OH Molecules Research Articles

Ab initio Calculation of the Dipole Moment Function of the OH Radical Ground State

Recently it has been theoretically predicted and experimentally confirmed that at altitudes of 80–110 km from the Earth’s surface the signals of the global navigation satellite systems (GNSS) are delayed as a result of a multiple resonance scattering by the Rydberg complexes. The attenuation of GNSS signals occurs mainly in the lower atmosphere layers, where the greatest effect is achieved through interaction with charged aerosol layers. A thunderstorm conditions are of particular interest whereby the presence of strong electric fields and hydration of aerosols can lead to the detachment of OH radicals from water clusters. Since the spectrum of radiation and absorption of these radicals for rotational transitions is part of the microwave range, they are expected to make an additional resonance contribution to the delay of radio signals thus necessitating a detailed study of the electronic structure of the OH radical. This paper offers the ab initio calculations of the dipole moment function for the ground X2Π state of the OH molecule using the dipole length approach and the finite-field method. The comparison with the experiment shows that in extended model spaces the approximation of the dipole length leads to more accurate values of the dipole moment than the finite-field method.

Influence of a liquid surface on the NOx production of a cold atmospheric pressure plasma jet

In this work a cold atmospheric pressure plasma jet was mounted above a distilled water reservoir to study the influence of a water surface on reactive oxygen and nitrogen species (RONS) generation. The RONS, namely O3 and NO2, were measured by multi-pass cell Fourier transform infrared absorption spectroscopy in the far-field of the jet. A shielding gas device was used to control the ambient environment of the effluent. The liquid surface has nearly no influence on the RONS dynamics as a function of shielding gas composition. It was found, however, that the interaction with the liquid surface increases the density of NOx by a factor of 2–3 due to local increase in H2O concentration through evaporation of water from the liquid surface. In contrast to the case of ambient humidity increase, this locally higher H2O density distinctly influences the ratio of water to oxygen and nitrogen which results in a higher generation of OH molecules.

Density functional theory study of the OH, Cl and H2O coadsorption on the step-defect Al2O3 film surface

Density functional theory has been performed on the step-defect Al2O3 film surfaces with the OH, Cl and H2O molecules coadsorption. Three kinds of step-defect (Al, O2, Al3) surfaces are optimized and the adsorption energy, the binding energies of film and adsorbates are calculated. The energy properties are similar in H2O or Cl coadsorption configurations, but have obvious differences for the OH group coadsorption configuration due to large numbers of adsorbate species and numbers. After structural relaxation, most of the step-defect surfaces could be easily hydroxylated. The Al3 step-defect surfaces are easier to be corroded by H2O and coadsorption molecules due to lots of unsaturated dangling bonds, some H2O molecules are located into the step-defect, surface Al atoms collapse inside the steps and several inner O atoms move outside the film. When the Cl exists in the aqueous solution, it would restrict H2O molecules from dissociating into OH groups. Moreover, the dissociation and recombination of H2O molecules could be promoted by OH groups.

Chemical kinetics in an atmospheric pressure helium plasma containing humidity.

Atmospheric pressure plasmas are sources of biologically active oxygen and nitrogen species, which makes them potentially suitable for the use as biomedical devices. Here, experiments and simulations are combined to investigate the formation of the key reactive oxygen species, atomic oxygen (O) and hydroxyl radicals (OH), in a radio-frequency driven atmospheric pressure plasma jet operated in humidified helium. Vacuum ultra-violet high-resolution Fourier-transform absorption spectroscopy and ultra-violet broad-band absorption spectroscopy are used to measure absolute densities of O and OH. These densities increase with increasing H2O content in the feed gas, and approach saturation values at higher admixtures on the order of 3 × 1014 cm-3 for OH and 3 × 1013 cm-3 for O. Experimental results are used to benchmark densities obtained from zero-dimensional plasma chemical kinetics simulations, which reveal the dominant formation pathways. At low humidity content, O is formed from OH+ by proton transfer to H2O, which also initiates the formation of large cluster ions. At higher humidity content, O is created by reactions between OH radicals, and lost by recombination with OH. OH is produced mainly from H2O+ by proton transfer to H2O and by electron impact dissociation of H2O. It is lost by reactions with other OH molecules to form either H2O + O or H2O2. Formation pathways change as a function of humidity content and position in the plasma channel. The understanding of the chemical kinetics of O and OH gained in this work will help in the development of plasma tailoring strategies to optimise their densities in applications.

