States Of O2 Research Articles

Global Model in Low-Pressure Oxygen Discharges

We use a global model (volume averaged) to study plasma discharges in molecular oxygen gas in the pressure range of 1- 78 mTorr. This model determines positive ions O2+, O+, negative ion O-, and electron densities, ground state O2 and O atoms and metastable O2(a1Δg) and O(1D) densities, and electron temperature as function of gas pressure and input power, for a cylindrical discharge. We apply the model to O2 discharges and the results are compared to experimental data and a more elaborated volume-averaged global model developed at the University of California at Berkeley.

Global model simulations of low-pressure oxygen discharges

We use a global model (volume averaged) to study plasma discharges in molecular oxygen gas in the 1-100 mTorr pressure range. This model determines densities of positive ions O2+ and O+ , negative ion O- , electrons, ground state O2 and O atoms, and metastables O2(a¹deltag) and O(¹D), and electron temperature as function of gas pressure and input power, for a cylindrical discharge. We apply the model to O2 discharges and the results are compared to a particle-in-cell simulation (PIC), experimental data and a volume-averaged global model developed at the University of California at Berkeley. We find that the total positive ion density increases with pressure at low pressures (up to approximately 30 mTorr), and decreases at higher pressures. The electronegativity decreases with increased power and increased pressure as predicted by the global models presented in the literature. The predictions for electron temperature are also in agreement with these models. However, there is a discrepancy betweeen these global models and PIC simulations and experimental data, for 20 and 40 mTorr cases, concerning electronegativity calculations. PIC simulations yield much higher electronegativities. There are strong indications that this is due to the assumption of Maxwellian electron energy distribution functions in the global model, while in the PIC simulations this is clearly not the case.

Model of daytime emissions of electronically-vibrationally excited products of O<sub>3</sub> and O<sub>2</sub> photolysis: application to ozone retrieval

Abstract. The traditional kinetics of electronically excited products of O3 and O2 photolysis is supplemented with the processes of the energy transfer between electronically-vibrationally excited levels O2(a1Δg, v) and O2(b1Σ+g, v), excited atomic oxygen O(1D), and the O2 molecules in the ground electronic state O2(X3Σg−, v). In contrast to the previous models of kinetics of O2(a1Δg) and O2 (b1Σ+g), our model takes into consideration the following basic facts: first, photolysis of O3 and O2 and the processes of energy exchange between the metastable products of photolysis involve generation of oxygen molecules on highly excited vibrational levels in all considered electronic states – b1Σ+g, a1Δg and X3Σg−; second, the absorption of solar radiation not only leads to populating the electronic states on vibrational levels with vibrational quantum number v equal to 0 – O2(b1Σ+g, v=0) (at 762 nm) and O2(a1Δg, v=0) (at 1.27 µm), but also leads to populating the excited electronic–vibrational states O2(b1Σ+g, v=1) and O2(b1Σ+g, v=2) (at 689 nm and 629 nm). The proposed model allows one to calculate not only the vertical profiles of the O2(a1Δg, v=0) and O2(b1Σ

Ab initio potential-energy surfaces of O2(XΣg−3,aΔg1,bΣg+1)+O2(XΣg−3,aΔg1,bΣg+1): Mechanism of quenching of O2(aΔg1)

Ab initio computational studies were carried out in order to explore the possible mechanisms of quenching of O2(aΔg1) by O2(XΣg−3): the self-quenching of O2(aΔg1) and other energy-transfer processes involving two O2 molecules. All eighteen states arising from two O2 molecules in the XΣg−3, aΔg1, and bΣg+1 states are considered. After scans at the state-averaged complete active space self-consistent field method to identify possible regions of crossing between states belonging to different asymptotes, complete active state second-order perturbation theory high-symmetry optimization and low-symmetry scans established that four different minima on the seams of crossing (MSXs), arising between the a+a manifold and the X+b manifold and responsible for self-quenching: O2(aΔg1)+O2(aΔg1)→O2(XΣg−3)+O2(bΣg+1), have coplanar C2h or C2v symmetries and are only 0.45eV barrier relative to the a+a asymptote and energetically easily accessible. The rate constant for this process was estimated based on the Landau-Zener formalism. The MSXs for quenching of O2(aΔg1) by the ground state O2(XΣg−3):O2(aΔg1)+O2(XΣg−3)→O2(XΣg−3)+O2(XΣg−3) require higher energies and the process is not likely to be important.

