Distribution Of OH Research Articles

Hydroxyl time-series measurements and simulations for turbulent premixed jet flames in the thickened preheat regime

Quantitative measurements of OH concentration time series are presented for turbulent lean-premixed, methane-air jet flames theoretically in the thickened preheat regime. Picosecond time-resolved laser-induced fluorescence (PITLIF) reveals unique differences between these premixed flames and previous non-premixed jet flames. Time-averaged [OH] measurements are used to identify mean flame structures and to discern how these structures are affected by varying bulk flow velocities and heat release. More importantly, hydroxyl time series are inspected to distinguish among three main regions in these turbulent premixed flames. These regions include the reacting side of the flame brush, the mixing side of the flame brush (radially outside the location of heat release), and above the flame tip. Although the main reaction zone appears to be broadened by its associated high turbulent intensity, a combination of statistical analysis plus flamelet simulations suggests that the primary internal structure responsible for the OH distribution remains constant across the mean flame brush. Therefore, the absolute concentration of OH depends principally on the intermittency of this instantaneous internal structure. Outside the mean flame brush, mixing of OH with co-flow air shifts the distribution of absolute OH concentrations. Distinct autocorrelation functions are found within the three different regions identified for these premixed flames. Across the flame brush, integral time scales are dominated by turbulent convection, as verified by flamelet simulations. Above the flame tip, integral time scales are determined by a competition between turbulent convection and the reaction rate for OH destruction.

Vibrationally mediated dissociation dynamics of H2O in the vOH=2 polyad

Vibrationally mediated photodissociation dynamics of jet-cooled H2O in the vOH=2 polyad is studied in a supersonic slit jet expansion. Single rotational states within |02〉− (≡ν1+ν3 in normal mode notation), |02〉+(≡2ν1), |11〉+(≡2ν3), and |01−2〉(≡ν3+2ν2) vibrational states of H2O are selectively prepared with near IR overtone pumping, photodissociated at 193 nm, and the resulting nascent internal state distribution of OH fragments probed via laser induced fluorescence. Strong oscillations in rotational, spin–orbit, and lambda-doublet distributions are observed, often in remarkably close agreement with H2O state-to-state photodissociation studies from both higher and lower vOH polyads. The influence of initially excited bending and JKaKc levels of H2O on spin–orbit, Λ-doublet, and rotational distributions of OH is examined in detail. Several new dynamical trends are identified, for example, a clear propensity at high N for a strong Λ+ versus Λ− inversion in the Π3/2 spin–orbit manifold, which reverses in the Π1/2 manifold, suggesting spin–orbit sensitive stereodynamics in the ejection process. Furthermore, the results highlight significant differences in photodissociation dynamics from gerade (e.g., |02〉+) versus ungerade (e.g., |02〉−) vibrational states, specifically with respect to OH(v=1)/OH(v=0) branching ratios, and signaling a breakdown of the “spectator” model at low vibrational excitation.

Experimental and numerical analyses of magnetic effect on OH radical distribution in a hydrogen-oxygen diffusion flame

The effect of magnetic field on OH radical distribution in a hydrogen-oxygen diffusion flame was experimentally and numerically investigated to explore the possibility of combustion control by magnetic force. In experiments, a coaxial type of burner was set between the magnet pieces. Two-dimensional (2D) cross-section distributions of OH* chemiluminescence intensity and OH fluorescence intensity were obtained with spectroscopic techniques using a CCD camera and a Planar Laser-Induced Fluorescence (PLIF) system, respectively. It was clearly seen that the high-density regions of OH* and OH radicals axisymmetrically migrated toward the central axis of the flame due to the influence of the magnetic field. In numerical simulations, such a phenomenon was qualitatively reproduced by solving the equations of reactive gas dynamics and magnetism. As a result, it was found that the magnetic force does not directly and selectively induce the diffusion velocity (the relative velocity) of OH itself. Alternatively, the magnetic force acting on O 2, whose mass density and magnetic susceptibility are much larger than those of other chemical species, causes the change in the mean velocity (the mass-average velocity) of mixture gas to transport the OH radical distribution indirectly and passively.

