Ground State Oxygen Atoms Research Articles

Quantitative measurements of oxygen atom and negative ion densities in a low pressure oxygen plasma by cavity ringdown spectroscopy

In this paper we report measurements of the absolute concentration of ground state oxygen atoms produced in a low pressure (≤100 mTorr) inductively coupled oxygen plasma. These experiments have utilised cavity ringdown spectroscopy, allowing line of sight absorption to be measured on the optically forbidden 1D ← 3P transition around 630 nm. Both the translational temperature and the absolute concentrations of the two most populated spin–orbit levels (J = 1 and 2) have been determined as a function of plasma pressure at a fixed operating power of 300 W, allowing accurate determination of dissociation fraction; in all cases, the dissociation fraction is considerable, ≥10%, maximising at 15% for 20 mTorr. Time-resolved measurements of the rate of loss of the oxygen atoms when the plasma is extinguished have allowed the probability for wall-loss in the plasma chamber, γ, to be determined; in this case, for an aluminium surface, γ is determined to be ca. a few ×10−3, with the exact value depending on pressure. In addition, the O− number density is shown to be an inverse function of pressure, showing a maximum of 1.6 × 1010 cm−3 at 10 mTorr, falling to 2 × 109 cm−3 at 100 mTorr, and characteristic of a discharge operating in the detachment regime. The measured number densities are interpreted using calculated electron energy distribution functions and yield physically reasonable values for the electron number density.

Potential for the measurement of mesosphere and lower thermosphere (MLT) wind, temperature, density and geomagnetic field with Superconducting Submillimeter-Wave Limb-Emission Sounder 2 (SMILES-2)

Abstract. Submillimeter-Wave Limb-Emission Sounder 2 (SMILES-2) is a satellite mission proposed in Japan to probe the middle and upper atmosphere (20–160 km). The main instrument is composed of 4 K cooled radiometers operating near 0.7 and 2 THz. It could measure the diurnal changes of the horizontal wind above 30 km, temperature above 20 km, ground-state atomic oxygen above 90 km and atmospheric density near the mesopause, as well as abundance of about 15 chemical species. In this study we have conducted simulations to assess the wind, temperature and density retrieval performance in the mesosphere and lower thermosphere (60–110 km) using the radiometer at 760 GHz. It contains lines of water vapor (H2O), molecular oxygen (O2) and nitric oxide (NO) that are the strongest signals measured with SMILES-2 at these altitudes. The Zeeman effect on the O2 line due to the geomagnetic field (B) is considered; otherwise, the retrieval errors would be underestimated by a factor of 2 above 90 km. The optimal configuration for the radiometer’s polarization is found to be vertical linear. Considering a retrieval vertical resolution of 2.5 km, the line-of-sight wind is retrieved with a precision of 2–5 m s−1 up to 90 km and 30 m s−1 at 110 km. Temperature and atmospheric density are retrieved with a precision better than 5 K and 7 % up to 90 km (30 K and 20 % at 110 km). Errors induced by uncertainties on the vector B are mitigated by retrieving it. The retrieval of B is described as a side-product of the mission. At high latitudes, precisions of 30–100 nT on the vertical component and 100–300 nT on the horizontal one could be obtained at 85 and 105 km (vertical resolution of 20 km). SMILES-2 could therefore provide the first measurements of B close to the electrojets' altitude, and the precision is enough to measure variations induced by solar storms in the auroral regions.

In vitro oxidations of low-density lipoprotein and RAW 264.7 cells with lipophilic O(3P)-precursors.

A beneficial property of photogenerated reactive oxygen species (ROS) is the capability of oxidant generation within a specific location or organelle inside a cell. Dibenzothiophene S-oxide (DBTO), which is known to undergo a photodeoxygenation reaction to generate ground state atomic oxygen [O(3P)] upon irradiation, was functionalized to afford localization within the plasma membrane of cells. The photochemistry, as it relates to oxidant generation, was studied and demonstrated that the functionalized DBTO derivatives generated O(3P). Irradiation of these lipophilic O(3P)-precursors in the presence of LDL and within RAW 264.7 cells afforded several oxidized lipid products (oxLP) in the form of aldehydes. The generation of a 2-hexadecenal (2-HDEA) was markedly increased in irradiations where O(3P) was putatively produced. The substantial generation of 2-HDEA is not known to accompany the production of other ROS. These cellular irradiation experiments demonstrate the potential of inducing oxidation with O(3P) in cells.

