Ground State Oxygen Atoms Research Articles

Kinetic Study of the Reactions of Ground State Atomic Carbon and Oxygen with Nitrogen Dioxide over the 50-296 K Temperature Range.

The kinetics of the reactions of nitrogen dioxide, NO2, with atomic oxygen and atomic carbon in their ground triplet states (3P) have been studied at room temperature and below using a supersonic flow (Laval nozzle) reactor. O(3P) and C(3P) atoms (hereafter O and C respectively) were created in situ by the pulsed laser photolysis of the precursor molecules NO2 at 355 nm and CBr4 at 266 nm, respectively. While the progress of the O + NO2 reaction was followed by detecting O atoms by a chemiluminescent tracer method, progress of the C + NO2 reaction was followed by detecting C atoms directly by vacuum ultraviolet laser-induced fluorescence at 116 nm. The measured rate constants for the O + NO2 reaction are found to be in excellent agreement with earlier work at higher temperatures and extend the available kinetic data for this process down to 50 K. The present work represents the first kinetics study of the C + NO2 reaction. Although both reactions display rate constants that increase as the temperature falls, a more substantial rate increase is observed for the O + NO2 reaction. The effects of these reactions on the simulated abundances of interstellar NO2 and related compounds were tested using a gas-grain model of the dense interstellar medium, employing expressions for the rate constants of the form, k(T) = α(T/300)β, with α = 1 × 10-11 cm3 s-1 and β = -0.65 for the O + NO2 reaction and α = 2 × 10-10 cm3 s-1 and β = -0.11 for the C + NO2 reaction. Although these simulations predict that gas-phase NO2 abundances are low in dense interstellar clouds, NO2 abundances on interstellar dust grains are predicted to reach reasonably high levels, indicating the potential for detection of this species in warmer regions.

Spatially resolved TALIF investigation of atomic oxygen in the effluent of a CO2 microwave discharge

A diagnostic setup for one-dimensionally spatially resolved two-photon absorption laser-induced fluorescence (TALIF) detection of ground state oxygen atoms ( 2p43P2,1,0 ) is developed. The goal of this study is to investigate the evolution of temperatures and absolute number densities of oxygen atoms along the effluent of a low-pressure CO2 microwave discharge in order to gain insights into some of the mechanisms governing the post-discharge regime. The plasma source is operated at conditions of 600 W– 1200 W of absorbed power with flow rates of 74 sccm and 370 sccm pure CO2 at pressures between 1.2 mbar and 5 mbar with specific energy inputs up to 111.9 eV/molecule. These operating conditions exhibit high CO2 conversions (up to 90%) at low energy efficiencies (2%–7.4%), due to direct electron impact dissociation driving the conversion process resulting in splitting of CO2 into CO and metastable oxygen atoms. The TALIF measurements yield spatially resolved translational temperatures between 1000 K– 1600 K for most operating conditions and axial positions along the effluent. Reference measurements with xenon 6p′[3/2]2 are used for absolute number density calibration. The resulting axially resolved number density profiles of ground state atomic oxygen increase along the effluent, even at considerable distances of several centimeters from the active discharge, before they reach a maximum between 5×1020 m−3 and 2.2×1021 m−3 depending on the condition, and decrease after that. This behavior indicates the potential significance of quenching of metastable oxygen atoms within the post-discharge regime of the investigated CO2 discharges. The measured spatially resolved number density evolutions are qualitatively consistent with quenching via wall collisions being the dominant deactivation mechanism, underlining the importance of particle-wall interactions.

Experimental and theoretical study of the Sn-O bond formation between atomic tin and molecular oxygen.

The merging of the electronic structure calculations and crossed beam experiments expose the reaction dynamics in the tin (Sn, 3Pj) - molecular oxygen (O2, X3Σ-g) system yielding tin monoxide (SnO, X1Σ+) along with ground state atomic oxygen O(3P). The reaction can be initiated on the triplet and singlet surfaces via addition of tin to the oxygen atom leading to linear, bent, and/or triangular reaction intermediates. On both the triplet and singlet surfaces, formation of the tin dioxide structure is required prior to unimolecular decomposition to SnO(X1Σ+) and O(3P). Intersystem crossing (ISC) plays a critical role in the reaction mechanism and extensively interosculates singlet and triplet surfaces. The studied reaction follows a mechanism parallel to that for the gas phase reaction of germanium and silicon with molecular oxygen, however, the presence of the tin atom enhances and expands ISC via the "heavy atom effect".

