OH Vibrational Distributions Research Articles

OH vibrational distributions: A comparison of Einstein [formula omitted] transition probabilities

A limited number of studies have measured OH vibrational temperatures due in part to the difficulty of simultaneously observing multiple ro-vibrational emission bands. With questions still remaining about the quenching processes of the vibrationally excited OH molecule, observations of vibrational temperatures are critical to better characterize OH vibrational distributions. The Einstein A transition probabilities are a key element of OH population measurements, and their impact upon these observations have not been fully examined. The Perkins Airglow Study (PAS) is a spectroscopic survey of airglow emission and OH vibrational temperature, in the H-band, at a latitude of 35∘ N. The spectra have a resolution of r=λ/Δλ≈600, and employ a 10 s integration time. Airglow emission was continuously observed over the course of twenty entire nights, and the spectra contain emission from the OH(6-4), OH(5-3), OH(4-2), and OH(3-1) ro-vibrational bands. Currently PAS has recorded nearly 55,000 airglow spectra, and these observations significantly increase the current number of OH vibrational temperature measurements. The Einstein A coefficients examined in this work were found to have a significant impact upon the measured populations by nearly a factor of two. The observed OH vibrational temperatures averaged 10,500 K, and the Einstein A coefficients were found to vary the average measured vibrational temperature by 1,100 K.

Insights into the Formation of Hydroxyl Radicals with Nonthermal Vibrational Excitation in the Meinel Airglow.

To understand night time airglow in the Meinel bands and heat conversion from the highly excited OH radicals in the upper atmosphere via the important atmospheric reaction H + O3 → OH + O2, we report here a quasi-classical trajectory study of the reaction dynamics on a recently developed full-dimensional potential energy surface (PES). Our results indicate that the reaction energy of this highly exoergic reaction is almost exclusively channeled into the vibration of the OH product, underscoring an extreme departure from the statistical limit. The calculated OH vibrational distribution is highly inverted and peaks near the highest accessible vibrational state, in excellent agreement with experimental observations, validating the accuracy of the PES. More importantly, the dynamical origin of the nonthermal excitation of the OH vibrational mode is identified by its large projection onto the reaction coordinate at a small potential barrier in the entrance channel, which controls the energy flow into various degrees of freedom in the products.

Venus night airglow: Ground-based detection of OH, observations of O 2 emissions, and photochemical model

Venus nightglow was observed at NASA IRTF using a high-resolution long-slit spectrograph CSHELL at LT = 21:30 and 4:00 on Venus. Variations of the O 2 airglow at 1.27 μm and its rotational temperature are extracted from the observed spectra. The mean O 2 nightglow is 0.57 MR at 21:30 at 35°S–35°N, and the temperature increases from 171 K near the equator to ∼200 K at ±35°. We have found a narrow window that covers the OH (1–0) P1(4.5) and (2–1) Q1(1.5) airglow lines. The detected line intensities are converted into the (1–0) and (2–1) band intensities of 7.2 ± 1.8 kR and <1.4 kR at 21:30 and 15.5 ± 2 kR and 4.7 ± 1 kR at 4:00. The f-component of the (1–0) P1(4.5) line has not been detected in either observation, possibly because of resonance quenching in CO 2. The observed Earth’s OH (1–0) and (2–1) bands were 400 and 90 kR at 19:30 and 250 and 65 kR at 9:40, respectively. A photochemical model for the nighttime atmosphere at 80–130 km has been made. The model involves 61 reactions of 24 species, including odd hydrogen and chlorine chemistries, with fluxes of O, N, and H at 130 km as input parameters. To fit the OH vibrational distribution observed by VEX, quenching of OH ( v > 3) in CO 2 only to v ⩽ 2 is assumed. According to the model, the nightside-mean O 2 emission of 0.52 MR from the VEX and our observations requires an O flux of 2.9 × 10 12 cm −2 s −1 which is 45% of the dayside production above 80 km. This makes questionable the nightside-mean O 2 intensities of ∼1 MR from some observations. Bright nightglow patches are not ruled out; however, the mean nightglow is ∼0.5 MR as observed by VEX and supported by the model. The NO nightglow of 425 R needs an N flux of 1.2 × 10 9 cm −2 s −1, which is close to that from VTGCM at solar minimum. However, the dayside supply of N at solar maximum is half that required to explain the NO nightglow in the PV observations. The limited data on the OH nightglow variations from the VEX and our observations are in reasonable agreement with the model. The calculated intensities and peak altitudes of the O 2, NO, and OH nightglow agree with the observations. Relationships for the nightglow intensities as functions of the O, N, and H fluxes are derived.