Rate rules for the reactions of oxygen atoms with terminal alkenes

The reaction of ground state atomic oxygen with terminal alkenes plays an important role in both atmospheric and combustion chemistry. Because of its complicated nature, which involves intersystem crossing from the triplet to the singlet potential energy surface, its channel specific reactivity has been explored only for a few species in restricted temperature and pressure ranges. Here, we report results of crossed molecular beam experiments and theoretical computations for the O(3P) + 1-butene reaction. It is found that traditional rate rules methods where the reactivity of larger size molecules is extrapolated from simpler homologues is not valid for this reaction class. A viable alternative is proposed that allows to predict the reactivity of the whole O(3P) + terminal alkenes reaction class across wide temperature and pressure ranges, encompassing atmospheric and combustion conditions. The present results suggest that all O(3P) reactions with unsaturated hydrocarbons implemented with extrapolated rate constants in kinetic mechanisms should be revised.

Molecular beam studies of elementary reactions relevant in plasma/combustion chemistry: O(3P) + unsaturated hydrocarbons

The identification of the primary products and the determination of their branching ratios as a function of translational energy (temperature) for multi-channel elementary (bimolecular) reactions of importance in combustion flames and plasma-assisted combustion still represent a challenge for traditional kinetics experiments. On the other hand, this kind of information is central for the detailed modeling of combustion/plasma systems. In this short review, the significant contribution provided in this area by the crossed molecular beam (CMB) scattering method with “universal” mass-spectrometric detection and time-of-flight analysis is illustrated. In particular, we describe the basics of the CMB technique empowered with “soft” electron-impact ionization as recently implemented in our laboratory, and report on its application to the study of the multi-channel elementary reactions of ground state atomic oxygen, O(3P), with unsaturated hydrocarbons containing two carbon atoms (acetylene and ethylene), three carbon atoms (propyne, propene, and allene), and also four carbon atoms (1-butene, 1,2-butadiene, and 1,3-butadiene), which are of paramount interest in combustion flames and plasma-assisted combustion of hydrocarbons. These studies are usually complemented in a synergistic manner by high-level electronic structure calculations of the underlying potential energy surfaces and related statistical (and dynamical when feasible) calculations of product branching ratios. The complementarity to kinetics studies and the implications of the dynamics results for the modeling of combustion/plasma chemistry will be commented on.

Ab Initio and Quasiclassical Trajectory Study of the O(3P) + 2-Propanol Hydrogen Abstraction Reaction.

We present a theoretical study of the hydrogen abstraction reaction from 2-propanol by ground-state oxygen atoms. First, ab initio calculations are used to characterize the stationary points of the potential energy surface. Rotation around the C-C-O-H dihedral affords two conformers in 2-propanol, which gives rise to 13 hydrogen abstraction reaction pathways grouped into three channels, Cα, Cβ, and O, depending on the abstraction site. Reaction at Cα exhibits the lowest barrier and largest exothermicity, followed by reaction at Cβ, and at 2-propanol's oxygen atom. Additional ab initio calculations beyond the stationary points are employed to obtain a grid of energies with which a specific-reaction-parameters (SRP) PM6 semiemipirical Hamiltonian is derived for the title reaction. The SRP-PM6 model captures the energetics of the reaction with higher accuracy than some conventional first-principles methods but is efficient enough to allow for extensive reaction dynamics calculations. Quasiclassical trajectories are subsequently propagated with the SRP-PM6 Hamiltonian to obtain reaction dynamics properties that are compared to experiments. Product translational energy and angular distributions for reaction at Cα with the two conformers of 2-propanol are in good agreement with recent molecular-beam measurements, and they exhibit largely backward scattering with modest energy release to relative translation. Most of the energy is deposited into the organic product, substantiating a reaction mechanism dominated by rebound dynamics.

