Energetic Oxygen in the Upper Atmosphere and the Laboratory

Significantly Improved Detection of Molecular Oxygen by Two-Color Resonance-Enhanced Multiphoton Ionization

We report a new spectroscopic detection scheme for molecular oxygen that achieves roughly two orders of magnitude higher sensitivity for fully rotationally resolved spectra than the current state of the art. Two-color (2 + 1') resonance-enhanced multiphoton ionization (REMPI) via the 3d Rydberg complex yields state-selective spectra with signal comparable to the intense but diffuse C 3sσ 3Πg ← X 3Σg- (2 + 1) REMPI bands without significant saturation or broadening. The resulting increase in sensitivity permitted observation of the very weak 3dπ 1Δ2 ← X 3Σg- transitions and is independent of the intermediate state. This advance in ionization efficiency and quantum state-selective sensitivity for O2 promises to aid physical and chemical studies across a wide variety of fields.

-Matrix Calculation of Electron Collisions with Molecular Oxygen in Its Electronically Excited States

Low-energy electron collisions with the X³Σ_g- ground state and a¹Δ_g and b¹Σ_g+, the Herzberg states (c¹Σ_u^-, A'³Δ_u, and A³Σ_u+), and B³Σ_u^- excited states of the O₂ molecules are studied using the fixed-nucleus R-matrix method. Integral elastic scattering and electronic excitation cross sections from the X³Σ_g^- ground state overall agree well with the available experimental and theoretical results. The electronic (de-)excitation cross sections for the electron impact with the Herzberg states and the B³Σ_u^- state are reported. The value of elastic cross sections for the six excited states decreases with the increment of electron energy, except for the resonance peaks. As the case of excitation from the X³Σ_g^- ground state, the O₂^{- 2}Π_u resonance makes a dominant contribution to the (de-)excitation cross sections from the a¹Δ_g, b¹Σ_g⁺, and the Herzberg states. The magnitude of the de-excitation cross sections at the location of the ²Π_u resonance from the Herzberg states to the ground state is about 2 to 8 times those of the excitation cross sections for the corresponding excitation transitions. These results should be significant for models of oxygen plasma and the dynamics of the Herzberg states of molecular oxygen in the earth's atmosphere.

Ozone destruction due to the recombination of oxygen atoms

Kinetics of ozone destruction due to the recombination of oxygen atoms produced by pulsed 266 nm laser photolysis of O₃/M (M = CO₂ and/or N₂) mixtures was studied using the absorption and emission spectroscopy to follow time evolutions of O₃ and electronically excited molecules O₂* formed in the recombination process 2O(³P) + M → O₂* + M. An unexpected high ozone destruction rate was observed when O₂* was present in the system. The kinetic model developed for the oxygen nightglow on the terrestrial planets was adapted to interpret the detected temporal profiles of the ozone number density and the O₂* emission intensities. It was deduced that the vibrationally excited singlet delta oxygen molecule O₂(a¹Δ, υ) formed in the secondary processes reacts efficiently with ozone in the process O₂(a¹Δ, υ ≥ 3) + O₃ → 2O₂ + O, and the rate constant of this process was estimated to be 3 × 10^-11 cm³ s^-1. Ab initio calculations at the CASPT2(14, 12)/cc-pVTZ/UωB97XD/cc-pVTZ level of theory were applied to find the reaction pathway from the reactants to products on the O₅ potential energy surface. These calculations revealed that the O₂(a¹Δ) + O₃ reaction is likely to proceed via singlet-triplet intersystem crossing exhibiting an energy barrier of 9.6 kcal/mol, which lies between two and three quanta of vibrational excitation of O₂(a¹Δ), and hence, O₂(a¹Δ, υ) with υ ≥ 3 could rapidly react with ozone.