Threshold ionization mass spectrometry study of singlet molecular oxygen in the deposition of SnO2 by PACVD

Threshold ionization mass spectrometry (TIMS) has been used to measure the excited molecular oxygen states O2 (1Δg) and during plasma-assisted chemical vapour deposition of tin oxide (SnO2) thin films. The latter, composed of nanosized features, was deposited by feeding in a mixture of Ar, O2 and tetramethyltin (Sn(CH3)4 or TMT) in a capacitively coupled RF discharge reactor. Langmuir probe measurements were performed along with TIMS to measure the electron temperature and density. The correlations between these two diagnostic methods have been investigated. The observed densities of O2 (1Δg) and in the γ mode of the discharge are maximum at a low electron temperature and high density. Furthermore, these results have been shown to be correlated to the trend of the electronic conductivity of the deposited SnO2 thin films.

Antioxidant Defenses in Fish: Biotic and Abiotic Factors

Oxygen in its molecular state O2, is essential for many metabolic processes that are vital to aerobic life. Aerobic organisms cannot exist without oxygen, which nevertheless is inherently dangerous to their lives. Like all aerobic organisms, fish are also susceptible to the effects of reactive oxygen and have inherent and effective antioxidant defenses that are well described in the literature. This review investigates the influence of different biotic and abiotic factors (age, phylogenetic position, feeding behavior, environmental factors, oxygen, temperature, presence of xenobiotics) on antioxidant defenses in fish. Studies of antioxidant activity in fish open a number of novel research lines providing greater knowledge of fish physiology, which will benefit various aspects of fish farming and artificial production.

Pulsed electron-beam-sustained discharge in oxygen-containing gas mixtures: electrical characteristics, spectroscopy,and singlet oxygen yield

The electrical and spectroscopic characteristics of electron-beam-sustained discharge (EBSD) in oxygen and oxygen-containing gas mixtures are studied experimentally under gas pressures up to 100 Torr in a large excitation volume (∼18 L). It is shown that the EBSD in pure oxygen and its mixtures with inert gases is unstable and is characterised by a small specific energy contribution. The addition of small amounts (∼1%-10%) of carbon monoxide or hydrogen to oxygen or its mixtures with inert gases considerably improves the stability of the discharge, while the specific energy contribution W increases by more then an order of magnitude, achieving ∼6.5 kJ L-1 atm-1 per molecular component of the gas mixture. A part of the energy supplied to the EBSD is spent to excite vibrational levels of molecular additives. This was demonstrated experimentally by the initiation of a CO laser based on the O2 : Ar : CO = 1 : 1 : 0.1 mixture. Experimental results on spectroscopy of the excited electronic states O2(a1Δg) and O2(b1Σg+), of oxygen formed in the EBSD are presented. A technique was worked out for measuring the concentration of singlet oxygen in the O2(a1Δg) state in the afterglow of the pulsed EBSD by comparing with the radiation intensity of singlet oxygen of a given concentration produced in a chemical generator. Preliminary measurements of the singlet-oxygen yield in the EBSD show that its value ∼3% for W ∼ 1.0 kJ L-1 atm-1 is in agreement with the theoretical estimate. Theoretical calculations performed for W ∼ 6.5 kJ L-1 atm-1 at a fixed temperature show that the singlet-oxygen yield may be ∼20%, which is higher than the value required to achieve the lasing threshold in an oxygen—iodine laser at room temperature.