Photodeoxygenation of phenanthro[4,5-bcd]thiophene S-oxide, triphenyleno[1,12-bcd]thiophene S-oxide and perylo[1,12-bcd]thiophene S-oxide

ABSTRACTSulfoxides, upon irradiation with ultraviolet (UV) light undergo α-cleavage, hydrogen abstraction, photodeoxygenation, bimolecular photoreduction, and stereo-mutation. The UV irradiation of dibenzothiophene S-oxide (DBTO) yields dibenzothiophene (DBT) as a major product along with ground-state atomic oxygen [O(3P)]. This is a common method for generating O(3P) in solution. The low quantum yield of photodeoxygenation and the requirement of UVA light are drawbacks of using this method. The sulfoxides benzo[b]naphtho-[1,2,d]thiophene S-oxide, benzo[b]naphtho [2,1,d]thiophene S-oxide, benzo[b] phenanthro[9,10-d]thiophene S-oxide, dinaphtho- [2,1-b:1’,2’-d]thiophene S-oxide, and dinaphtho[1,2-b:2’,1’-d]thiophene S-oxide have shown to deoxygenate up to three times faster than DBTO upon UVA irradiation; however, the photodeoxygenation of these sulfoxides does not appear to be limited to the production of O(3P). In this work, phenanthro[4,5-bcd]thiophene S-oxide, triphenyleno[1,12-bcd]thiophene-S-oxide, and perylo[1,12-bcd]thiophene-S-oxide were synthesized and their photodeoxygenation was studied. Phenanthro[4,5-bcd]thiophene-S-oxide, triphenyleno[1,12-bcd]thiophene-S-oxide, and perylo[1,12-bcd]thiophene-S-oxide deoxygenated upon UVA irradiation. However, the common intermediate experiments did not conclusively identify the photodeoxygenation mechanism of these sulfoxides.

Rate constants of the reactions of O(3P) atoms with Br2 and NO2 over the temperature range 220‐950 K

AbstractThe kinetics of the reactions of Br2 and NO2 with ground state oxygen atoms have been studied over a wide temperature range, T = 220‐950 K, using a low‐pressure flow tube reactor coupled with a quadrupole mass spectrometer: O + NO2 → NO + O2 (1) and O + Br2 → Br + BrO (2). The rate constant of reaction (1) was determined under pseudo–first‐order conditions, either monitoring the kinetics of O‐atom or NO2 consumption in excess of NO2 or of the oxygen atoms, respectively: k1 = (6.1 ± 0.4) × 10−12 exp((155 ± 18)/T) cm3 molecule−1 s−1 (where the uncertainties represent precision at the 2σ level, the estimated total uncertainty on k1 being 15% at all temperatures). The temperature dependence of k1, found to be in excellent agreement with multiple previous low‐temperature data, was extended to 950 K. The rate constant of reaction (2) determined under pseudo–first‐order conditions, monitoring the kinetics of Br2 consumption in excess of O‐atoms, showed upward curvature at low and high temperatures of the study and was fitted with the following three‐parameter expression: k2 = 9.85 × 10−16 T1.41 exp(543/T) cm3 molecule−1 s−1 at T = (220‐950) K, which is recommended from the present study with an independent of temperature conservative uncertainty of 15% on k2.

Spatially resolved ground state atomic oxygen density during the mode transition of inductively coupled oxygen plasmas

The ground state atomic oxygen density (nOg) and its distribution were systematically studied by two-photon absorption laser induced fluorescence (TALIF) during the discharge mode transition in inductively coupled oxygen plasmas (ICP). After distinguishing discharge mode by experiments, absolute value of nOg under different heating powers/gas pressures were measured. Increasing the heating power in discharges with a total gas pressure of 10, 60 and 80 Pa leads to E→H transition at 0.5, 1.1 and 1.3 kW, respectively. When pressure increased in 10–80 Pa, 0.1 and 2 kW plasmas maintained in E and H mode respectively, and 1 kW plasmas experienced H→E transition at 60 Pa nOg in H-modes were one order of magnitude larger than E-mode. The power-pressure composed mode transition boundaries were obtained through measuring nOg. Spatial distribution of nOg showed that spatial difference of H-mode was more obvious than E-mode. By 0.1 mm spatial resolution TALIF measurement, density gradients of rear 5 mm region in H-mode are one order of magnitude larger than E-mode. These results provide references for processing parameter selection. High spatial resolution TALIF measurement will contribute key technology for acquiring important parameters of surface reactions.