Reaction of the N Atom with Electronically Excited O2 Revisited: A Theoretical Study

The kinetics of the reaction of N with electronically excited O₂ (singlet a¹Δ_g and b¹Σ_g⁺ states), potentially relevant for NO_x formation in nonthermal air plasma, is theoretically studied using the multireference second-order perturbation theory. The corresponding thermodynamically and kinetically favored reaction pathways together with possible intersystem crossings are identified. It has been revealed that the energy barrier for the N + O₂(a¹Δ_g) → NO + O reaction is approximately twice the barrier height for the counterpart process with O₂(X³Σ_g^-). The molecular oxygen in the b¹Σ_g⁺ state, in turn, proved to be even less reactive to atomic nitrogen than O₂(a¹Δ_g). Appropriate thermal rate constants for specified reaction channels are calculated by the variational transition-state theory incorporating corrections for the tunneling effect, nonadiabatic transitions, and anharmonicity of vibrations for transition states and reactants. The corresponding three-parameter Arrhenius expressions for the broad temperature range (T = 300-4000 K) are reported. At last, post-transition-state molecular dynamics simulations indicate that the N + O₂(a¹Δ_g) reaction produces vibrationally much colder NO molecules than the N + O₂(X³Σ_g^-) process.

Revisiting spontaneous Raman scattering for direct oxygen atom quantification

In this Letter, the counterintuitive and largely unknown Raman activity of oxygen atoms is evaluated for its capacity to determine absolute densities in gases with significant O-density. The study involves ${\rm CO}_2$ microwave plasma to generate a self-calibrating mixture and establish accurate cross sections for the $^3{\!P_2}{\leftrightarrow ^3}{\!P_1}$ and $^3{\!P_2}{\leftrightarrow ^3}{\!P_0}$ transitions. The approach requires conservation of stoichiometry, confirmed within experimental uncertainty by a 1D fluid model. The measurements yield ${\sigma _{J = 2 \to 1}} = 5.27 \pm _{{\rm sys}:0.53}^{{\rm rand}:0.17} \times {10^{- 31}}\;{{\rm cm}^2}/{\rm sr}$ and ${\sigma _{J = 2 \to 0}} = 2.11 \pm _{{\rm sys}:0.21}^{{\rm rand}:0.06} \times {10^{- 31}}\;{{\rm cm}^2}/{\rm sr}$, and the detection limit is estimated to be $1 \times {10^{15}}\;{{\rm cm}^{- 3}}$ for systems without other scattering species.

New Aspects of the Airglow Problem and Reactivity of the Dioxygen Quintet O2(5Πg) State in the MLT Region as Predicted by DFT Calculations

Dioxygen in the quintet O₂(⁵Π_g) state is a weakly bound species near the entrance of the O(³P) + O(³P) recombination channel. It was predicted by ab initio calculations in 1977 and detected experimentally in 1999. Meantime, the O₂(⁵Π_g) species was tentatively assumed as intermediate in transport properties calculations for the rarefied gases of the Earth's upper atmosphere, though its potential energy curve is still debated. Besides six other strongly bound low-lying states of dioxygen, the O₂(⁵Π_g) state is an important potential candidate for modeling energy transfer and airglow of the upper atmosphere. A number of photochemical kinetic schemes designed to simulate energy flow upon atomic and molecular oxygen collisions in the rarefied mesosphere take into account a participation of the O₂(⁵Π_g) state in energy relaxation processes responsible for terrestrial nightglow. All mechanisms of energy redistribution are based on the hard-sphere collision models. The possibility of chemical interactions between the quintet excited state of dioxygen and other atmospheric components has not been considered so far in photochemistry of the upper atmosphere. In the present paper, the chemical reactivity of the quintet O₂(⁵Π_g) species is calculated for the first time in the framework of the density functional theory. Definitely, O₂(⁵Π_g) is the most reactive species among all other metastable dioxygen states below 5.1 eV. Quintet products of the O₂(⁵Π_g) state association with heavy inert gases, H₂O, N₂, and CO₂ are predicted to be chemically significant, while the complexes with abundant H₂ and He species are rather weak and not important even in the mesopause low-temperature region. The complex with N₂ molecule is unexpectedly stable with dissociation energy 4 kJ/mol, which can strongly influence the abundant termolecular association O + O + N₂ → O₂ + N₂ process. Reaction with meteoritic ablated Mg atom produces metastable ⁵A₁ excited state of MgO₂ being more strongly bound than the ground ³A₂ state of magnesium peroxide.