On-surface chemical dynamics of monolayer, bilayer, and many-layered graphene surfaces probed with supersonic beam scattering and STM imaging.

We have developed the capability to elucidate interfacial reaction dynamics using an arguably unique combination of supersonic molecular beams combined with in situ STM visualization. These capabilities have been implemented in order to reveal the complex spatiotemporal correlations that govern the oxidation of graphitic systems spanning atomic-, nano-, and meso-length scales. In this study, the 3 nm periodic moiré pattern of monolayer and bilayer graphene on Ru(0001) provides a diverse palette of potential scattering and binding sites at the interface for ground state atomic oxygen. We resolve the site-specificity of atomic oxygen placement on the moiré lattice for both monolayer and bilayer graphene on Ru(0001) with atomic resolution. Angle- and energy-controlled scattering of O(3P) on these interfaces reveals an incisive side-by-side comparison of preferential reactivity of the monolayer surface compared to a more free-standing bilayer graphene ruthenium interface. Morphologically dependent reactivity of many layered graphene (HOPG) and monolayer graphene on Ru(0001) reveal anisotropic on-surface reactivity dependent on the presence of proximal reacted sites or local regions. The kinetics of on-surface oxidation are additionally shown to influence the morphology of surface products by varying the temperature of the interface and flux of reactant species. Such correlations are important in chemisorption, catalysis, materials oxidation and erosion, and film processing-and tunable moiré templated adsorption is a route to well-ordered self-assembled 2D materials for use in next-generation platforms for quantum devices and catalysis. Taken together, these results highlight a new direction in the examination of interfacial reaction dynamics where incident beam kinetic energy and angle of incidence can be used as reaction control parameters, with outcomes such as site-specific reactivity, changes for overall time-evolving mechanisms, and the relative importance of non-adiabatic channels in adsorption all linked to the on-surface fate of chemisorbed species.

Validation of THz absorption spectroscopy by a comparison with ps-TALIF measurements of atomic oxygen densities

Terahertz (THz) absorption spectroscopy has recently been developed as a diagnostic technique for measuring absolute ground-state atomic oxygen densities in plasmas. To demonstrate the validity of this approach, we present in this Letter a benchmark against a more established method. Atomic oxygen densities were measured with THz absorption spectroscopy and compared to those obtained from picosecond (ps) two-photon absorption laser induced fluorescence (TALIF) measurements on the same capacitively coupled radio frequency oxygen discharge. Similar changes in the atomic oxygen density were observed with both diagnostics when varying the applied power (20–100 W) and the gas pressure (0.7–1.3 mbar). Quantitatively, the results are in good agreement as well, especially when considering the total margin of error of the two diagnostics. For example, for a gas pressure of 1.3 mbar and an applied power of 30 W, atomic oxygen densities measured with THz absorption spectroscopy and TALIF were (7.0 ± 1.7)×1014 cm−3 and (5.3 ± 3.2)×1014 cm−3, respectively. This shows that THz absorption spectroscopy is an accurate technique that can be reliably used for real-world applications to determine atomic oxygen densities in plasmas.

Chemical scanning of atomic oxygen at the gas-liquid interface of a NaCl solution via quantum mechanics/molecular mechanics molecular dynamics simulations