A new model of tropospheric hydroxyl radical concentrations

A chemistry model of the global troposphere is presented which focuses on the hydroxyl radical, OH. Global distributions of OH are calculated based on known chemical reaction pathways, experimentally measured values of precursor species O 3, H 2O, NO x (defined as NO + NO 2), CO, CH 4, and actinic flux (which includes the effects of cloud cover and O 3 column absorption). Model grid resolution is 1 km in altitude by 10° latitude, and zonally divided into land or ocean. Species are calculated as seasonal averages. Global annual mean OH in the troposphere (up to 14 km altitude) is calculated to be 9.2 × 10 5 mol cm −3 with averages of 9.8 × 10 5 in the northern hemisphere, and 8.5 × 10 5 in the southern hemisphere. Global CO and CH 4 oxidation rates by OH are calculated to be 1840 Tg year −1 and 580 Tg year −1, respectively. OH is found to be most sensitive to O 3 and H 2O concentrations, as well as to the photolysis rate of O 3 to O( 1D). Sensitivity of CO and CH 4 oxidation rates to cloud presence shows an inverse relationship to cloud amount and optical depth. Model results are shown to be consistent with results from two other published models.

Dependence of magnetically induced change in oh distribution in a methane-air premixed flame on equivalence ratio

In order to explore the possibility of combustion control by magnetic field, the magnetically induced change in the spatial distribution of OH radicals in a methane-air premixed flame was visualized via planar laser-induced fluorescence measurements. Then equivalence ratio was changed from 0.89 to 1.8 under a constant total flow rate. The results clearly showed that the change in OH fluorescence intensity distribution depended on the equivalence ratio. At equivalence ratios smaller than 1.0 (fuel-lean conditions), the OH concentration was decreased in the downstream region of the flame by the magnetic field, while it was increased in the upstream region. At ratios greater than 1.0 (fuel-rich conditions), the decrease region existed in the peripheral region of the flame, while the increase region was inside the flame. The change rate of the OH distribution tended to become larger as the equivalence ratio became larger. The mechanisms of these phenomena were discussed in conjunction with O 2 flows induced magnetically.

Absorption and Emission in the Four Ground-State OH Lines Observed at 18 cm with the VLA Towards the Galactic Centre

The OH distribution in the Sgr A Complex has been observed in the 1612, 1665, 1667, and 1720 MHz OH transitions with the Very Large Array (VLA), in the BnA and DnC configurations. Spectral line maps have been produced with a channel velocity resolution of about 9 km s–1 and with angular resolutions of 4″ × 3″, and 24″ × 22″, respectively. Some clear results are highlighted here: i) the existence of an OH streamer inside the Circumnuclear Disk (CND) near Sgr A*, ii) absorption from the CND, iii) strong absorption towards the eastern and most of the western parts of the Sgr A East shell, iv) lack of absorption towards both Sgr A West and the compact H II-regions to the east of Sgr A East, v) a double-lobed structure of the High Negative Velocity Gas (HNVG) oriented to the northeast and southwest of Sgr A*, and vi) compact point-like maser emission in all four transitions. In particular, a 1720 MHz maser at –132 km s–1 in the CND as counterpart to a 1720 MHz maser at +132 km s–1 in the CND, was observed.