Singlet Oxygen Generation via UV-A, -B, and -C Photoexcitation of Isoprene-Oxygen (C5H8-O2) Encounter Complexes in the Gas Phase

The formation of singlet oxygen ¹O₂ provided by the photoexcitation of the encounter complexes of isoprene with oxygen (C₅H₈-O₂) in the gas phase within the spectral region 253.5-355 nm has been observed at the elevated pressure of oxygen. Singlet oxygen has been detected with its NIR luminescence centered near 1.27 μm. The photogeneration of ¹O₂ is found to be a one-photon process. In the UV-C region (253-278 nm) the quantum yield of ¹O₂ is measured. This yield of ¹O₂ is governed mainly by photoexcitation of O₂ molecules to the Herzberg III (³Δ_u) state via enhanced absorption by C₅H₈-O₂ collision complexes. So excited triplet O₂ gives rise to singlet oxygen because of triplet-triplet annihilation in the collisions with unexcited O₂ molecules. In the UV-B (308 nm) region the appearance of ¹O₂ is attributed to the excitation of a double spin-flip (DSF) transition in complex C₅H₈-O₂. In the UV-A region (355 nm) besides DSF the O₂-assisted T₁ ← S₀ excitation of isoprene to the triplet state takes place, which is a sensitizer of ¹O₂ formation. The contribution of the encounter complexes C₅H₈-O₂ to the production of singlet oxygen and to the lifetime of isoprene in the Earth's troposphere are estimated.

Singlet (1Δg) O2 as an efficient tropospheric oxidizing agent: the gas phase reaction with the simplest Criegee intermediate

The reaction between CH₂OO and ¹Δ_g O₂ has been investigated by means of high level quantum chemical and chemical kinetic calculations. Post-CCSD(T) corrections in terms of full triplets and partial quadratic excitations, along with core corrections have been employed to estimate the reaction energetics. The title reaction was found to be effectively barrierless with the transition state lying -22.85 kcal mol^-1 below the isolated reactants. Rate coefficients under tropospheric conditions have been calculated using the master equation. The calculated rate coefficient was found to be marginally over the gas kinetic limit, implying that the reaction rate would be limited by the upper limit of bimolecular collision frequency. When compared against ˙OH and O₃, ¹O₂ was found to compete efficiently with the two well known tropospheric oxidants.

Reactivities of singlet oxygen: open-shell or closed-shell?

This work explores the reactivity of singlet oxygen with respect to two typical reactions: cycloaddition to anthracene and excitation energy transfer (EET) to a carotenoid using diabatic states with multistate density functional theory (MSDFT). Noticeably, the degenerate state 1Δg has distinct open-shell (OS) and closed-shell (CS) components, and the closed-shell component showed more reactivity than the open-shell one due to the strong diabatic couplings to the product diabatic states. The diabatic perspective presented in this work could also apply to general singlet fission processes.

Theoretical Investigations on the Possibility of Prebiotic HCN Formation via O-Addition Reactions

Until now, reactions between methane photolysis products (CH₃^•, CH₂) and active N atom or reactive NO radical are proposed as routes of HCN formation in the prebiotic Earth. Scientists think that the reducing atmosphere of primitive Earth was made of H₂, He, N₂, NO, CH₄, H₂O, CO₂, etc., and there was no molecular oxygen. However, it has been evident from experiments that the vacuum ultraviolet (VUV) photolysis of CO₂ can produce atomic oxygen. Therefore, it can be presumed that atomic oxygen was likely present in early Earth's atmosphere. Was there any impact of atomic oxygen in production of early atmospheric HCN for the emergence of life? To hunt for the answer, we have employed computational methods to study the mechanism and kinetics of CH₃NO + O(¹D) and CH₂NO^• + O(³P) addition reactions. Current study suggests that the addition of O(¹D) into nitrosomethane (CH₃NO) and the addition of O(³P) into nitrosomethylene radical (CH₂NO^•) can efficiently produce HCN through an effectively barrierless pathway. At STP, Bartis-Widom phenomenological loss rate coefficients of O(¹D) and O(³P) are obtained as 2.47 × 10^-12 and 4.67 × 10^-11 cm³ molecule^-1 s^-1, respectively. We propose that addition reactions of atomic oxygen with CH₃NO and CH₂NO^• might act as a potential source for early atmospheric HCN.