Atmospheric pressure plasmas can serve as double phase reactors to produce plasma activated water for water treatment. However, the physical-chemical processes involving plasma-supplied atomic oxygen and reactive oxygen species in an aqueous solution remain unclear. In this work, quantum mechanics (QM)/molecular mechanics (MM) molecular dynamics simulations (MDs) have been performed to directly observe the chemical reactions occurring between atomic oxygen and a NaCl solution at the gas-liquid interface using a model containing 10,800 atoms. During simulations, the atoms in the QM and MM Parts are dynamically adjusted. To examine the effects of local microenvironments on the chemical processes, atomic oxygen is used as a chemical probe to scan the gas-liquid interface. The excited atomic oxygen reacts with water molecules and Cl− ions to produce H2O2, OH, HOCl, ClO−, and HO2−/H3O+ species. The ground-state atomic oxygen is significantly more stable than the excited atomic oxygen, although it can react with water molecules to produce OH radicals. However, the branch ratio of ClO− computed for triplet atomic oxygen is significantly larger than that determined for singlet atomic oxygen. This study can help achieve a better understanding of the fundamental chemical processes during plasma-treated solution experiments and promotes advances in applications of QM/MM calculations at the gas-liquid interface.

Ion-molecule studies of energetic oxygen allotropes in flow tubes: .

Starting in the 1960s, flow tube apparatuses have played a central role in the study of ion-molecule kinetics, allowing for immense chemical diversity of cationic, anionic, and neutral reactants. Here, we review studies of oxygen allotropes, excluding ground state O2 ( ), and focusing instead on reactions of cations, anions, and metal chemi-ionization reactions with ground state atomic oxygen (O 3 P), vibrationally excited molecular oxygen (O2 (v)), electronically excited molecular oxygen (O2 ( )), and ozone (O3 ). Historical outlines of work over several decades are given along with a focus on more recent work by our group at the Air Force Research Laboratory.

Efficient inactivation of amoeba spores and their intraspore bacteria by solar/chlorine: Kinetics and mechanisms

Amoebae are widespread in water and serve as environment vectors for pathogens, which may threaten public health. This study evaluated the inactivation of amoeba spores and their intraspore bacteria by solar/chlorine. Dictyostelium discoideum and Burkholderia agricolaris B1qs70 were selected as model amoebae and intraspore bacteria, respectively. Compared to solar irradiation and chlorine, solar/chlorine enhanced the inactivation of amoeba spores and intraspore bacteria, with 5.1 and 5.2-log reduction at 20 min, respectively. The enhancement was similar in real drinking water by solar/chlorine under natural sunlight. However, the spore inactivation decreased to 2.97-log by 20 min solar/chlorine under oxygen-free condition, indicating that ozone played a crucial role in the spore inactivation, as also confirmed by the scavenging test using tert‑butanol to scavenge the ground-state atomic oxygen (O(3P)) as a ozone precursor. Moreover, solar/chlorine induced the shape destruction and structural collapse of amoeba spores by scanning electron microscopy. As for intraspore bacteria, their inactivation was likely ascribed to endogenous reactive oxygen species. As pH increased from 5.0 to 9.0, the inactivation of amoeba spores decreased, whereas that of intraspore bacteria was similar at pH 5.0 and 6.5 during solar/chlorine treatment. This study first reports the efficient inactivation of amoeba spores and their intraspore pathogenic bacteria by solar/chlorine in drinking water.

Comparison of Low-Density Lipoprotein Oxidation by Hydrophilic O(3 P)-Precursors and Lipid-O(3 P)-Precursor Conjugates.

Lipid oxidation by reactive oxygen species (ROS) provide several different oxidation products that have been implicated in inflammatory responses. Ground state atomic oxygen [O(3 P)] is produced by the photodeoxygenation of certain heterocyclic oxides and has a reactivity that is unique from other ROS. Due to the reactive nature of O(3 P), the site of O(3 P)-generation is expected to influence the products in heterogenous solutions or environments. In this work, the oxidation of low-density lipoprotein (LDL) by lipids with covalently bound O(3 P)-photoprecursors was compared to more hydrophilic O(3 P)-photoprecursors. Lipid oxidation products were quantified after Bligh-Dyer extraction and pentafluorobenzyl bromide (PFB) derivatization by GC-MS. Unlike the more hydrophilic O(3 P)-photoprecursors, the oxidation of LDL during the irradiation of lipid-(O3 P)-photoprecursor conjugates showed little quenching by the addition of the O(3 P)-scavenging sodium allyl sulfonate. This indicated that lipophilic O(3 P)-photoprecursors are expected to generate lipid oxidation products where other more hydrophilic O(3 P)-photoprecursors could be quenched by other reactive groups present in solution or the environment.