High fluorine pargasites in ultrahigh temperature granulites from Tonagh Island in the Archean Napier Complex, East Antarctica

Pargasites (F/(F+Cl+OH) ratio ( X F) of up to 0.48) from Tonagh Island in Enderby Land, East Antarctica are closely associated with typical high-grade minerals such as orthopyroxene in quartzo-feldspathic, mafic, and ultramafic granulites, and is regarded as a stable mineral at the peak metamorphic conditions (>1100 °C) calculated for the ultrahigh-temperature Archean Napier Complex. Although experimental investigations have suggested that the upper thermal stability limit of F-free pargasite is below 1050 °C, thermodynamic calculations for the present pargasite+quartz assemblage indicate that the thermal stability limit of pargasite with X F=0.5 is about 150 °C higher than that of the hydroxyl end member. Fluorine substitution in the pargasite therefore allowed the mineral to survive the ultrahigh-temperature metamorphism at Tonagh Island. A positive correlation between the F content of pargasite and coexisting biotite indicates that the minerals approach chemical equilibrium in terms of F–OH distribution. Although the fluorine composition of pargasites ( X F=0.12–0.48) and bulk rock (300–2500 ppm) varies widely, the log( f H 2O / f HF) values calculated for these rocks are relatively constant (3.2–3.7), which is consistent with infiltration of an F-bearing fluid during prograde metamorphism. The infiltration of such a fluid is also supported by the higher bulk F content of most of the analyzed samples compared to those of continental and oceanic basaltic rocks, that is, F had been added from an external source. A positive correlation between bulk MgO and F content suggests that F may have been selectively trapped in high- X Mg pargasite in MgO-rich rocks.

Multi-point time-series observation of optical emissions for flame-front motionanalysis

A simple optical diagnostic technique to analyse flame-front movement isproposed. Time-series, multi-point optical emission measurements using anaberration-free Cassegrain probe coupled with multiple fibres were performedsuccessfully in turbulent premixed flames. The spatial resolution of theCassegrain optics was examined using a CCD camera in free space and also inturbulent flame conditions. Spatial intensity distributions of OH∗, CH∗ and C2∗chemiluminescence measured in a two-dimensional laminar flame were comparedwith the numerical calculation of one-dimensional profiles of these species,demonstrating the ability of the present optical technique to detect a flame front.The probability density of the instantaneous speed and orientation angle offlame-front displacement were determined locally through a geometric motionanalysis of simultaneous, time-evolutional OH* signals at three points. Flow fieldsmeasured by PIV were compared with the flame-front velocities to examinedifferences between gas flow and flame-front dynamics. Errors due to flamecurvature, thermal density change and inclined propagation were alsoestimated.

Influence of Four Kinds of Gradient Magnetic Fields on Hydrogen-Oxygen Flame

To explore the possibility of flame control by magnetic force, the influence of four kinds of gradient magnetic fields on OH density distribution in a hydrogen-oxygen jet diffusion flame was investigated by numerically solving the equations of reactive gasdynamics and magnetism. Vertically decreasing (case 1), vertically increasing (case 2), horizontally decreasing (case 3), and horizontally increasing (case 4) gradients of magnetic field were considered as model configurations. According to the numerical analysis, because the mass density and the magnetic susceptibility of O 2 gas are much higher than those of other chemical species, the surrounding air containing a lot of O 2 gas is strongly influenced by the magnetic force, and the magnetically induced airflow changes the OH distribution in the flame indirectly. The direction and magnitude of the OH density change significantly depends on the configurations of the gradient magnetic field. Specifically, in cases 1, 3, and 4, the OH distribution migrated to the inside of the flame, but in case 2, it moved toward the outside. However, when the horizontally decreasing gradient of the magnetic field was set up near the burner outlet, as in case 3, the OH density change becomes largest among those in the four cases.

Hydrogen in some natural garnets studied by nuclear reaction analysis and vibrational spectroscopy