An unrestricted approach for the accurate calculation of the interaction potentials of open-shell monomers: The case of O2-O2

The properties of molecular oxygen including its condensed phases continue to be of great relevance for the scientific community. The richness and complexity of its associated properties stem from the fact that it is a very stable diradical. Its open-shell nature leads to low-lying multiplets with total electronic spin S = 0, 1, 2 in the case of the dimer, (O₂)₂, and the accurate calculation of the intermolecular potentials represents a challenge to ab initio electronic structure methods. In this work, we present intermolecular potentials calculated at a very high level, thus competing with the most accurate restricted potentials obtained to date. This is accomplished by drawing on an analogy between the coupled and uncoupled representations of angular momentum and restricted vs unrestricted methodologies. The S = 2 state can be well represented by unrestricted calculations in which the spins of the unpaired electrons are aligned in parallel; however, for the state where they are aligned in antiparallel fashion, it would seem that the total spin is not well defined, i.e., the well-known spin contamination problem. We show that its energy corresponds to that of the S = 1 state and perform unrestricted coupled cluster calculations for these two states. Then, we obtain the S = 0 state through the Heisenberg Hamiltonian and show that this is very reliable in the well region of the potentials. We make extensive comparisons with the best restricted potentials [Bartolomei et al., Phys. Chem. Chem. Phys. 10(35), 5374-5380 (2008)] and with reliable experimental determinations, and a very good agreement is globally found.

Model of Daytime Oxygen Emissions in the Mesopause Region and Above: A Review and New Results

Atmospheric emissions of atomic and molecular oxygen have been observed since the middle of 19th century. In the last decades, it has been shown that emissions of excited oxygen atom O(1D) and molecular oxygen in electronically–vibrationally excited states O2(b1Σ+g, v) and O2(a1Δg, v) are related by a unified photochemical mechanism in the mesosphere and lower thermosphere (MLT). The current paper consists of two parts: a review of studies related to the development of the model of ozone and molecular oxygen photodissociation in the daytime MLT and new results. In particular, the paper includes a detailed description of formation mechanism for excited oxygen components in the daytime MLT and presents comparison of widely used photochemical models. The paper also demonstrates new results such as new suggestions about possible products for collisional reactions of electronically–vibrationally excited oxygen molecules with atomic oxygen and new estimations of O2(b1Σ+g, v = 0–10) radiative lifetimes which are necessary for solving inverse problems in the lower thermosphere. Moreover, special attention is given to the “Barth’s mechanism” in order to demonstrate that for different sets of fitting coefficients its contribution to O2(b1Σ+g, v) and O2(a1Δg, v) population is neglectable in daytime conditions. In addition to the review and new results, possible applications of the daytime oxygen emissions are presented, e.g., the altitude profiles O(3P), O3 and CO2 can be retrieved by solving inverse photochemical problems when emissions from electronically vibrationally excited states of O2 molecule are used as proxies.

Plasma-Exposure-Induced Mobility Enhancement of LiTFSI-Doped Spiro-OMeTAD Hole Transport Layer in Perovskite Solar Cells and Its Impact on Device Performance

2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-OMeTAD) film currently prevails as hole transport layer (HTL) employed in perovskite solar cells (PSCs). However, the standard preparation method for spin-coated, Lithium bis(trifluoromethylsulfony) imide (LiTFSI)-doped, spiro-OMeTAD HTL depends on a time-consuming and uncontrolled oxidation process to gain desirable electrical conductivity to favor device operation. Our previous work demonstrated that ~10 s oxygen or oxygen containing gas discharge plasma exposure can oxidize spiro-OMeTAD HTL effectively and make PSCs work well. In this communication, hole-only devices are fabricated and in-situ current density-voltage measurements are performed to investigate the change in hole mobility of LiTFSI-doped spiro-OMeTAD films under plasma exposure. The results reveal that hole mobility values can be increased averagely from ~5.0 × 10^-5 cm²V^-1s^-1 to 7.89 × 10^-4 cm²V^-1s^-1 with 7 s O₂ plasma exposure, and 9.33 × 10^-4 cm²V^-1s^-1 with 9 s O₂/Ar plasma exposure. The effects on the photovoltaic performance of complete PSC devices are examined, and optical emission spectroscopy (OES) is used for a diagnostic to explain the different exposure effects of O₂ and O₂/Ar plasma. High efficiency, fine controllability and good compatibility with current plasma surface cleaning techniques may make this method an important step towards the future commercialization of photovoltaic technologies employing spiro-OMeTAD hole transport material.