Terahertz absorption spectroscopy for measuring atomic oxygen densities in plasmas

This paper describes the first implementation of terahertz (THz) quantum cascade lasers for high-resolution absorption spectroscopy on plasmas. Absolute densities of ground state atomic oxygen were directly obtained by using the fine structure transition at approximately 4.75 THz. Measurements were performed on a low-pressure capacitively coupled radio frequency oxygen discharge. The detection limit in this arrangement was found to be cm−3, while the measurement accuracy was within 5%, as demonstrated by reference measurements of a well-defined ammonia transition. The results show that the presented method is well suited to measure atomic oxygen densities, and it closes the THz gap for quantitative atomic density measurements in harsh environments such as plasmas.

Cavity ringdown studies of the E–H transition in an inductively coupled oxygen plasma: comparison of spectroscopic measurements and modelling

The absolute number density of ground state oxygen atoms, O(3P), present in a 100 mTorr oxygen plasma has been determined as a function of operating power using cavity ringdown spectroscopy (CRDS). The dissociation fraction increases by an order of magnitude from ∼0.8% at 50 W to 8% at 250 W and reflects a similar increase in the electron density over this power range. Emission spectra show that the E–H switchover is accompanied by increased rotational heating of O2 and this behaviour is also observed in the translational temperatures determined by fitting the Doppler limited O(3P) CRDS data. The measurements are contextualised via a volume averaged kinetic model that uses the measured absolute densities of O(3P) and O2(a 1Δg, v = 0) as a function of power as its benchmarks. Despite the inherent spatial inhomogeneity of the plasma, the volume averaged model, which uses a minimal set of reactions, is able to both reproduce previous measurements on the absolute density of O− and to infer physically reasonable values for both the electron temperature and number density as the E–H switch over is traversed. Time-resolved emission measurements return a value of 0.2 for the wall loss coefficient for O2(b 1Σg +); as a consequence, the number density of O2(b 1Σg +) is (at least) one order of magnitude less than O2(a 1Δg).

The role of highly vibrationally excited H2 initiating the nitrogen chemistry

The formation of hydrides by gas-phase reactions between H2 and a heavy element atom is a very selective process. Reactions with ground-state neutral carbon, oxygen, nitrogen, and sulfur atoms are very endoergic and have high energy barriers because the H2 molecule has to be fragmented before a hydride bond is formed. In cold interstellar clouds, these barriers exclude the formation of CH, OH, NH, and SH radicals through hydrogen abstraction reactions. Here we study a very energetically unfavorable process, the reaction of N(4S) atoms with H2 molecules. We calculated the reaction rate coefficient for H2 in different vibrational levels, using quantum methods for v = 0−7 and quasi-classical methods up to v =12; for comparison purposes, we also calculated the rate coefficients of the analogous reaction S (3P)+ H2(v) → SH + H. Owing to the high energy barrier, these rate coefficients increase with v and also with the gas temperature. We implemented the new rates in the Meudon photodissociation region (PDR) code and studied their effect on models with different ultraviolet (UV) illumination conditions. In strongly UV-irradiated dense gas (Orion Bar conditions), the presence of H2 in highly vibrationally excited levels (v ≥ 7) enhances the NH abundance by two orders of magnitude (at the PDR surface) compared to models that use the thermal rate coefficient for reaction N(4S) + H2 → NH + H. The increase in NH column density, N(NH), across the PDR is a factor of ~25. We investigate the excitation and detectability of submillimeter NH rotational emission lines. Being a hydride, NH excitation is very subthermal (Trot ≪ Tk) even in warm and dense gas. We explore existing Herschel/HIFI observations of the Orion Bar and Horsehead PDRs. We report a 3σ emission feature at the ~974 GHz frequency of the NH NJ = 12 − 01 line toward the Bar. The emission level implies N(NH) ≃ 1013 cm−2, which is consistent with PDR models using the new rate coefficients for reactions between N and UV-pumped H2. This formation route dominates over hydrogenation reactions involving the less abundant N+ ion. JWST observations will quantify the amount and reactivity of UV-pumped H2 in many interstellar and circumstellar environments.

