatures are determined simulating the OH(A2Σ+(v″=0)→X2Π(v′=0)) bands with the aid of LIFBASE simulation software. The excitation temperature is obtained from the CuI transition taking non-thermal equilibrium condition into account employing intensity ratio method. The electron density is approximated from the Hα Stark broadening using the Voigt profile fitting method. It is observed that the rotational and vibrational temperatures decrease with increasing electrode spacing and O2 flow rate, but increase with the applied voltage. The excitation temperature is found to increase with increasing applied voltage and O2 flow rate, but decrease with electrode spacing. The electron density increases with increasing applied voltage while it seems to be in a downward trend with increasing electrode spacing and O2 flow rate.

Is it possible to deduce the ground state OH density from relative optical emission intensities of the OH(A2Σ+-X2Πi) transition in atmospheric pressure non-equilibrium plasmas?—An analysis of self-absorption

The measurement of absolute densities of reactive species and radicals such as OH is of growing interest for many plasma applications. In this paper, we extend the use of a self-absorption model for atomic emission spectroscopy to molecular emission spectroscopy. The proposed analysis of self-absorbed molecular emission spectra is a simple and inexpensive method to determine OH(X) densities and rotational temperatures compared to laser induced fluorescence. We compare the recorded absolute OH density in a non-equilibrium diffuse atmospheric-pressure RF glow discharge by this method with broadband UV absorption considering a number of rotational lines with J′ ⩽ 6.5, the detection limit of the line integrated OH(X) density with this method is of the order of 2 × 1019 m−2. The accuracy of the density is sensitive to the rotational temperature of the OH(A) state and the non-equilibrium rotational population distribution.

Spectroscopic and electrical characters of SBD plasma excited by bipolar nanosecond pulse in atmospheric air

In this paper, an atmospheric surface barrier discharge (SBD) generated by annular electrodes in quartz tube is presented through employing bipolar nanosecond pulse voltage in air. The discharge images, waveforms of pulse voltage and discharge current, and optical emission spectra emitted from the discharges are recorded and calculated. A spectra simulation method is developed to separate the overlap of the secondary diffraction spectra which are produced by grating in monochromator, and N2 (B3Πg→A3Σu+) and O (3p5P→3s5S2o) are extracted. The effects of pulse voltage and discharge power on the emission intensities of OH (A2Σ+→X2Пi), N2+ (B2Σu+→X2Σg+), N2 (C3Πu→B3Πg), N2 (B3Πg→A3Σu+), and O (3p5P→3s5S2o) are investigated. It is found that increasing the pulse peak voltage can lead to an easier formation of N2+ (B2Σu+) than that of N2 (C3Πu). Additionally, vibrational and rotational temperatures of the plasma are determined by comparing the experimental and simulated spectra of N2+ (B2Σu+→X2Σg+), and the results show that the vibrational and rotational temperatures are 3250±20K and 350±5K under the pulse peak voltage of 28kV, respectively.

Toward a new paradigm for sonochemistry: Short review on nonequilibrium plasma observations by means of MBSL spectroscopy in aqueous solutions

This review summarizes recent studies of multibubble sonoluminescence (MBSL) in aqueous media in order to highlight new insights into the origin of the sonochemical activity. The observation of OH(C2Σ+–A2Σ+) emission band and a spectroscopic analysis of OH(A2Σ+–X2Πi) emission band in MBSL of water pre-equilibrated with noble gases revealed the formation of a nonequilibrium plasma inside the collapsing bubble (Te>Tv>Tr, where Te is an electron temperature, Tv is a vibrational temperature and Tr is a rotational (gas) temperature). The Te and Tv estimated using OH(A2Σ+–X2Πi) emission band increase with ultrasonic frequency. In Xe the Tv of OH(A2Σ+) state is much higher than in Ar most probably due to the lower ionization potential of Xe. The MBSL of C2∗ Swan band (d3Πg–a3Πu) measured in aqueous tert-butanol (t-BuOH) solutions correlates with the data obtained for OH(A2Σ+–X2Πi) emission band. Analysis of the gaseous products of t-BuOH sonolysis revealed a significant sonochemical activity even at high t-BuOH concentration when MBSL is totally quenched, indicating that drastic intrabubble conditions (plasma) are not necessarily accompanied by sonoluminescence. The nonequilibrium plasma model of cavitation allows to explain the reverse carbon isotope effect observed during the sonolysis of water in the presence of Ar/CO gas mixture.

Origin of activated combustion in steady-state premixed burner flame with superposition of dielectric barrier discharge

The objective of this work is to understand the mechanism of plasma-assisted combustion in a steady-state premixed burner flame. We examined the spatiotemporal variation of the density of atomic oxygen in a premixed burner flame with the superposition of dielectric barrier discharge (DBD). We also measured the spatiotemporal variations of the optical emission intensities of Ar and OH. The experimental results reveal that atomic oxygen produced in the preheating zone by electron impact plays a key role in the activation of combustion reactions. This understanding is consistent with that described in our previous paper indicating that the production of “cold OH(A2Σ+)” via CHO + O → OH(A2Σ+) + CO has the sensitive response to the pulsed current of DBD [K. Zaima and K. Sasaki, Jpn. J. Appl. Phys. 53, 110309 (2014)].