Photoassociation reaction of OH molecules through reverse ladder transition

Photoassociation via reverse ladder transition controlled by two and four laser pulses is investigated using the time-dependent quantum wave packet method. The calculated results show that the amplitudes of the pulses have an enormous effect on the target population and total yield of association. For the target state with a high energy level, the population of background states can reduce the state-selectivity. Although, the total yield of association is decreased, the four pulses can induce the population transferring to low vibrational levels, and the state-selectivity of the target state is high.

Controlling spin flips of molecules in an electromagnetic trap

Doubly dipolar molecules exhibit complex internal spin-dynamics when electric and magnetic fields are both applied. Near magnetic trap minima, these spin-dynamics lead to enhancements in Majorana spin-flip transitions by many orders of magnitude relative to atoms, and are thus an important obstacle for progress in molecule trapping and cooling. We conclusively demonstrate and address this with OH molecules in a trap geometry where spin-flip losses can be tuned from over $200 \text{ s}^{-1} $ to below our $2\text{ s}^{-1}$ vacuum limited loss rate with only a simple external bias coil and with minimal impact on trap depth and gradient.

Rotationally cold (> 99% J = 0) OH− molecular ions in a cryogenic storage ring

We store a 10 keV OH− ion-beam at 13.5 ± 0.5 K in one of the DESIREE storage rings. Using photodetachment thermometry we measure the effective relative photodetachment cross section at different storage times and determine the rotational temperature of the ions to be 13.4 ± 0.2 K in agreement with the macroscopic temperature. A model cross section in the threshold range taking into account the formation of excited neutral OH molecules is calculated as a function of rotational temperature in order to justify the use of the rotational thermometry method developed earlier by the group of Roland Wester at Innsbruck University in the present case. In addition, we apply a selective photodetachment technique to produce an ion beam with more than 99% of the ions in the rotational ground state. The intrinsic lifetime of the J = 1 rotational level is measured to be 145 ± 28 s.

What Controls Selectivity of Hydroxyl Radicals in Aqueous Solution? Indications for a Cage Effect.

The oxidation of three isotopologues of methylcyclohexane (MCH: C7H14, C7D14, c-C6D11-CH3) by OH-radicals (•OH) in aqueous solution was investigated. Intermolecular and intramolecular H/D kinetic isotope effects (KIE = kH:kD) for the abstraction of H and D atoms by •OH were measured. These KIEs reflect inter- and intramolecular selectivities of hydrogen abstraction, i.e., the selection of •OH attack on carbon-hydrogen bonds in different molecules and in different positions of one molecule, respectively. The intermolecular selectivity of •OH attack in aqueous solution is largely discriminated against in comparison with the intramolecular selectivity. The observed extent of discrimination cannot be explained by partial diffusion control of the overall reaction rates. A cage model, where •OH and hydrocarbon molecules are entrapped in a solvent cage, is more appropriate. The much higher intramolecular KIEs compared to the intermolecular KIEs of the same chemical reaction, R-H + •OH → R• + H2O, indicate a high degree of mobility of the two reaction partners inside of the solvent cage. This mobility is sufficient to develop an intramolecular selectivity comparable to that of gas-phase reactions of •OH. Furthermore, literature data on KIEs of H-abstraction by •OH in aqueous and gas phases are discussed. There is a general tendency toward lower selectivities in the aqueous phase.