Transmission of oxygen concentration fluctuations through the diffusive boundary layer overlying aquatic sediments

Even though the O2 concentration in the diffusive boundary layer overlying natural aquatic sediments usually varies over time, microprofiles with little or no error bars are abundant in the literature. These temporal concentration fluctuations are not accounted for in the conceptual models used to describe the diffusive boundary layer, and their effects on phenomena investigated through microprofile measurements are unclear. This paper focuses on concentration fluctuations in the diffusive boundary layer above a marine sediment. One microsensor was inserted into a laboratory flume from below and positioned with the sensing tip precisely at the sediment surface. A second microelectrode was deployed from above through the flowing water column and positioned within the diffusive boundary layer directly above the tip of the lower sensor. Time series of the fluctuating O2 concentration at the two points were measured simultaneously with the two sensors and demonstrated a tight coupling of concentration fluctuations across the diffusive boundary layer. A dynamic model accounting only for molecular diffusion plus zero‐order O2 consumption within the sediment explained the observed coupling through the diffusive boundary layer and enabled calculation of instantaneous O2 profiles from the top of the true diffusive boundary layer to the depth of O2 penetration. Model results confirmed that concentration fluctuations were linked from the top of the true diffusive boundary layer down to about 0.5 mm sediment depth. The transient profiles moved back and forth, without losing the general shape derived from the averaged steady state O2 distribution. Apart from this description of the temporal structure of the diffusive boundary layer, the model shows that concentration fluctuations in the layer do not necessarily indicate turbulence within the true diffusive boundary layer. This dynamic description is used to discuss experimental procedures when measuring microprofiles.

The formation of fluorescent and metastable fragments by photoexcitation of somediatomic molecules in the vacuum ultraviolet region

The formation of fluorescent and metastable fragments from four diatomic molecules,i.e. O2, N2, NO and CO, has been investigated in the vacuum ultraviolet region. The neutralparticles are detected by using a microchannel plate stack, where retardingelectric potentials prevent charged particles from reaching the detector. Everydiatomic molecule investigated here shows the formation of fluorescent andmetastable fragments in particular photon energy regions. Three Rydberg states ofO2 convergingto O2+(a 4Πu) undergo both neutral dissociation processes forming fluorescent[O+(4S)]3s 3S1 andmetastable [O+(4S)]3s 5S2; the yield curve for each fragment is determined. Direct and cascade formation of the fluorescent[N+(3P)]3s 4P fragmentfrom N2 are separated, and it is found that the dissociation of the4s σ Rydberg state convergingto N2+(C 2Σu+) preferentiallyproduces [N+(3P)]3p fragments,but not or weakly the [N+(3P)]3s 4P fragment. High -n Rydbergstates converging to NO+(c 3Π) and CO+(D 2Π) undergo neutral dissociation described by the core ion model, resulting in large peaks forneutral particle formation.

Emission Spectra and Relaxation Mechanism of b State O2 in Low Temperature Ar Solids

Abstract The b → X emission of electronically excited O2 isolated in Ar solids was observed under the pulsed KrF excimer laser irradiation at 248 nm. The relaxation proceeds along the b → a → b interstate transition. The time evolution of each b → X (v′, v′′) band was fitted by multi-exponential curves. The nonradiative rate is limited at the b → a transition due to the energy gap, which is larger than that of the a → b transition. The relaxation rate constants of individual vibrational levels (v ≤ 8) of the b state were determined. The vibrational quantum number and temperature dependence of the nonradiative rate constants could be explained based on a multiphonon relaxation theory.