Detection of the O2 A'3ΔU Herzberg III state by photofragment imaging

Photofragment imaging is shown to provide a sensitive method for detection of the O2 A'3Δu Herzberg III state using a one-laser dissociation/O(1D) resonance enhanced multiphoton ionization (REMPI) scheme with a focused nanosecond dye laser beam tuned to 203.8 or 205.2 nm, combined with velocity map imaging of the atomic oxygen photofragment. O2 populated in the Herzberg states is generated by photodesorption at 250 nm of solid O2 ice held at 15 K and by an electric discharge in a pulsed molecular beam of pure O2. Ice photo-desorption results in Herzberg state products with higher translational, vibrational and rotational energy spreads, yielding the same signal as the discharge source but with lower velocity resolution. A clear signal with parallel character (β ∼ 0.9) assigned to dissociation of O2 A'3Δu(v = 0, 1 Ω = 1) was observed when using a pulsed electric discharge source under specific 'cold' conditions with O(1D) detection, driving one-photon dissociation around 205 nm. No products corresponding to O2 A'3Δu state dissociation were observed for 225.625 or 200.32 nm dissociation with O(3P2) detection, which implies that the O2 A'3Δu state dissociates exclusively to the third (O1D + O1D) dissociation limit. Dissociation is suggested to take place through the 21Πg upper state to the O1D + O1D limit where spin-orbit coupling of the A'3Δu state with the 11Πu state accesses the allowed parallel 1Πu → 1Πg transition. While the absence of a parallel-type photodissociation signal from the c1Σ-u state may be expected, the A3Σ+u should spin-orbit couple through the same pathway as the A'3Δu state. The fact that no clear A3Σ+u signal is observed suggests a faster deactivation process compared to the A'3Δu state in the discharge and ice desorption process.

A previously unrecognized source of the O2 Atmospheric band emission in Earth's nightglow

Earth's night sky continuously produces a faint chemiluminescence known as nightglow. Two prominent nighttime emissions around 90 km are the O₂ Atmospheric and the OH Meinel band systems. Despite a plethora of studies since their identification seven decades ago, substantial gaps persist in our understanding of the mechanisms that control them. This report shows that oxygen atoms connect these two emissions: Fast, multiquantum, vibrational-to-electronic relaxation of OH(v) by O atoms activates a pathway that generates O₂ Atmospheric band emission. This newly discovered source exhibits a strong altitude dependence and can contribute a majority of the observed O₂ Atmospheric band emission when the peaks of the OH and O-atom layers overlap. The new findings call for a reinterpretation of Earth's nightglow emissions and a revision of relevant atmospheric models.

Rate constants for collision-induced emission of O2(a1Δg) with He, Ne, Ar, Kr, N2, CO2 and SF6 as collisional partners

Rate constants for singlet oxygen collision induced emission of the a1Δg-X3Σ-g transition at 1.27 μm were measured for CO2, N2, SF6, and rare gases as collisional partners. Photolysis of ozone by 266 nm laser radiation produced singlet oxygen. We performed direct measurements of pressure dependences of the 1.27 μm emission intensity for partner gases. The measured rate constants kMa-X in the units of 10-24 cm3 s-1 are as follows: CO2 - 10 ± 2; N2 - 3.2 ± 0.6; SF6 - 7 ± 1; He - 1.1 ± 0.3; Ne - 1.3 ± 0.3; Ar - 2.8 ± 0.6; Kr - 6 ± 1. The measured values of kMa-X are close to the values calculated from absorption measurements. Considering the known rate constants kMb-a for the b1Σg+-a1Δg transition in the gas phase we found that the ratio kMa-X/kMb-a was constant and independent of a collisional partner according to the "spin-orbit based" mechanism of intensity borrowing proposed by Minaev (THEOCHEM, 1989, 183, 207). However, this ratio amounted to (1.3 ± 0.2) × 10-4, which is considerably lower than the theoretically predicted value of (3-6) × 10-4.

Detailed kinetics of tetrafluoroethene ozonolysis

The C2F4 + O3 reaction plays an important role in the oxidation process of perfluoroalkenes in the atmosphere. The detailed reaction mechanism was explored using the accurate electronic structure method, CCSD(T)/CBS//B3LYP/aug-cc-pVTZ. The 1,3-cycloaddition of O3 with C2F4 to form the primary ozonide was found to be the rate-determining step of the oxidation process with a small barrier (i.e., 7.3 kcal mol-1 at 0 K). The temperature- and pressure-dependent behaviors of the title reaction were characterized in the range of 200-1000 K & 0.1-760 Torr using the integrated deterministic and stochastic master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) rate model with the inclusion of the corrections for anharmonicity and tunneling treatments. It is found that the anharmonic effect plays a role in the kinetic behaviors (e.g., lower the rate by a factor of ∼ two at 298 K) while the tunneling correction is insignificant. The total rate constants were found to be pressure-independent under the considered conditions, shown as ktot(T) = 4.80 × 10-23 × T2.69 × exp(-2983.4 K/T) (cm3 per molecule per s), which confirms the latest experimental data by Acerboni et al. (G. Acerboni, N. R. Jensen, B. Rindone and J. Hjorth, Chem. Phys. Lett., 1999, 309, 364-368); thus this study helps to resolve a long-term controversy among the previous measurements. The sensitivity analyses on the derived rate coefficients and time-resolved species mole fraction with respect to the ab initio input parameters were also performed to further understand as well as quantify the kinetic behaviors for the title reaction.