A suite of 11 gem-quality, optically completely clear garnet crystals with a broad variety of compositions in the space of the end members pyrope–almandine–spessartine–grossular–andradite–goldmanite were analyzed for trace amounts of “water” by nuclear reaction analysis, NRA, based on the reaction 1H(15N, αγ)12C, and by single-crystal absorption spectroscopy in the νOH vibrational range using microscope-FTIR-spectroscopic methods. The aim was to establish a calibration of the highly sensitive IR method with high areal resolution for “water” determination in garnets, by studying garnets of a wide compositional range, and to check for compositional dependencies of the integral molar absorptivities of the “water” component, ɛint[1molH 2 O −1cm−2], in the nominally “water”-free garnets. The results of NRA show a broad variation of water contents in the range (14 ± 3) to (950 ± 80) wt ppmH 2 O, the values being low and very high for the garnet solid solutions (PyrAlm)SS and close-to-end-member GrossSS, respectively. There were no indications of inhomogeneities in the OH distribution, except possibly for one of the garnets (grossular, variety hessonite, from Tanzania). The quantitative evaluation of the complex νOH spectra, which showed similar shape only for members of the (PyrAlm)SS, yielded integral absorption coefficients, αint (cm−2), which allowed the calculation of integral molar absorptivities, ɛint, using the “water” values of NRA. The ɛint values obtained varied in a wide range but with no obvious correlation with the composition of the garnet except for the extremely high values, in the 104 range, of the two specimen with compositions close to end-member grossular. In all other garnets, ɛint was in the 103 range with an average of ɛint=3630±1580[1molH 2 O −1cm−2]. Therefore, this value is proposed for the use in routine “water” determinations of compositionally different garnets by the micro-IR method, except for garnets near to end-member grossular.

18-cm VLA observations of OH towards the Galactic Centre

The OH distribution in the Sgr A Complex has been observed in the 1612-, 1665-, 1667- and 1720-MHz OH transitions with the Very Large Array (VLA) in BnA configuration. Maps are presented with a cha ...

Comparison of OH(A–X) chemiluminescence spectra from photo-initiated intra-cluster reaction of HBr–N 2O isomers prepared with He and Ar seeding gases

The internal energy distribution of product OH(A) was measured for the photo-initiated intra-cluster reaction of HBr–N 2O prepared using either Ar or He seeding gas. We find that rotation of OH(A) produced from HBr–N 2O seeded in Ar is more excited than that from HBr–N 2O seeded in He. This difference could be ascribed to the difference in relative abundance of HBr–N 2O geometric isomers in the cluster beam. The present result is consistent with the previous experimental finding in the focusing experiment of HBr–N 2O using the electrostatic hexapole field, namely the preferential formation of the linear isomer under Ar seeding condition.

ArF Laser Photodissociation Dynamics of 1,4-Pentadien-3-ol: Laser-Induced Fluorescence Observation of OH Rovibrational States

On photoexcitation at 193 nm, the 1 (π, π*) excited 1,4-pentadien-3-ol appears to be undergoing rapid internal conversion producing a highly energized ground electronic state, which is followed by dissociation to CH 2 = CH-CH-CH=CH 2 (pentadienyl) and OH radicals as primary products. While the laser-induced fluorescence (LIF) showed that only 1.1% of the nascent OH (X 2 Π) are produced in the vibrationally excited state with v = 1, there is no OH produced with v = 2. The rotational state distribution of OH is found to fit a Boltzmann distribution, characterized by a rotational temperature, T r o t , of 1250 ′ 100 K for the v = 0 and T c o t of 1020 ′ 100 K for the v = 1 vibrational states. By measuring the Doppler spectroscopy of the v = 0 and v = 1 states of OH, an average relative translational energy of the photofragments is found to be 41.8 ′ 5.0 and 37.4 ′ 5.0 kJ mol - 1 , respectively. The real time formation of OH shows a dissociation rate constant of the 1,4-pentadien-3-ol to be (2.0 ′ 0.4) x 10 7 s - 1 . The above dissociation rate in relation to statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory suggests a resonance stabilization energy of the pentadienyl radical to be 70 kJ mol - 1 .