Measurements of trap dynamics of cold OH molecules using resonance-enhanced multiphoton ionization

Trapping cold, chemically important molecules with electromagnetic fields is a useful technique to study small molecules and their interactions. Traps provide long interaction times that are needed to precisely examine these low density molecular samples. However, the trapping fields lead to non-uniform molecular density distributions in these systems. Therefore, it is important to be able to experimentally characterize the spatial density distribution in the trap. Ionizing molecules in different locations in the trap using resonance enhanced multiphoton ionization (REMPI) and detecting the resulting ions can be used to probe the density distribution even with the low density present in these experiments because of the extremely high efficiency of detection. Until recently, one of the most chemically important molecules, OH, did not have a convenient REMPI scheme. Here, we use a newly developed 1 + 1' REMPI scheme to detect trapped cold OH molecules. We use this capability to measure trap dynamics of the central density of the cloud and the density distribution. These types of measurements can be used to optimize loading of molecules into traps, as well as to help characterize the energy distribution, which is critical knowledge for interpreting molecular collision experiments.

Quenching of Premixed Flames at Cold Walls: Effects on the Local Flow Field

The presence of a turbulent premixed flame strongly influences the properties of the adjacent velocity boundary layer. This influence is studied here using a generic configuration where at atmospheric pressure turbulent premixed methane/air flames interact with a temperature stabilized wall. The experiment is optimized for well-defined boundary conditions and optical accessibility in the zone where the flame impinges at the wall. Laser based diagnostic methods are used to measure two components of the velocity field by particle image velocimetry simultaneously with the flame front position using laser induced fluorescence of the OH molecule. Two measurement planes are selected that are aligned perpendicularly to the surface of the wall. Based on this data, the flow field near the wall is analyzed by different methodologies using laboratory-fixed and flame-conditioned statistics, a quadrant splitting analysis of the Reynolds stresses and an evaluation of the production term of the turbulent kinetic energy. The results of chemically reactive cases are compared to their corresponding non-reactive flows for otherwise identical inflow conditions. In the zone of flame-wall interactions the boundary layer structure and its turbulence are dominated by the turbulent flame. Important features are that the flame compresses the boundary layer already upstream the location where the flame is finally quenched and that ejection and sweeps are no longer the dominant mechanisms as in non-reactive boundary layers. This experimental data may serve additionally as a database for model development for near wall reactive flows.

Effect of flow distortion on fuel/air mixing and combustion in an upstream-fueled cavity flameholder for a supersonic combustor

This paper describes an experimental study of the effects of an incident shockwave on the flow field, fuel distribution and combustion within a cavity flameholder with upstream fuel injection. Two impingement locations are employed: (1) near the fuel injector (the so-called shock-on-jet case) and (2) on the cavity shear layer (the shock-on-cavity case). Shadowgraph is used to characterize the flow field. Air seeded with nitric oxide (NO) is used as the simulated fuel and the resulting planar laser-induced fluorescence (NO-PLIF) from NO molecules is used to characterize fuel/air mixing while planar laser-induced fluorescence of OH molecules to characterize the actual combustion process. The shadowgraph and NO-PLIF images are compared with a CFD (Computational Fluid Dynamics) solution of the Reynolds-averaged-Navier Stokes (RANS) for assessment and explanation of experimental results of non-reacting tests. The effect of the shock on the cavity shear layer is to control the fuel distribution within the cavity. The effect of the shock on the jet is to force the shear layer deep within the cavity, which results in higher fuel concentrations near the cavity centerline. The shock-on-cavity case causes the shear layer to separate upstream of the cavity. Mixing uniformity is enhanced by the increased breakup of the fuel plume. Combustion is stronger and more uniform with the shock impinging on the cavity, while it is limited to the edges of the cavity with shock impingement on the jet. The greater mixing afforded in the shock-on-cavity case reduces the fuel concentration near the centerline and allows stronger burning in the center of the cavity. Doubling the fuel injection momentum flux ratio does not strongly affect the pattern of fuel distribution in either case, but combustion in the shock-on-cavity case is reduced, because the fuel concentration at the centerline is high.

Confirmation of OH as good thermometric species for gas temperature determination in an atmospheric pressure argon plasma jet

We have proved that the temperatures determined from OH rotational spectra accurately describe the actual gas temperatures in an atmospheric pressure argon plasma jet, expanding in a hot ambient atmosphere. For this, we have operated the radiofrequency jet discharge inside a heated oven at various decreasing forwarded powers and increasing gas flow rates. We compared the rotational temperature values with the temperature of the oven. When extrapolated to zero power or infinite gas flow, the data describing the dependence of the rotational temperatures upon power and mass flow rate indicate the correct oven temperature. We demonstrate that the population of the rotational levels of the OH molecule is in equilibrium with the translational motion of the argon atoms in the plasma jet, from room temperature (RT) to 500 °C.