Laboratory Studies of Vibrational Excitation in O2( a1Δg, v) Involving O2, N2, and CO2

Collisional removal of electronic energy from O₂ in the low-lying a¹Δ_g state is typically an extremely slow process for the v = 0 level. In this study, we report results on the deactivation of O₂( a¹Δ_g, v = 1-3) in collisions with O₂ and CO₂. Ozone photodissociation in the 200-310 nm Hartley band is the source of O₂( a, v), and resonance-enhanced multiphoton ionization is used to probe the vibrational-level populations. Deactivation of the a( v = 1-3) levels in collisions with O₂ at 300 K is fast, with rate coefficients of (5.6 ± 1.1) × 10^-11, (3.6 ± 0.4) × 10^-11, and (1.9 ± 0.4) × 10^-11 cm³ s^-1 (2σ) for v = 1, 2, and 3, respectively. The relaxation process appears to involve a near-resonant electronic energy transfer pathway analogous to that observed in vibrationally excited O₂( b¹Σ_g⁺). With CO₂ collider gas, the removal rate coefficient at 300 K is (1.8 ± 0.4) × 10^-14 and (4.4 ± 0.6) × 10^-14 cm³ s^-1 (2σ) for v = 1 and 2, respectively. Despite the small mole fraction of O₂ in the atmospheres of Mars and Venus, O₂ is at least as important as CO₂ in the final stages of collisional relaxation within the O₂ vibrational-level manifold.

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.

O2(b1Σg+) Quenching by O2, CO2, H2O, and N2 at Temperatures of 300-800 K

Rate constants for the removal of O₂(b¹Σ_g⁺) by collisions with O₂, N₂, CO₂, and H₂O have been determined over the temperature range from 297 to 800 K. O₂(b¹Σ_g⁺) was excited by pulses from a tunable dye laser, and the deactivation kinetics were followed by observing the temporal behavior of the b¹Σ_g⁺-X³Σ_g^- fluorescence. The removal rate constants for CO₂, N₂, and H₂O were not strongly dependent on temperature and could be represented by the expressions k_CO2 = (1.18 ± 0.05) × 10^-17 × T^1.5 × exp[Formula: see text], k_N2 = (8 ± 0.3) × 10^-20 × T^1.5 × exp[Formula: see text], and k_H2O = (1.27 ± 0.08) × 10^-16 × T^1.5 × exp[Formula: see text] cm³ molecule^-1 s^-1. Rate constants for O₂(b¹Σ_g⁺) removal by O₂(X), being orders of magnitude lower, demonstrated a sharp increase with temperature, represented by the fitted expression k_O2 = (7.4 ± 0.8) × 10^-17 × T^0.5 × exp[Formula: see text] cm³ molecule^-1 s^-1. All of the rate constants measured at room temperature were found to be in good agreement with previously reported values.

Direct 1O2 optical excitation: A tool for redox biology

Molecular oxygen (O2) displays very interesting properties. Its first excited state, commonly known as singlet oxygen (1O2), is one of the so-called Reactive Oxygen Species (ROS). It has been implicated in many redox processes in biological systems. For many decades its role has been that of a deleterious chemical species, although very positive clinical applications in the Photodynamic Therapy of cancer (PDT) have been reported. More recently, many ROS, and also 1O2, are in the spotlight because of their role in physiological signaling, like cell proliferation or tissue regeneration. However, there are methodological shortcomings to properly assess the role of 1O2 in redox biology with classical generation procedures. In this review the direct optical excitation of O2 to produce 1O2 will be introduced, in order to present its main advantages and drawbacks for biological studies. This photonic approach can provide with many interesting possibilities to understand and put to use ROS in redox signaling and in the biomedical field.