A discussion on the determination of atmospheric OH and its trends

Abstract. The oxidation efficiency of the troposphere is largely determined by the hydroxyl radical and its global distribution. Its presence limits the lifetime of most trace gases. Because of the great importance of several of these gases for climate, ozone budget and OH itself, it is of fundamental importance to acquire knowledge about atmospheric OH and possible trends in its concentrations. In the past, average concentrations of OH and trends were largely derived using industrially produced CH3CCl3 as a chemical tracer. The analyses have given valuable, but also rather uncertain results. In this paper we describe an idealized computer aided tracer experiment which has as one of its goals to derive tracer concentration weighted, global average <k(OH)>, where the temporal and spatial OH distribution is prescribed and k is the reaction rate coefficient of OH with a hitherto never produced (Gedanken) tracer, which is injected at a number of surface sites in the atmosphere in well known amounts over a given time period. Using a three-dimensional (3-D) time-dependent chemistry transport model, <k(OH)> can be accurately determined from the calculated 3-D tracer distribution. It is next explored how well <k(OH)> can be retrieved solely from tracer measurements at a limited number of surface sites. The results from this analysis are encouraging enough to actually think about the feasibility to carry out a global dedicated tracer experiment to derive <k(OH)> and its temporal trends. However, before that, we propose to test the methods that are used to derive <k(OH)>, so far largely using CH3CCl3, with an idealized tracer experiment, in which a global chemistry transport model is used to calculate the ``Gedanken'' tracer distribution, representing the real 3-D world, from which <k(OH)> is derived, using only the tracer information from a limited set of surface sites. We propose here that research groups which are, or will be, involved in global average OH studies to participate in such an inter-comparison of methods, organized and over-seen by a committee appointed by the International Global Atmospheric Chemistry (IGAC) program.

Photodissociation dynamics of enolic-acetylacetone at 266, 248, and 193 nm: Mechanism and nascent state product distribution of OH

The photodissociation dynamics of acetylacetone (H3C–CO–CH2–CO–CH3), which exists predominantly as an enolic form [H3C–COCH=C(OH)–CH3] in gas phase, is studied using pulsed laser photolysis laser induced fluorescence (LIF) “pump-and-probe” technique at room temperature. Although two pathways for OH formation have been observed, we have focused on the nascent state of the primary OH radical, formed after photo-excitation of the molecule to its (π,π*) and Rydberg states. The (π,π*) and Rydberg transitions are prepared by excitation with fourth harmonic of Nd:YAG (266 nm)/KrF (248 nm) and ArF (193 nm) lasers, respectively. The ro-vibrational distribution of the nascent OH photofragment is measured in collision-free conditions using LIF. The OH fragments are formed in vibrationally cold state at all the above wavelengths of excitation, but differ in rotational state distributions. The rotational distribution is Boltzmann-like, and characterized by rotational temperatures of 950±50, 1130±60, and 1010±80 K at 266, 248, and 193 nm photodissociation, respectively. The spin–orbit and Λ-doublets ratios of OH fragments formed in the dissociation process are also measured. The average translational energy partitioned into the photofragment pairs in the center-of-mass co-ordinate is found to be 16.0−4.0+1.0, 17.3±4.2, and 19.2±4.7 kcal/mol at 266, 248, and 193 nm excitation, respectively. The energy partitioning into various degrees of freedom of products is interpreted with the help of different models, namely, statistical, impulsive, and hybrid models. To understand the nature of the dissociative potential energy surface involved in the OH production channel, detailed ab initio calculations are performed using configuration interaction-singles method. Although acetylacetone is initially prepared in the (1ππ*) state at 266 and 248 nm excitation, it is concluded that the OH fragment is formed from the lowest (3ππ*) state. However, upon excitation at 193 nm, the initially prepared Rydberg state of acetylacetone crosses over fast to the nearby σ* repulsive state along the C–OH bond, and dissociates to give the OH radical.