De-excitation dynamics of Rydberg states in O2:I. Total cross sections for O I fluorescence emissionfollowing predissociation of 2σ-1u(c4Σ-u)nσg3Σ-u states

Absolute cross sections for the total fluorescence emissionbetween 97 and 131 nm by atomic oxygen formed after thephotoexcitation of 2σ-1u(c4Σ-u)nσg3Σ-u Rydberg states inO2 and their successive predissociation were measured forthe first time. The dissociation lifetimes and fluorescenceyields were estimated using a stepwise model and the potentialcurve of the O2+2σ-1u(c4Σ-u)state known from Beebe et al (Beebe N H F,Thulstrup E W and Andersen A 1976 J. Chem. Phys. 64 2080).The calculated lifetimes of the vibrational levels v = 0 and1 of the 2σ-1u(c4Σ-u) state are3.4×10-12 and 6.3×10-14 s, respectively.They agree with the measured data of Evans et al (Evans M, Stimson S, Ng C Yand Hsu C-W 1998 J. Chem. Phys. 109 1285) and are consistent withthe results of the present experiment. The calculated lifetimessupport the model of `fast' dissociation of theO2+2σ-1u(c4Σ-u),v states in contrast to the earlier model of`slow' dissociation of Tanaka and Yoshimine (Tanaka K andYoshimine M 1979 J. Chem. Phys. 70 1626).

Benthic diatoms of a high Arctic fjord (Young Sound, NE Greenland): importance for ecosystem primary production

Patches of benthic diatoms covered 23 to 73% of the sediment area at water depths down to 30 m of a high Arctic fjord system (Young Sound, NE Greenland). Mapping of the in situ chlorophyll (chl a) fluorescence by a pulse amplitude modulated (PAM) fluorometer demonstrated the presence of diatom-covered sediment patches with a characteristic size scale of 5 to 8 m. The benthic diatoms were well adapted to the ambient irradiance as demonstrated by in situ measurements of the relative elec- tron transport rate (ETR) versus irradiance. However, the characteristics of the PE (photosynthesis ver- sus downwelling irradiance) relations changed within minutes after exposure to changed light condi- tions neutralizing any depth related light acclimation. This demonstrated that the diatoms efficiently optimized their photosynthetic apparatus to the current light conditions. Steady state O2 microprofiles measured in situ and in the laboratory showed a gradual increase in the O2 penetration depth and O2 concentration at the sediment surface with increasing irradiance. Net photosynthesis showed a com- pensation irradiance of 4.5 µmol photons m -2 s -1 and a net photosynthetic capacity (Pmax) of 85 O2 mmol m -2 d -1 . No photoinhibition was observed at the maximum irradiance of 140 µmol photons m -2 s -1 . To- tal exchange rates of O2 and DIC mirrored each other and confirmed the depth distribution of benthic diatoms and the microsensors measurements. The compensation irradiance for the integrated benthic community (including macrofauna) decreased with water depth to a minimum of 12 µmol photons m -2 s -1 at 30 m water depth. The mean O2/DIC exchange ratio across the benthic interface was approxi- mately 1.2 and was independent of water depth. Extrapolation of our productivity data to the outer fjord system by accounting for the in situ light regime showed that the primary production of the ben- thic diatoms was equivalent to that of benthic macroalgae. Benthic net photosynthesis was almost 7 times higher than the gross photosynthetic rates of the pelagic community for water depths <30 m. However, integrated for the entire outer Young Sound, benthic net photosynthesis only accounted for ~70% of the pelagic production as measured by the 14 C-incubation technique.

The role of O2 (a1Δg) metastables and associativedetachment in discharges in oxygen

A model of discharges in oxygen is developed which takes intoconsideration the role that the long-lived metastable state O2 (a1Δg)can play in their dynamics. This enables the electron temperature (Te)versus pressure×plasma dimension (pL) characteristicto be found over the range 1 mTorr cm to 1 Torr cm. The conclusion isreached that only at the very low pressure end andhigh electron densities is oxygen a recombination-dominated gas and thatgenerally at higher pressures it is detachment dominated. Inbetween, both recombination and detachment are important to describe theloss of negative ions. The paper brings together two separate approaches todischarges in oxygen and demonstrates that each has its own place, while atthe same time making it clear that oxygen is a special case rather than anexemplar for electronegative gases in general.