Theoretical Study of the Reactions of Methane and Ethane with Electronically Excited N2(A(3)Σu(+))

Comprehensive quantum chemical analysis with the usage of density functional theory and post-Hartree-Fock approaches were carried out to study the processes in the N2(A(3)Σu(+)) + CH4 and N2(A(3)Σu(+)) + C2H6 systems. The energetically favorable reaction pathways have been revealed on the basis of the examination of potential energy surfaces. It has been shown that the reactions N2(A(3)Σu(+)) + CH4 and N2(A(3)Σu(+)) + C2H6 occur with very small or even zero activation barriers and, primarily, lead to the formation of N2H + CH3 and N2H + C2H5 products, respectively. Further, the interaction of these species can give rise the ground state N2(X(1)Σg(+)) and CH4 (or C2H6) products, i.e., quenching of N2(A(3)Σu(+)) by CH4 and C2H6 molecules is the complex two-step process. The possibility of dissociative quenching in the course of the interaction of N2(A(3)Σu(+)) with CH4 and C2H6 molecules has been analyzed on the basis of RRKM theory. It has been revealed that, for the reaction of N2(A(3)Σu(+)) with CH4, the dissociative quenching channel could occur with rather high probability, whereas in the N2(A(3)Σu(+)) + C2H6 reacting system, an analogous process was little probable. Appropriate rate constants for revealed reaction channels have been estimated by using a canonical variational theory and capture approximation. The estimations showed that the rate constant of the N2(A(3)Σu(+)) + C2H6 reaction path is considerably greater than that for the N2(A(3)Σu(+)) + CH4 one.

Voigt deconvolution method and its applications to pure oxygen absorption spectrum at 1270 nm band

Experimental spectral lines of pure oxygen at 1270 nm band were analyzed by Voigt deconvolution method. The method gave a total Voigt profile, which arises from two overlapping bands. Deconvolution of total Voigt profile leads to two Voigt profiles, the first as a result of O2 dimol at 1264 nm band envelope, and the second from O2 monomer at 1268 nm band envelope. In addition, Voigt profile itself is the convolution of Lorentzian and Gaussian distributions. Competition between thermal and collisional effects was clearly observed through competition between Gaussian and Lorentzian width for each band envelope. Voigt full width at half-maximum height (Voigt FWHM) for each line, and the width ratio between Lorentzian and Gaussian width (ΓLΓG(-1)) have been investigated. The following applied pressures were at 1, 2, 3, 4, 5, and 8 bar, while the temperatures were at 298 K, 323 K, 348 K, and 373 K range.

Luminescence of the (O2(a(1)Δ(g)))2 collisional complex in the temperature range of 90-315 K: Experiment and theory

Experimental and theoretical studies of collision induced emission of singlet oxygen molecules O2(a(1)Δg) in the visible range have been performed. The rate constants, half-widths, and position of peaks for the emission bands of the (O2(a(1)Δg))2 collisional complex centered around 634 nm (2) and 703 nm (3) have been measured in the temperature range of 90-315 K using a flow-tube apparatus that utilized a gas-liquid chemical singlet oxygen generator. The absolute values of the spontaneous emission rate constants k2 and k3 are found to be similar, with the k3/k2 ratio monotonically decreasing from 1.1 at 300 K to 0.96 at 90 K. k2 slowly decreases with decreasing temperature but a sharp increase in its values is measured below 100 K. The experimental results were rationalized in terms of ab initio calculations of the ground and excited potential energy and transition dipole moment surfaces of singlet electronic states of the (O2)2 dimole, which were utilized to compute rate constants k2 and k3 within a statistical model. The best theoretical results reproduced experimental rate constants with the accuracy of under 40% and correctly described the observed temperature dependence. The main contribution to emission process (2), which does not involve vibrational excitation of O2 molecules at the ground electronic level, comes from the spin- and symmetry-allowed 1(1)Ag←(1)B3u transition in the rectangular H configuration of the dimole. Alternatively, emission process (3), in which one of the monomers becomes vibrationally excited in the ground electronic state, is found to be predominantly due to the vibronically allowed 1(1)Ag←2(1)Ag transition induced by the asymmetric O-O stretch vibration in the collisional complex. The strong vibronic coupling between nearly degenerate excited singlet states of the dimole makes the intensities of vibronically and symmetry-allowed transitions comparable and hence the rate constants k2 and k3 close to one another.