The seasonal cycle of cosmogenic 14CO at the surface level: A solar cycle adjusted, zonal‐average climatology based on observations

Mainly three processes determine the 14CO content of the troposphere: the cosmogenic production of 14C throughout the atmosphere, the removal (oxidation) by the hydroxyl radical (OH), and the transport of 14CO from the stratosphere into the troposphere. These characteristics make 14CO an interesting tracer for application in atmospheric dynamics and chemistry research. If 14CO observations from different times have to be compared for changes in OH or the stratosphere–troposphere exchange (STE), the varying global source strength of 14CO caused by variations in solar activity has to be taken into account. Three approaches for parameterizing solar activity with respect to the effect on the global average atmospheric 14CO production rate are developed and compared. These methods involve the sunspot number, the heliospheric potential, and atmospheric neutron monitor data. Applying further results from previous studies about the atmospheric response, a rescaling procedure for 14CO observations of different epochs to the same solar activity conditions is derived. All three approaches yield consistent results, whereby small differences provide an uncertainty estimate. The rescaling is used to compile the first solar cycle adjusted, zonal‐average 14CO climatology (cosmogenic contribution) from all currently available 14CO observations at the surface level. For this, in a first step, the smaller contribution of 14CO of noncosmogenic, that is, biogenic origin (secondary source) is estimated from CO observations and subtracted from the measurements. The resulting climatology can be used to trace changes if compared to future observations, and especially for the evaluation of OH distributions and the stratosphere–troposphere exchange in three‐dimensional global atmospheric models.

Photodissociation Dynamics of Propiolic Acid at 193 nm: The State Distribution of the Nascent OH Product

Photolysis of propiolic acid (HC⋮C−COOH) upon π−π* excitation at 193 nm leads to the C−O bond fission generating the primary product OH in a good yield. The partitioning of the available energy into the internal states and the translation of OH(v,J) is evaluated by measuring the relative intensities of ro-vibronic lines employing laser-induced fluorescence and their Doppler profiles, respectively. The vibrational distribution of nascent OH(v‘ ‘=0 and 1) corresponds to the vibrational temperature of 1030 ± 80 K. The rotational population of OH(v‘ ‘=0 and 1) is characterized by rotational temperatures of 800 ± 30 and 620 ± 30 K, respectively. Using widths of the Doppler-broadened lines, the OH(v‘ ‘=0) average translational energy was measured to be 24.4 ± 3.0 kcal/mol, which implies about 75% of the total available energy goes to the translation of the fragments. The observed high translational energy is due to the presence of a barrier in the exit channel, suggesting the C−O bond cleavage occurs on an electronically excited potential energy surface. The measured partitioning of the available energy between the fragments OH and HCCCO is explained using the hybrid model with 37.5 kcal/mol of barrier in the exit channel. We employed ab initio molecular orbital theory to calculate structures and energetics of the ground and the excited electronic states of propiolic acid.

Ultraviolet and visible dispersed spectroscopy for the photofragments produced from H2O in the extreme ultraviolet

The photofragmentation of H2O has been studied by fluorescence spectroscopy at photon energies between Ehν=16.9–54.5 eV. The primary photon beam was monochromatized undulator radiation supplied from the UVSOR synchrotron radiation facility. The fluorescence in the wavelength range of 280–720 nm was dispersed with an imaging spectrograph. The dispersed spectra exhibit the hydrogen Balmer lines of H*[n2LJ′′→2 2LJ″″(n=3–9)] and the emission band systems of H2O+[Ã 2A1(0,v2′,0)→X̃ 2B1(0,0,0)], OH+(Ã 3ΠΩ,v′→X̃ 3Σ−,v″), and OH(Ã 2Σ+,v′→X̃ 2ΠΩ,v″). The fluorescence cross sections for these transitions have characteristic dependences on Ehν and vibrational quantum numbers. The cross section summed over the Balmer lines takes a minimum value at Ehν=21.7 eV and is very small even at 24.9 eV beyond which it steadily increases with increasing Ehν. This behavior is understood as that the superexcited states correlating with H*(n⩾3)+OH(Ã 2Σ+) are too repulsive to be accessible below Ehν∼30 eV by the Franck–Condon transitions from H2O(X̃ 1A1) and as that the Balmer emission below 30 eV is mainly due to the H*(n⩾3)+H(n=1)+O(3Pg) channel. The appearance energy of the OH+(Ã 3ΠΩ,v′→X̃ 3Σ−,v″) transitions is found to be ca. 25.5±0.3 eV. This value is much higher than the dissociation limit of 21.5 eV for the OH+(Ã 3ΠΩ)+H(n=1) channel, but is consistent with the vertical ionization energy to H2O+[(1b1)−2(4a1)1 2A1] that has been assumed to correlate with the above dissociation limit in the literature. The vibrational distribution of OH+(Ã 3ΠΩ) evaluated from the OH+(Ã 3ΠΩ,v′→X̃ 3Σ−,v″) band intensities is similar to the prior distribution in the rigid-rotor harmonic-oscillator approximation.