Effects of Oxygen Concentration on the Reactive Oxygen Species Density Under Different Operating Conditions in Atmospheric-Pressure Helium/Oxygen Pulsed Dielectric Barrier Discharge

In this paper, a numerical investigation on the effects of oxygen concentration on the reactive oxygen species (ROS) density under different operating conditions in the atmospheric-pressure helium/oxygen pulsed dielectric barrier discharge (DBD) has been performed by means of a one-dimensional (1-D) fluid model. The involved ROS are ground state oxygen atom O, excited state oxygen atom O( 1 D), single delta metastable oxygen O 2 ( 1 Δg) (SDO), and ozone molecule O 3 . The present work gives the following significant results. For a given operating condition, the averaged O density is evidently larger than the other three ROS densities. There exist characteristic oxygen concentrations where the averaged densities of O and SDO reach their maximum with the increase in the oxygen concentration, respectively. The averaged O 3 density increases with the oxygen concentration, but the reverse is true for the averaged O(1D) density. In addition, the ROS density is increased when increasing the applied voltage amplitude Va, decreasing the rising time of the applied voltage pulse t r , or reducing the frequency of the applied voltage pulse f. Also, the characteristic oxygen concentrations for the averaged densities of both the O and SDO move towards the higher oxygen concentration when increasing the V a , decreasing the t r , or increasing the f. The effect of the operating conditions on the averaged O 3 density is weak under lower oxygen concentrations, and it is increasingly affected by the operating conditions with the increasing oxygen concentration. However, the variation trend of the averaged O( 1 D) density versus oxygen concentration is hardly affected by the operating conditions.

Evidence for diffusing atomic oxygen uncovered by separating reactants with a semi-permeable nanocapsule barrier.

Ground-state atomic oxygen [O(3P)] is an oxidant whose formation in solution was proposed but never proven. Polymer nanocapsules were used to physically separate dibenzothiophene S-oxide (DBTO), a source of O(3P), from an O(3P)-accepting molecule. Irradiation of polymer nanocapsules loaded with DBTO resulted in oxidation of the O(3P)-acceptor placed outside nanocapsules. The results rule out a direct oxygen atom transfer mechanism and are consistent with freely diffusing O(3P) as the oxidant.

Asymmetric Dibenzothiophene Sulfones as Fluorescent Nuclear Stains.

Asymmetric dibenzothiophene S, S-dioxides (DBTOOs) were synthesized and their photophysical properties examined. Through examination, the molecules fluoresced at wavelengths between 371 and 492 nm with quantum yields of fluorescence nearing 0.59. Three of the sulfonic acid sodium salt analogues were chosen to be introduced to HeLa cells, resulting in illumination of the nucleus by fluorescent microscopy. These compounds function as nuclear stains while also affording the ability to predict the localization of the corresponding sulfoxide precursor to ground-state atomic oxygen.

Potential energy surface of triplet O4.

We present a global ground-state potential energy surface (PES) for the triplet spin state of O4 that is suitable for treating high-energy vibrational-rotational energy transfer and collision-induced dissociation in electronically adiabatic spin-conserving O2-O2 collisions. The surface is based on MS-CASPT2/maug-cc-pVTZ electronic structure calculations with scaled external correlation; the active space has 16 electrons in 12 orbitals. The global ground-state potential energy surface was fitted by a many-body approach with an accurate O-O pairwise interaction and a fit of the many-body interaction potential to 10 180 electronic structure data points. The many-body fit is based on permutationally invariant polynomials in terms of bond-order functions of the six interatomic distances; the bond-order functions are mixed exponential-Gaussian functions. The geometries calculated and used for the fit include geometry scans corresponding to dissociative and vibrationally excited diatom-diatom collisions of O2, scans corresponding to O3 interacting with O, additional geometries identified by running trajectories, and geometries along linear synchronous transit paths connecting randomly selected points. The global O4 PES includes subsurfaces describing the interaction of diatomic molecules with other diatomic molecules or interactions of triatomic molecules and an atom. The interaction of ozone with a ground-state oxygen atom occurs on the triplet O4 surface, and our surface includes high-energy points with O3-O geometries as well as O2-O2 geometries and O2-O-O geometries.