Effect of operational conditions on sonoluminescence and kinetics of H2O2 formation during the sonolysis of water in the presence of Ar/O2 gas mixture

Ultrasonic frequency is a key parameter determining multibubble sonoluminescence (MBSL) spectra of water saturated with Ar/O2 gas mixtures. At 20 kHz, the MBSL is quenched by oxygen. By contrast, at high-frequency ultrasound the maximal MBSL intensity is observed in the presence of Ar/20%O2 gas mixture. Nevertheless, oxygen has no influence on the shape of MBSL spectra. The effect of oxygen on MBSL is explained by oxygen dissociation inside the collapsing bubble which is much more effective at high ultrasonic frequency compared to 20 kHz ultrasound. In contrast to MBSL, a higher yield of H2O2 is observed in Ar/20%O2 gas mixture whatever the ultrasonic frequency. At 20°C and 20% of oxygen the maximal yield of H2O2 is observed at 204-362 kHz. The maximal yield of H2O2 is shifted to 613kHz when the bulk temperature is raised up to 40°C. Coupling of high-frequency ultrasound with mechanical stirring and intensive Ar/O2 bubbling improves H2O2 production. Comparison of MBSL and sonochemistry allowed to conclude that H2O2 is formed from non-excited OH (X(2)Π) and HO2 radicals. Finally, it was shown that at the studied conditions the efficiency of ultrasonic degassing is hardly influenced by frequency.

Physics and chemistry of the influence of excited molecules on combustion enhancement

The paper addresses detailed analysis of kinetic processes in the H(2)-O(2), CO-O(2) and CH(4)-O(2)-reactive systems upon the presence of singlet oxygen molecules O(2)(a(1)Δg) and [Formula: see text] and the influence of the activation of oxygen molecules in electric discharge on the acceleration of ignition in the H(2)-O(2) and CH(4)-O(2) mixtures. The possibility of the intensification of CO oxidation due to excitation of O(2) and N(2) molecule vibrations and generation of singlet oxygen molecules is also considered. It is shown that the effect of accelerating the ignition strongly depends on the reduced electric field and, as a consequence, on the composition of discharge plasma as well as on the features of chain mechanism development in oxy-fuel systems. It is revealed that the most effective approach for the intensification of CO oxidation both in the moist air and in the products of hydrocarbon combustion in air is the generation of O(2)(a(1)Δg) molecules by electric discharge. Computations showed that the presence of 1% O(2)(a(1)Δg) in the total oxygen allowed one to convert CO to CO(2) even at the temperature T=850-900 K in the time of 10(-2) s. The excitation of O(2) and N(2) molecule vibrations is less effective for such a conversion.

Visible-ultraviolet vibronic emission of silica nanoparticles

We report the study of the visible-ultraviolet emission properties and the structural features of silica nanoparticles prepared through a laboratory sol–gel technique.

Vibrational relaxation of O2(X3Σ(-)g, v = 6-8) by collisions with O2(X3Σ(-)g, v = 0): solution of the problems in the integrated profiles method

The linear kinetic analysis called the integrated profiles method (IPM) makes it simple to analyze the multistep relaxation processes of vibrational manifold. The problem that plots for linear regression in the IPM analysis cannot be made, however, has been found in the study of self relaxation of O2(X3Σ(-)g, v = 6-8). The cause of the problem is the identical time-dependence of the populations of the adjacent vibrational levels. An addition of CF4 into the system made a difference in the time profiles and enabled us to make IPM analysis and determine the rate coefficients. In the experiments, a gaseous mixture of O3/O2/CF4 in an Ar carrier at 298 K was irradiated at 266 nm, and the direct photoproduct O2(X3Σ(-)g, v = 6-9) from O3 was detected by laser-induced fluorescence (LIF)in the B3Σu-X3Σ(-)g transition. Time-resolved LIF intensities of O2(X3Σ(-)g, v) at various pressures of O2 and fixed pressure of CF4 were recorded. The resulting rate coefficients for v = 6−8 correlate smoothly with those for v ≤ 5 and v ≥ 8 reported previously.The vibrational-level dependence (v = 2-13) of the rate coefficients for relaxation of O2(X3Σ(-)g, v) by O2 is accounted for by the balance between the harmonic transition probabilities and the energy defect in the V-V energy-transfer mechanism.