Rate constant and products of the reaction of O(3P) atoms with thiirane over the temperature range 220–950 K

AbstractA discharge‐flow modulated molecular beam mass spectrometry technique has been used to determine the rate constant for the reaction of ground state oxygen atoms with C2H4S in the temperature range 220–950 K. The reaction rate constant has been determined directly by monitoring C2H4S in an excess of O atoms and through competitive reaction of oxygen atoms with C2H4S and Br2. Weak, negative temperature dependence is observed below 300 K, whereas positive temperature dependence is observed above 380 K: k = 1.13 × 10–12 (T/298)2.18 exp(733/T) cm3 molecule–1 s–1. We estimate this expression to be accurate within 15% over the investigated temperature range. SO and C2H4 have been detected as reaction products and a branching ratio of unity (with uncertainty of 10%) has been found for the channel forming these two species in the temperature range 300–900 K, confirming that the title reaction is an efficient source of SO radicals.

Combined Spectroscopic and Computational Investigation on the Oxidation of exo-Tetrahydrodicyclopentadiene (JP-10; C10H16) Doped with Titanium-Aluminum-Boron Reactive Metal Nanopowder.

We report the results on the combustion of single, levitated droplets of exo-tetrahydrodicyclopentadiene (JP-10) doped with titanium-aluminum-boron (Ti-Al-B) reactive metal nanopowders (RMNPs) in an oxygen (60%)-argon (40%) atmosphere by exploiting an ultrasonic levitator with droplets ignited by a carbon dioxide laser. Ultraviolet-visible (UV-vis) emission spectroscopy revealed the presence of gas-phase aluminum (Al) and titanium (Ti) atoms. These atoms can be oxidized in the gas phase by molecular oxygen to form spectroscopically detected aluminum monoxide (AlO) and titanium monoxide (TiO) transients. Analysis of the optical ignition videos supports that the nanoparticles are ignited before JP-10. The detection of boron monoxide (BO) further proposes an active surface chemistry through the oxidation of the RMNPs and the release of at least BO into the gas phase. The oxidation of gas-phase BO by molecular oxygen to boron dioxide (BO2) plus atomic oxygen might operate in the gas phase, although the involvement of surface oxidation processes of RMNPs to BO2 cannot be discounted. The UV-vis emission spectra also revealed the key reactive intermediates (OH, CH, C2, and HCO) of the oxidation of JP-10. Electronic structure calculations reveal that the presence of reactive radicals has a profound impact on the oxidation of JP-10. Although titanium monoxide (TiO) reacts to produce titanium dioxide (TiO2), it does not engage in an active JP-10 chemistry as all abstraction pathways are endoergic by more than 217 kJ mol-1. This is similar for atomic aluminum and titanium, whose hydrogen abstraction reactions from JP-10 were revealed to be endoergic by at least 77 kJ mol-1. Therefore, aluminum and titanium react preferentially with molecular oxygen to produce their monoxides. However, the formation of BO, AlO, and BO2 supplies a pool of highly reactive radicals, which can abstract hydrogen from JP-10 via transition states ranging from only 1 to 5 kJ mol-1 above the separated reactants, forming JP-10 radicals along with the hydrogen abstraction products (boron hydride oxide, aluminum monohydroxide, and metaboric acid) in the overall exoergic reactions. These abstraction barriers are well below the barriers of abstractions for ground-state atomic oxygen and molecular oxygen. In this sense, gas-phase BO, AlO, and BO2 catalyze the oxidation of gas-phase JP-10 via hydrogen abstraction, forming highly reactive JP-10 radicals. Overall, the addition of RMNPs to JP-10 not only provides a higher energy density fuel but is also expected to lead to shorter ignition delays compared to pure JP-10 due to the highly reactive pool of radicals (BO, AlO, and BO2) formed in the initial stage of the oxidation process.

Study of the Synchrotron Photoionization Oxidation of Alpha-Angelica Lactone (AAL) Initiated by O(3P) at 298, 550, and 700 K.