Application of high-resolution continuum source flame atomic absorption spectrometry to reveal, evaluate and overcome certain spectral effects in Pb determination of unleaded gasoline

High-resolution continuum source and line source flame atomic absorption spectrometry (HR-CS FAAS and LS FAAS, respectively) were applied for Pb determination in unleaded aviation or automotive gasoline that was dissolved in methyl-isobutyl ketone. When using HR-CS FAAS, a structured background (BG) was registered in the vicinity of both the 217.001nm and 283.306nm Pb lines. In the first case, the BG, which could be attributed to absorption by the OH molecule, directly overlaps with the 217nm line, but it is of relatively low intensity. For the 283nm line, the structured BG occurs due to uncompensated absorption by OH molecules present in the flame. BG lines of relatively high intensity are situated at a large distance from the 283nm line, which enables accurate analysis, not only when using simple variants of HR-CS FAAS but also for LS FAAS with a bandpass of 0.1nm. The lines of the structured spectrum at 283nm can have “absorption” (maxima) or “emission” (minima) character. The intensity of the OH spectra can significantly depend on the flame character and composition of the investigated organic solution. The best detection limit for the analytical procedure, which was 0.01mgL−1 for Pb in the investigated solution, could be achieved using HR-CS FAAS with the 283nm Pb line, 5pixels for the analyte line measurement and iterative background correction (IBC). In this case, least squares background correction (LSBC) is not recommended. However, LSBC (available as the “permanent structures” option) would be recommended when using the 217nm Pb line. In LS FAAS, an additional phenomenon related to the nature of the organic matrix (for example, isooctane or toluene) can play an important role. The effect is of continuous character and probably due to the simultaneous efficient correction of the continuous background (IBC) it is not observed in HR-CS FAAS. The fact that the effect does not depend on the flame character indicates that it is not radiation scattering. For LS FAAS, the determination of Pb using the 283nm line, a 0.1nm bandpass and a fuel lean flame is strongly recommended. The analysis of certified reference materials, recovery studies and the analysis of real samples with low Pb content supported the satisfactory accuracy of Pb determination in automotive or aviation gasoline when the recommended analytical variants are applied.The studies in this work shed new light on spectral phenomena in air-acetylene flames. The structured background due to absorption by the OH molecules must be taken into account during Pb determination in other materials as well as in some other elemental determinations, especially at low absorbance levels. The usefulness of HR-CS FAAS for revealing and investigating a structured background was demonstrated. HR-CS FAAS does not reveal fully corrected spectral effects with a continuous character, which can be found in LS FAAS.

Laser cooling of OH molecules in theoretical approach

Laser cooling of OH molecules in theoretical approach

Flame-Flow Interaction in Premixed Turbulent Flames During Transient Head-On Quenching

This paper reports on experimental investigations of turbulent flame-wall interaction (FWI) during transient head-on quenching (HOQ) of premixed flames. The entire process, including flame-wall approach and flame quenching, was analyzed using high repetition rate particle image velocimetry (PIV) and simultaneous flame front tracking based on laser-induced fluorescence (LIF) of the OH molecule. The influence of convection upon flame structures and flow fields was analyzed qualitatively and quantitatively for the fuels methane (CH4) and ethylene (C2H4) at ϕ = 1. For this transient FWI, flames were initialized by laser spark ignition 5 mm above the burner nozzle. Subsequently, flames propagated against a steel wall, located 32 mm above the burner nozzle, where they were eventually quenched in the HOQ regime due to enthalpy losses. Twenty ignition events were recorded and analyzed for each fuel. Quenching distances were 179 μm for CH4 and 159 μm for C2H4, which lead by nondimensionalization with flame thickness to Peclet numbers of 3.1 and 5.5, respectively. Flame wrinkling and fresh gas velocity fluctuations proved flame and flow laminarization during wall approach. Velocity fluctuations cause flame wrinkling, which is higher for CH4 than C2H4 despite lower velocity fluctuations. Lewis number effects explained this phenomenon. Results from flame propagation showed that convection dominates propagation far from the wall and differences in flame propagation are related to the different laminar flame speeds of the fuels. Close to the wall flames of both fuels propagate similarly, but experimental results clearly indicate a decrease in intrinsic flame speed. In general, the experimental results are in good agreement with other experimental studies and several numerical studies, which are mainly based on direct numerical simulations.