Data for: Wavelength Dependent Photochemistry of Expanded Chromophore and Asymmetric Dibenzothiophene S-Oxides Derivatives

Abstract Irradiation of dibenzothiophene S-oxide (DBTO) is believed to generate ground state atomic oxygen [O(3P)]. The irradiation conditions and the efficiency of the deoxygenation event are not ideal when working with biological systems. Dinaphtho[2,1-b:1′,2′-d]thiophene S-oxide (DNTO) and several other analogs were shown to allow visible light to drive deoxygenation due to red-shifted absorbance of the DBTO derivatives. Additionally, DNTO and its analogs did not substantially improved quantum yield of deoxygenation (Φdeox.). To improve both Φdeox. and absorbance, changes were made to the DBTO scaffold. The first change was an addition of an electron withdrawing group (EWG) to weaken the S O bond strength of the sulfoxide to improve Φdeox.. The second change was the addition of an aromatic substituent to enhance and red-shift the absorbance. Common intermediate experiments were performed in toluene to examine if these new DBTO analogs produced a common oxidant under both UV and visible light irradiation. The separate light sources produced different product ratios suggesting these asymmetric DBTO derivatives have wavelength-dependent mechanisms of oxidation.

Wavelength dependent photochemistry of expanded chromophore and asymmetric dibenzothiophene S-Oxide derivatives

Irradiation of dibenzothiophene S-oxide (DBTO) is believed to generate ground state atomic oxygen [O(3P)]. The irradiation conditions and the efficiency of the deoxygenation event are not ideal when working with biological systems. Dinaphtho[2,1-b:1′,2′-d]thiophene S-oxide (DNTO) and several other analogs were shown to allow visible light to drive deoxygenation due to red-shifted absorbance of the DBTO derivatives. Additionally, DNTO and its analogs did not substantially improved quantum yield of deoxygenation (Φdeox.). To improve both Φdeox. and absorbance, changes were made to the DBTO scaffold. The first change was an addition of an electron withdrawing group (EWG) to weaken the SO bond strength of the sulfoxide to improve Φdeox.. The second change was the addition of an aromatic substituent to enhance and red-shift the absorbance. Common intermediate experiments were performed in toluene to examine if these new DBTO analogs produced a common oxidant under both UV and visible light irradiation. The separate light sources produced different product ratios suggesting these asymmetric DBTO derivatives have wavelength-dependent mechanisms of oxidation.

Dynamics of Graphite Oxidation at High Temperature

Interactions of ground-state atomic and molecular oxygen, O(3P) and O2(3Σg–), with a highly oriented pyrolytic graphite surface were investigated for a broad range of surface temperatures from 1100 K to approximately 2300 K. A molecular beam composed of 89% O atoms and 11% O2, with average translational energies of 472.1 and 944.4 kJ mol–1, respectively, was directed at the surface with an incidence angle, θi, of 45°. Angle- and velocity-resolved distributions were collected for nonreactively and reactively scattered products with the use of a rotatable mass spectrometer detector. Four scattered products were observed: O, O2, CO, and CO2. O atoms that exited the surface without reacting exhibited both impulsive scattering (IS) and thermal desorption (TD) components. The primary reaction product observed was carbon monoxide (CO). Carbon dioxide (CO2) was measured only with surface temperatures below 1400 K, and O2 was attributed to IS of O2 that was present in the incident beam. Although there is evidence ...

Thiol Reactivity toward Atomic Oxygen Generated during the Photodeoxygenation of Dibenzothiophene S-Oxide.

Aromatic heterocyclic oxides, such as dibenzothiophene S-oxide (DBTO), have been suggested to release ground state atomic oxygen [O(3P)] upon irradiation, and as such, they have been used to create a condensed phase reactivity profile for O(3P). However, thiols, which are highly reactive with O(3P) in the gas phase, were not previously investigated. An earlier study of O(3P) with proteins in solution indicated a preference for thiols. A further investigation of the apparent thiophilicity provided the subject for this study. DBTO was employed as a putative O(3P)-precursor. However, the effective rate of O(3P) formation was found to be dependent on reactant concentrations in certain cases. All reactants were found to increase the rate of deoxygenation to some extent, but in the presence of reactants containing an alcohol linked to a reactive functional group, deoxygenation occurred substantially more rapidly. The rate enhancement was quantified and attributed to the reaction of activated O atom within the solvent cage prior to escape into the bulk solution. Through competition experiments, the relative rate constants of O(3P) with thiols and other functional groups were found. A small preference for primary thiols was observed over other thiols, sulfides, and alkenes. A much larger preference was observed for thiols, sulfides, and alkenes over aromatic groups. In summary, DBTO was successfully used as an O(3P)-precursor, and the thiophilicity of O(3P) was confirmed and quantified.