In recent years, biofuels have been receiving significant attention because of their potential for decreasing carbon emissions and providing a long-term renewable solution to unsustainable fossil fuels. Currently, lactones are some of the alternatives being produced. Many lactones occur in a range of natural substances and have many advantages over bioethanol. In this study, the oxidation of alpha-angelica lactone initiated by ground-state atomic oxygen, O(3P), was studied at 298, 550, and 700 K using synchrotron radiation coupled with multiplexed photoionization mass spectrometry at the Lawrence Berkeley National Lab (LBNL). Photoionization spectra and kinetic time traces were measured to identify the primary products. Ketene, acetaldehyde, methyl vinyl ketone, methylglyoxal, dimethyl glyoxal, and 5-methyl-2,4-furandione were characterized as major reaction products, with ketene being the most abundant at all three temperatures. Possible reaction pathways for the formation of the observed primary products were computed using the CBS–QB3 composite method.

Collisional excitation kinetics for O(3s^{5}S^{o}) and O(3p^{5}P_{3}) states using laser absorption spectroscopy in shock-heated weakly ionized O_{2}-Ar mixture.

Collisional excitation kinetics for atomic oxygen is studied behind reflected shock waves in 1%O_{2}/Ar mixtures over 10 000-11000K using laser absorption spectroscopy of the O(3s^{5}S^{o}) to O(3p^{5}P_{3}) transition at 777nm and the O(3p^{5}P_{3}) to O(3d^{5}D_{2,3,4}^{o}) transitions at 926nm. Four time histories are inferred simultaneously from the absorbance of the two transitions: the population density of level 4 of atomic oxygen, i.e., the O(3s^{5}S^{o}) state, n_{4}; the population density of level 6 of atomic oxygen, i.e., the O(3p^{5}P_{3}) state, n_{6}; the electron number density, n_{e}; and the heavy-particle translational temperature, T_{tr}. Atomic oxygen in the levels 4 and 6 are not in equilibrium with the ground-state atomic oxygen as the measurements of n_{4} and n_{6} are generally 3-20 times smaller than the corresponding values under Boltzmann equilibrium at T_{tr}. However, these two states are close to partial equilibrium with each other within the test time, indicating strong heavy-particle cross coupling between levels 4 and 6 of atomic oxygen. A simplified two-temperature collisional-radiative (CR) model is developed to study the thermal and chemical nonequilibrium of atomic oxygen following shock heating. The four measured time histories are used to optimize the 12 collisional rate constants in the CR model using a stochastic gradient descent (SGD) algorithm. The time-history results, diagnostic methods, and collisional-radiative model presented in the current study are potentially useful in studies of high-enthalpy air, plasma processing, or other applications involving weakly ionized oxygen.

Investigation of non-thermal atmospheric plasma for the degradation of avermectin solution

Increasing concern with regard to food safety in the presence of pesticide residues (PRs) on the surface of agricultural products has resulted in the rapid development of practical degrading technologies for corresponding PRs. In this paper, an unconventional method of degrading pesticides, non-thermal atmospheric plasma (NTAP), was proposed to degrade the avermectin (AVM) in aqueous solution. Optical emission spectroscopy shows that NTAP, consisting of filamentary streamers, contains a variety of reactive oxygen species (ROS) that may interact with AVM. The high-performance liquid chromatography (HPLC)-MS/MS results indicate that the efficiency of AVM degradation seriously depends on multiple operation parameters of the NTAP, including the applied voltage, treatment time and gas flow rate. The maximum degradation rate of AVM was observed to be 97.47% after 240 s exposure under NTAP with an applied voltage of 18 kV and gas flow rate of 1 l min−1. Molecular dynamics simulation based on a reactive force field for the interaction between O (ground state atomic oxygen) and AVM was performed to analyze the underpinning mechanisms. The simulation result shows the possible pathways of the NTAP-generated O degrading AVM by destroying the glycosyl group or fracturing the ester group.

Quantum study of reaction O(3 P) + H2 (v, j) → OH + H: OH formation in strongly UV-irradiated gas.