State Selective Electron Capture by State Prepared Beams of Multiply Charged Ions in Atomic Hydrogen

State selective electron capture by state prepared ground state ions of C2+, N2+ and O2+ in collisions with atomic hydrogen has been studied in the energy range 1.5–6 keV using the technique of double translational energy spectroscopy. In the case of C2+/H system it is clear from this comparison that whilst experiment and theory agree that the C2+(2s2p2) 2D state is the dominant product state there are considerable differences for the other C2+ product states. For the N2+/H system the experimental measurements confirm the predicted dominance of N+(2s2p2) 3Do formation but show that significant yields of other product states are formed. The only significant O+ product state resulting from the one-electron capture process by ground state O2+ 3P with H at 6 keV is O+ (2s2p4) 4P confirming theoretical calculations.

Nonresonant photofragmentation/ionization dynamics of O2 using picosecond and femtosecond laser pulses at 248 nm

Photodissociation/ionization of molecular oxygen in a cold molecular beam is studied using a short-pulse laser beam at 248 nm and velocity map imaging. Both photoelectron and O+ images are recorded for laser pulsewidths of 5 and 0.5 ps. Most of the observed ionization dynamics takes place after absorption of four laser photons, equivalent to 20 eV excitation energy, in an above threshold ionization process. Two main channels are identified: postdissociative ionization creating O(3P)+O**3s″(1P, 3P) atoms where O** is an electronically excited autoionizing atom, and molecular (auto)ionization to create a range of highly vibrationally excited ground electronic state O2+ ions. The observed O+ signals then arise from resonance-enhanced two-photon dissociation of O2+ or autoionization of O** atoms, while the electron signals arise from ionization of O2 or autoionization of O**. The latter channel can be used to directly scale the photoelectron and O+ signal strengths. The O+ images show strong differences for 0.5 ps and 5 ps pulse which could arise from ac Stark shifting of levels involved in the A 2Πu←X 2Πg transition of O2+. With resonance enhancement, two-photon dissociation of O2+ is the dominant process producing O+ ions. The O+ angular distributions show an anisotropy that is more extreme than a simple two-step dissociation, which is attributed to alignment effects.

KrF Excimer Laser-induced Ozone Formation in Supercritical Carbon Dioxide

Laser-induced reactions by a pulsed KrF excimer laser were studied using UV absorption spectroscopy in sub- and supercritical O2/CO2 mixtures up to the pressure of 15 MPa (corresponding density, 17 mol dm-3). Although the 248 nm excimer laser photon energy is smaller than the energy required for dissociating O2, ozone formation was observed in O2/CO2 mixtures. Under the laser irradiation, O3 concentration increased monotonically with the increase of the irradiation time and then stayed constant, which is satisfactorily expressed by the equation d[O3]/dt = a − b[O3]. a corresponds to O3 formation rate and b to O3 decomposition rate constant. The value of a increased with the increase of CO2 density up to 3 mol dm-3 and was then kept almost constant with further increase. O2 absorbs a photon to yield an oxygen molecule in the Herzberg III state O2(A‘ 3Δu), being augmented along with the increase of CO2 density. In pure O2, the predominant pathway of O3 formation is the reaction between excited O2 in Herzberg states and ground state O2 to yield O3 and atomic oxygen. In high-density O2/CO2 mixtures, O3 is considered to be produced through reaction between the Herzberg states O2 and CO2. Taking account of the quenching effect for the above reaction together with the augmentation of O2 absorption of laser light by the high-density CO2, the behavior of a with respect to CO2 density was satisfactorily explained. The behavior of b suggested a certain inhibition of O3 recovery in high-density CO2 after the photodecomposition of the product O3, which was ascribed to the formation of CO3 from the O(1D) reaction with CO2. A certain cage effect for the O3 photodecomposition was also suggested. No specific pressure effect was observed near the critical point.