Quantitative measurements of ground state atomic oxygen in atmospheric pressure surface micro-discharge array

The generation of atomic oxygen in an array of surface micro-discharge, working in atmospheric pressure He/O2 or Ar/O2 mixtures, is investigated. The absolute atomic oxygen density and its temporal and spatial dynamics are studied by means of two-photon absorption laser-induced fluorescence. A high density of atomic oxygen is detected in the He/O2 mixture with up to 10% O2 content in the feed gas, whereas the atomic oxygen concentration in the Ar/O2 mixture stays below the detection limit of 1013 cm−3. The measured O density near the electrode under the optimal conditions in He/1.75% O2 gas is 4.26 × 1015 cm−3. The existence of the ground state O (2p4 3P) species has been proven in the discharge at a distance up to 12 mm away from the electrodes. Dissociative reactions of the singlet O2 with O3 and deep vacuum ultraviolet radiation, including the radiation of excimer , are proposed to be responsible for O (2p4 3P) production in the far afterglow. A capability of the surface micro-discharge array delivering atomic oxygen to long distances over a large area is considered very interesting for various biomedical applications.

Ground state atomic oxygen in high-power impulse magnetron sputtering: a quantitative study

The ground state density of oxygen atoms in reactive high-power impulse magnetron sputtering discharges has been studied quantitatively. Both time-resolved and space-resolved measurements were conducted. The measurements were performed using two-photon absorption laser-induced fluorescence (TALIF), and calibrated by optical emission actinometry with multiple Ar emission lines. The results clarify the dynamics of the O ground state atoms in the discharge afterglow significantly, including their propagation and fast decay after the plasma pulse, as well as the influence of gas pressure, O2 admixture, etc.

Crossed beam polyatomic reaction dynamics: recent advances and new insights.

Over the past ten years or so, great advances in our understanding of the dynamics of elementary (bimolecular) polyatomic reactions in the gas-phase have occurred. This has been made possible by critical improvements (a) in crossed molecular beam (CMB) instruments with rotating mass spectrometric detection and time-of-flight analysis, especially following the implementation of soft ionization (by tunable low energy electrons or vacuum-ultraviolet synchrotron radiation) for product detection with increased sensitivity and universal detection power, and (b) in REMPI-slice velocity map ion imaging (VMI) detection techniques in pulsed CMB experiments for obtaining product pair-correlated information through high-resolution measurements directly in the center of mass system. The improved universal CMB method is permitting us to identify all primary reaction products, characterize their formation dynamics, and determine the branching ratios (BRs) for multichannel non-adiabatic reactions, such as those of ground state oxygen atoms, O(3P), with unsaturated hydrocarbons (alkynes, alkenes, dienes). The improved slice VMI CMB technique is permitting us to explore at an unprecedented level of detail, through pair-correlated measurements, the reaction dynamics of a prototype polyatomic molecule such as CH4 (and isotopologues) in its ground state with a variety of important X radicals such as F, Cl, O, and OH. In this review, we highlight this recent progress in the field of CMB reaction dynamics, with an emphasis on the experimental side, but with the related theoretical work, at the level of state-of-the-art calculations of both the underlying potential energy surfaces and the reaction dynamics, noted throughout. In particular, the focus is (a) on the effect of molecular complexity and structure on product distributions, branching ratios and role of intersystem crossing for the multichannel, addition-elimination reactions of unsaturated hydrocarbons with O atoms, and (b) on the very detailed dynamics of the abstraction reactions of ground-state methane (and isotopologues) with atoms (F, Cl, O) and diatoms (OH), with inclusion of also rotational mode specificity in the vibrationally excited methane reactions.