Evaluation of stratosphere–troposphere exchange and the hydroxyl radical distribution in three‐dimensional global atmospheric models using observations of cosmogenic 14CO

A new method for the quantitative evaluation of global atmospheric transport and the hydroxyl radical (OH)‐based oxidation in three‐dimensional (3‐D) atmospheric chemistry transport models (CTMs) and general circulation models (GCMs) is developed. The method is based on a cosmogenic 14CO climatology that has been previously derived from a large number of 14CO observations. Using 14CO measurements to constrain model OH distributions and the simulated stratosphere–troposphere exchange (STE) provides a challenging test for 3‐D atmospheric models. Here, the evaluation method is applied to the CTMs MATCH and TM3. Whereas MATCH overestimates the STE in both hemispheres, TM3 does reproduce the STE in the Southern Hemisphere (SH) but underestimates it in the Northern Hemisphere (NH). The STE phase in MATCH is 1 month too early, whereas no significant phase shift for TM3 is revealed. These characteristic deficiencies in both models were consistently determined, i.e., with the same boundary conditions (OH distribution and 14CO source distribution). The robustness of the results is tested by various sensitivity studies, involving the 14CO source distribution and strength, the tropospheric OH distribution, the stratospheric OH abundance, and the applied numerical advection scheme. Consistency is further checked by comparison of the model simulated vertical 14CO profiles to 14CO observations from a number of aircraft campaigns. The 14CO simulations do not support an interhemispheric asymmetry in the OH abundance with an on average higher concentration in the SH.

Direct Observation of OH Formation and Luminescent Emission from Photoexcited Acetaldoxime

On photoexcitation at 193 nm, the 1(π, π*) excited acetaldoxime (CH3−CHN−OH) appears to be undergoing intersystem crossing producing a highly energized triplet state, which is followed by parallel processes of the emission of a UV photon at 310 nm and the dissociation to CH3−CHN and OH radicals as primary products. While the laser-induced fluorescence showed that only 1.5% of the nascent OH (X2Π) is produced in the vibrationally excited state with v = 1, there is no OH produced with v = 2. The rotational state distribution of OH is found to fit a Boltzmann distribution, characterized by a rotational temperature Trot of 1200 ± 120 K for the v = 0 and Trot of 990 ± 100 K for the v = 1 vibrational states, respectively. By measuring the Doppler spectroscopy of the v = 0 and v = 1 states of OH, the translational energy of the photofragments is found to be 40.0 ± 5.0 and 32.2 ± 4.0 kcal mol-1, respectively. While 20 kcal mol-1 of translational energy is expected statistically, imparting such a large amount of translational energy into the products suggests that the dissociation occurs on the excited state potential energy surface. The real time formation of OH shows a dissociation rate of the acetaldoxime to be (1.5 ± 0.3) × 106 s-1. The above dissociation rate vis-à-vis statistical Rice−Ramsperger−Kassel−Marcus theory suggests that the acetaldoxime dissociates from the triplet state, with a threshold dissociation energy of about 49 kcal mol-1. The decay of the triplet acetaldoxime emission at 310 nm with a rate of (1.2 ± 0.3) × 106 s-1, similar to that of its dissociation to form OH, further suggests that both the competitive decay processes occur from the triplet state potential energy surface.