Post-column photochemical reaction detection in liquid chromatography based on photosensitized generation of singlet molecular oxygen: Study of reaction parameters and application to the determination of polychlorinated biphenyls

The parameters effecting the sensitivity and selectivity of a photochemical reaction detection scheme based on the reaction of 3-substituted pyrroles with singlet molecular oxygen (1O2) in HPLC are reported. Polychlorinated biphenyls (PCBs) were chosen as model compounds for the detection scheme. Following separation by reverse-phase chromatography, PCBs are excited by a Hg pen-ray lamp in a crocheted PTFE photochemical reactor. PCBs that are efficient1O2-sensitizers promote ground state O2 (3Σ-3) to an excited state (1Σg+ or1Δg) which rapidly oxidizes 3-substituted pyrroles which are added to the mobile phase. Detection is based on the loss of pyrrole. The reaction is catalytic in nature since one analyte molecule may absorb light many times, producing large amounts of1O2. Detection limits for 4,4′-dichlorobiphenyl and Aroclors 1242, 1248 and 1254 were improved by 1–2 orders of magnitude over optimized UV-absorbance detection. Configuration of the photochemical reactor and judicious choice of the wavelength used to follow the loss of pyrrole were determined to be the most important factors in terms of sensitivity of the detection scheme. Comparison of three reagents (i.e. 3-substituted-pyrroles) used for trapping1O2 demonstrates the effect of substitution on sensitivity and to some extent selectivity of PCB determination.

Non-steady state monitoring by respiratory gas exchange.

Traditionally, the study of CO2 and O2 kinetics in the body has been mostly confined to equilibrium conditions. However, the peri-anesthesia period and the critical care arena often involve conditions of non-steady state. The detection and explanation of CO2 kinetics during non-steady state pathophysiology have required the development of new methodologies, including the CO2 expirogram, average alveolar expired PCO2, and CO2 volume exhaled per breath. Several clinically relevant examples of non-steady state CO2 kinetics perturbations are examined, including abrupt decrease in cardiac output, application of positive end-expiratory pressure during mechanical ventilation, and occurrence of pulmonary embolism. The lesser known area of non-steady state O2 kinetics is introduced, including the measurement of pulmonary O2 uptake per breath. Future directions include the study of the respiratory quotient per breath, where the anaerobic threshold during anesthesia is identified by increasing respiratory quotient.

Photochemical Reaction of Ozone and Dimethylacetylene: An Infrared Matrix Isolation and ab Initio Investigation

The photolysis of Ar/ozone/dimethylacetylene matrices (200/1/1) at λ ≥ 280 nm and 12 K yields the following stable products: dimethylketene, the E and Z configurational isomers of methyl vinyl ketone, butane-2,3-dione, and acetic anhydride. The reaction of O(3P) with dimethylacetylene to form products of molecular formula C4H6O, including dimethyloxirene, was modeled using the second-order perturbation and CCSD(T) theories. In all cases, it is concluded that spin crossing takes place early on the reaction surface to take advantage of low singlet state activation barriers. The formation of butane-2,3-dione from singlet state O2 molecules and dimethylacetylene, as well as the photolysis of DMA/O3 reactant pairs, is discussed.

Unraveling the mysteries of metastable O4*

A recent report from our laboratory described 1+1 resonant photoionization spectra of an energetic, metastable O4 species produced in a dc discharge [Bevsek et al., Faraday Discuss. 108, 131 (1998)]. Although a definitive assignment of the spectra was lacking, the long-predicted covalent forms of O4, either cyclic (D2d) or “pinwheel” (D3h), were adduced as possible candidates. We here present rotationally resolved photoionization spectra, photoelectron spectra, and ab initio calculations providing strong evidence for the identity of this species as a novel complex between a ground state O2 molecule and one in the c(1Σu−) state, which is excited via an allowed transition to the 1(1Πg)valence state. The latter then couples to the d(1Πg) Rydberg state, shifted in energy owing to the presence of the adjacent O2 molecule, from which it then ionizes. The results underscore the potential importance of the fully allowed but overlooked 1(1Πg)←c(1Σu−) electronic transition in O2 in the near ultraviolet, and provide a simple interpretation of puzzling results in an earlier study of electron transfer to O4+ [H. Helm and C. W. Walter, J. Chem. Phys. 98, 5444 (1993)].