Combined quantum chemistry and Monte Carlo simulation of competitive adsorption of O2 and OH on Pt surfaces

To obtain a microscopic explanation on the difference of oxygen reduction reaction activity on different Pt low index surfaces, we simulated competitive adsorptions of O2 and OH on four Pt low index surfaces. Firstly, all possible chemical adsorption configurations of the O2 and OH molecules on the three surfaces were acquired through density functional theory. The distribution of these configurations on the different surfaces was collected from Monte Carlo simulations. Our results demonstrated that the adsorption energy order of O2 on different surfaces was (110)(1×2)>(110)>(100)>(111) and that the adsorption energy order of the OH molecules on Pt surfaces was the same. Considering the competitive adsorption of O2 and OH on Pt surfaces, the final O2 adsorption efficiencies order of three surfaces was (111)>(110)>(100)>(110)(1×2), which was consistent with the experimental activities of oxygen reduction. Our study provided theoretical references for previous experimental studies and had important significance for the understanding of oxygen adsorption on Pt surfaces.

Non-thermal production and escape of OH from the upper atmosphere of Mars

We present a theoretical analysis of formation and kinetics of hot OH molecules in the upper atmosphere of Mars produced in reactions of thermal molecular hydrogen and energetic oxygen atoms. Two major sources of energetic O considered are the photochemical production, via dissociative recombination of O2+ ions, and energizing collisions with fast atoms produced by the precipitating Solar Wind (SW) ions, mostly H+ and He2+, and energetic neutral atoms (ENAs) originating in the charge-exchange collisions between the SW ions and atmospheric gases. Energizing collisions of O with atmospheric secondary hot atoms, induced by precipitating SW ions and ENAs, are also included in our consideration. The non-thermal reaction O + H2(v, j) → H + OH(v′, j′) is described using recent quantum-mechanical state-to-state cross sections, which allow us to predict non-equilibrium distributions of excited rotational and vibrational states (v′, j′) of OH and expected emission spectra. A fraction of produced translationally hot OH is sufficiently energetic to overcome Mars’ gravitational potential and escape into space, contributing to the hot corona. We estimate its total escape flux from the dayside of Mars for low solar activity conditions at about 1.1 × 1023 s−1, or about 0.1% of the total escape rate of atomic O and H. The described non-thermal OH production mechanism is general and expected to contribute to the evolution of atmospheres of the planets, satellites, and exoplanets with similar atmospheric compositions.

On the origin of the ionosphere at the Moon using results from Chandrayaan‐1 S band radio occultation experiment and a photochemical model

AbstractThe origin of the Moon's ionosphere has been explored using Chandrayaan‐1 radio occultation (RO) measurements and a photochemical model. The electron density near the Moon's surface, obtained on 31 July 2009 (∼300 cm−3), is compared with results from a model which includes production and recombination of 16 ions, solar wind proton charge exchange, and the electron impact ionization. The model calculations suggest that in the absence of transport, inert ions, namely Ar+, Ne+, and He+, dominate lunar ionosphere (density ∼5 × 104 cm−3). Interaction with solar wind, however, leads to their complete removal (∼2–3 cm−3). Assuming the Moon's exosphere to have CO2, H2O, O, OH, H2, CH4, and CO molecules in addition to the inert gases, the model calculations suggest that the lunar ionosphere is dominated by molecular ions, namely H2O+, CO , and H3O+, with near‐surface density ∼250 cm−3. We surmise that lunar ionosphere can be molecular in nature.