The reaction between atomic oxygen and molecular hydrogen is an important one in astrochemistry as it regulates the abundance of the hydroxyl radical and serves to open the chemistry of oxygen in diverse astronomical environments. However, the existence of a high activation barrier in the reaction with ground state oxygen atoms limits its efficiency in cold gas. In this study we calculate the dependence of the reaction rate coefficient on the rotational and vibrational state of H2 and evaluate the impact on the abundance of OH in interstellar regions strongly irradiated by far-UV photons, where H2 can be efficiently pumped to excited vibrational states. We use a recently calculated potential energy surface and carry out time-independent quantum mechanical scattering calculations to compute rate coefficients for the reaction O(3 P) + H2 (v, j) → OH + H, with H2 in vibrational states v = 0-7 and rotational states j = 0-10. We find that the reaction becomes significantly faster with increasing vibrational quantum number of H2, although even for high vibrational states of H2 (v = 4-5) for which the reaction is barrierless, the rate coefficient does not strictly attain the collision limit and still maintains a positive dependence with temperature. We implemented the calculated state-specific rate coefficients in the Meudon PDR code to model the Orion Bar PDR and evaluate the impact on the abundance of the OH radical. We find the fractional abundance of OH is enhanced by up to one order of magnitude in regions of the cloud corresponding to A V = 1.3-2.3, compared to the use of a thermal rate coefficient for O + H2, although the impact on the column density of OH is modest, of about 60%. The calculated rate coefficients will be useful to model and interpret JWST observations of OH in strongly UV-illuminated environments.

High-Efficiency and Ground-State Atomic Oxygen-Dominant Photodegradation of Carbamazepine by Coupling Chlorine and g-C3N4

In this study, a solar/chlorine/graphitic carbon nitride (g-C3N4) system for the highly efficient degradation of carbamazepine (CBZ) was constructed, and the degradation mechanism of CBZ in this system was elucidated. The removal rate of CBZ by g-C3N4 combined with solar/chlorine was as high as 97.8%, which was more than twice that of the solar/chlorine combined system (43.8%). Electron spin resonance analysis revealed that the hydroxyl radicals (HO•) in the solar/chlorine/g-C3N4 system were remarkably less than that in the solar/chlorine photocatalytic system, wherein HO• was the dominant participant. However, the former system had higher efficiency than the latter. Scavenging tests confirmed the existence of a synergistic effect between solar/chlorine and g-C3N4 in CBZ degradation, and ground-state atomic oxygen (O(3P)) played a more important role than HO• in the solar/chlorine/g-C3N4 system. A novel O3-related pathway of CBZ degradation was proposed on the basis of the determination of intermediates. The intermediates detected via liquid chromatography quadrupole time of flight mass spectrometry (LC/Q-TOF-MS)/MS analysis indicated that O3 attacked the double bond of CBZ via the Criegee mechanism and produced intermediates with two new aldehydes. This study not only improved the knowledge of the effects of O(3P) and the O(3P)-related degradation mechanism of organic pollutants but also provided a reminder that O(3P) might be overlooked in the photocatalytic processes, especially in processes that occur in the presence of dissolved oxygen or oxygen-containing anions.

Modeling of the thermal migration mechanisms of atomic oxygen in Ar, Kr, and Xe crystals.

Accommodation and migration of the ground-state (2s22p4 3P) oxygen atom in the ideal Ar, Kr, and Xe rare gas crystals are investigated using the classical model. The model accounts for anisotropy of interaction between guest and host atoms, spin-orbit coupling, and lattice relaxation. Interstitial and substitutional accommodations are found to be the only thermodynamically stable sites for trapping atomic oxygen. Mixing of electronic states coupled to lattice distortions justifies that its long-range thermal migration follows the adiabatic ground-state potential energy surface. Search for the migration paths reveals a common direct mechanism for interstitial diffusion. Substitutional atoms are activated by the point lattice defects, whereas the direct guest-host exchange meets a higher activation barrier. These three low-energy migration mechanisms provide plausible interpretation for multiple migration activation thresholds observed in Kr and Xe free-standing crystals, confirmed by reasonable agreement between calculated and measured activation energies. An important effect of interaction anisotropy and a minor role of spin-orbit coupling are emphasized.