Chemical Composition, Physical Properties and Populating Mechanism of Some O(I) States for a DC Discharge in Oxygen with Water Cathode

This paper reports the results of the experimental study of parameters for a DC oxygen discharge with water cathode in the pressure range of 0.1–1 bar and the discharge current of 40 mA. The radius of positive column, the cathode voltage drop, the cathode current density and the electric field strength were measured. Rotational temperatures of N2 (C3Πu, V = 0) and OH (A2Σ, V = 0) and absolute line intensities of atomic oxygen with wave length of 845 and 777 nm were determined as well. Plasma composition modeling was carried out by the combined solution of the Boltzmann equation for electrons, the equations of vibrational kinetics for ground states of N2, O2, H2O molecules, and the equations of chemical kinetics, and the plasma conductivity equation. Calculations were carried out taking into consideration the discharge radial heterogeneity and using experimental values of E/N and gas temperatures. The main particles being formed in plasma were shown to be ·OH, H2O2, O(3P), O2(a1Δg), O2(b1Σ g + ), H(1S). On the basis of this calculation and experimental values of line intensities, the populating mechanism of (3p 3P) level of atomic oxygen was discussed. The comparison of some properties of discharges in O2, N2 and air was done.

Measurements of the number density of water molecules in plasma by using a combined spectral−probe method

A novel method for measuring the number density of water molecules in low-temperature plasma is developed. The absolute intensities of rotational lines of the (0,0) band of the OH(A2Σ–X2П) transition are used. Lines with sufficiently large rotational quantum numbers referring to the so-called “hot” group of molecules produced by electron-impact dissociative excitation of water molecules are chosen for measurements. The excitation rate of a process with a known cross section is determined by measuring the parameters of plasma electrons by means of the probe method. The measured number densities of molecules are compared with those in the initial plasma-forming mixture. The time evolution of the particle densities in plasma is investigated. The problems of the sensitivity and applicability of the absolute spectral method are considered.

Contribution of electrons, Ar(3P0,2), H2O+, and H3O+ to production of OH(A2Σ+) in a micro-dielectric barrier discharge of Ar/H2O

The numerical simulation of a micro-dielectric barrier discharge of an Ar/H2O gas mixture has been performed to understand the behavior of OH(A) in three-dimensionally integrated micro-solution plasma. During the discharge ignition sequence in the period of 0–200 ns, the major OH(A) production reactions are the dissociation of H2O by high-electron-temperature electrons and metastable excited Ar(3P). After the main discharge, OH(A) production is dominated by the dissociative recombination of H3O+ and low-electron-temperature electrons.

Responses of OH (X2Π) and OH (A2Σ+) to high-energy electrons of dielectric barrier discharge in plasma-assisted burner flame

We examined the responses of OH(X2Π) and OH(A2Σ+) in a premixed CH4 flame to high-energy electrons produced by a dielectric barrier discharge. The density of OH(X2Π) did not respond to the pulsed production of high-energy electrons; however, we observed a pulsed increase in the density of chemically produced OH(A2Σ+). In addition, we observed that OH(A2Σ+) produced at the same time as high-energy electrons had a lower rotational temperature. We discussed possible key reactions in plasma-assisted combustion on the basis of the experimental observation showing the production of cold OH(A2Σ+).

Temperature Dependence Kinetic Studies of the Reaction of O(3P) with CHI3 and C2H5I and the 298 K Reaction of OH(X2Π) with CHI3

ABSTRACTA flash photolysis–resonance fluorescence technique was used to investigate the kinetics of the OH(X2Π) radical and O(3P) atom‐initiated reactions with CHI3 and the kinetics of the O(3P) atom‐initiated reaction with C2H5I. The reactions of the O(3P) atom with CHI3 and C2H5I were studied over the temperature range of 296 to 373 K in 14 Torr of helium, and the reaction of the OH (X2Π) radical with CHI3 was studied at T = 298 K in 186 Torr of helium. The experiments involved time‐resolved resonance fluorescence detection of OH (A2Σ+ → X2Π transition at λ = 308 nm) and of O(3P) (λ = 130.2, 130.5, and 130.6 nm) following flash photolysis of the H2O/He, H2O/CHI3/He, O3/He, and O3/C2H5I/He mixtures. A xenon vacuum UV (VUV) flash lamp (λ &gt; 120 nm) served as a photolysis light source. The OH radicals were produced by the VUV flash photolysis of water, and the O(3P) atoms were produced by the VUV flash photolysis of ozone. Decays of OH radicals and O(3P) atoms in the presence of CHI3 and C2H5I were observed to be exponential, and the decay rates were found to be linearly dependent on the CHI3 and C2H5I concentrations. Measured rate coefficients for the reaction of O(3P) atoms with CHI3 and C2H5I are described by the following Arrhenius expressions (units are cm3 s−1): kO+C2H5I(T) = (17.2 ± 7.4) × 10−12 exp[−(190 ± 140)K/T] and kO+CHI3(T) = (1.80 ± 2.70) × 10−12 exp[−(440 ± 500)K/T]; the 298 K rate coefficient for the reaction of the OH radical with CHI3 is kOH+CHI3(298 K) = (1.65 ± 0.06) × 10−11 cm3 s−1. The listed uncertainty values of the Arrhenius parameters are 2σ‐standard errors of the calculated slopes by linear regression.