Heavy metal concentrations in soil from San Luis Potosí, México

High-resolution continuum source and line source flame atomic absorption spectrometry (HR-CS FAAS and LS FAAS, respectively) were applied for Pb determination in unleaded aviation or automotive gasoline that was dissolved in methyl-isobutyl ketone. When using HR-CS FAAS, a structured background (BG) was registered in the vicinity of both the 217.001 nm and 283.306 nm Pb lines. In the first case, the BG, which could be attributed to absorption by the OH molecule, directly overlaps with the 217 nm line, but it is of relatively low intensity. For the 283 nm line, the structured BG occurs due to uncompensated absorption by OH molecules present in the flame. BG lines of relatively high intensity are situated at a large distance from the 283 nm line, which enables accurate analysis, not only when using simple variants of HR-CS FAAS but also for LS FAAS with a bandpass of 0.1 nm. The lines of the structured spectrum at 283 nm can have “absorption” (maxima) or “emission” (minima) character. The intensity of the OH spectra can significantly depend on the flame character and composition of the investigated organic solution. The best detection limit for the analytical procedure, which was 0.01 mg L− 1 for Pb in the investigated solution, could be achieved using HR-CS FAAS with the 283 nm Pb line, 5 pixels for the analyte line measurement and iterative background correction (IBC). In this case, least squares background correction (LSBC) is not recommended. However, LSBC (available as the “permanent structures” option) would be recommended when using the 217 nm Pb line. In LS FAAS, an additional phenomenon related to the nature of the organic matrix (for example, isooctane or toluene) can play an important role. The effect is of continuous character and probably due to the simultaneous efficient correction of the continuous background (IBC) it is not observed in HR-CS FAAS. The fact that the effect does not depend on the flame character indicates that it is not radiation scattering. For LS FAAS, the determination of Pb using the 283 nm line, a 0.1 nm bandpass and a fuel lean flame is strongly recommended. The analysis of certified reference materials, recovery studies and the analysis of real samples with low Pb content supported the satisfactory accuracy of Pb determination in automotive or aviation gasoline when the recommended analytical variants are applied.The studies in this work shed new light on spectral phenomena in air-acetylene flames. The structured background due to absorption by the OH molecules must be taken into account during Pb determination in other materials as well as in some other elemental determinations, especially at low absorbance levels. The usefulness of HR-CS FAAS for revealing and investigating a structured background was demonstrated. HR-CS FAAS does not reveal fully corrected spectral effects with a continuous character, which can be found in LS FAAS.

Experimental investigation of flame stabilization inside the quarl of an oxyfuel swirl burner

A more detailed understanding of coal combustion as required for predictive engineering is still lacking. To gain insights into physically relevant sub-processes the community follows a stepwise approach. Experiments have been conducted starting from lab-scale gas-assisted coal flames up to self-sustaining industrial-scale coal combustors in the MW-range. In the intermediate range, however, experiments are sparse. To close this gap in this contribution a new generic test rig is presented that is suitable for operating gas-assisted coal flames. In close similarity to a burner designed for oxycoal combustion the nozzle and quarl assembly exhibit most important characteristics of a state-of-the-art combustor. Quarl and combustion chamber provide excellent optical access such that advanced laser diagnostic methods can be applied for detailed studies of flow and scalar fields. Geometrical requirements for future numerical simulations such as easy meshing of the nozzle have been considered during the re-design of the burner. Although prepared for gas-assisted oxycoal firing, in a first step various gas flames operated in air and oxyfuel atmospheres are investigated here. The focus of the present study is on flame stabilization. The leading edge of the flame is located inside the quarl such that the optical access to this most important region is a mandatory requirement. For non-reacting and reacting conditions flow fields inside the quarl and the combustor are studied using stereoscopic particle image velocimetry (SPIV). To visualize reaction zones and flame stabilization regions planar laser induced fluorescence of the OH molecule (OH-PLIF) and broadband chemiluminescence (CL) measurements are performed for reacting conditions. It is observed that close to the nozzle the leading edge of the flame stabilizes as a diffusion flame in the slow moving shear layer located between the central recirculation zone and the fuel inlet. Further downstream the flame stabilizes in the high-momentum annular jet flow generated by the primary and secondary flow. Most probably it has switched to the premixed regime.