Modeling Reactive Scattering of F(2P) at a Liquid Squalane Interface: A Hybrid QM/MM Molecular Dynamics Study

To better understand the reactivity of gases with liquid surfaces, experimentalists have recently probed the reactive scattering of atomic fluorine at the surface of liquid squalane (C(30)H(62)). In this paper we further this research by simulating this scattering process at collision energies of 0.5 and 1.0 eV using a hybrid QM/MM molecular dynamics scheme. To model the structure of the liquid surface, classical molecular dynamics calculations were performed utilizing the OPLS-AA force field. During the F + squalane molecular dynamics simulation, QM/MM calculations are performed at every trajectory step by combining the MSINDO semiempirical Hamiltonian with OPLS-AA and using a dynamic partitioning of the atoms in the QM or MM regions via a "seed atom" method. This computational model provides a type of "on-the-fly" direct dynamics applicable to larger scale chemical processes that include the making/breaking of chemical bonds not available in standard force field models. Our results show that H abstraction is the only reactive scattering pathway and that most trajectories result in reactive scattering. Reaction statistics at the squalane surface are discussed, including variation of the results with incident angle and collision energy, and the probability of reaction as a function of carbon atom type, collision depth, and residence time. Product states, including angular distributions and final translational and rovibrational energies, are also considered and found to be significantly affected by the exothermic reaction energy for H abstraction. The vibrational distributions are in good agreement with recent experiments, but the rotational distributions are dominated by a nonthermal component while the experiments, which involve thermal incident energies, show comparable thermal and nonthermal contributions. Results for O + squalane at 1.0 eV, which we also present, show analogous comparisons with experiment, with OH vibrational distributions which are cold and match experiment, while the calculated rotational distributions are dominated by nonthermal behavior.

Nascent vibrational distributions and relaxation rates of diatomic products of the reactions of O( 1D) with CH 4, C 2H 6, CH 3F, CH 2F 2 and CHF 3 studied by time resolved Fourier transform infrared emission

Time resolved Fourier transform infrared (TRFTIR) emission has been used to study the reactions of CH 4, C 2H 6, CH 3F, CH 2F 2 and CHF 3 with O( 1D). One hundred and ninety-three nanomters photolysis of N 2O was used to prepare O( 1D), and emission analysed from OH( ν = 1–4) for the two hydrocarbons and HF( ν = 1–6) from CH 3F, CH 2F 2 and CHF 3. For the O( 1D) + CH 4 reaction, the nascent OH vibrational distribution showed a population inversion between ν = 1 and 2, and was in excellent agreement with previous laser induced fluorescence and TRFTIR data, as well as with quasi-classical trajectory calculations. Time resolved populations were analysed to yield rate constants for vibrational relaxation of OH( ν) with CH 4, and found to be consistent with stepwise deexcitation rather than chemical removal being dominant. Reaction with C 2H 6 produced a monotonically decreasing population in ν = 1–4 and more rapid relaxation rates than those with methane. For the fluorinated methanes, nascent vibrational populations in HF( ν = 1–6) were measured and shown to be very similar, all monotonically decreasing with ν, and fitting the same vibrational surprisal plot, showing a larger than statistical partitioning of the available energy in vibration. Relaxation rate constants of HF( ν) with the parent fluorinated methane showed values, which increased with increasing H atom content.

A quantum-mechanical study of the dynamics of the O(1D)+H2→OH+H insertion reaction

We have studied the quantum dynamics of the O(1D)+H2(X 1Σg+)→ OH(X 2Π)+H(2S) reaction at low collision energies. The hyperspherical method has been used in a time-independent formalism. We present rotational and vibrational distributions. OH vibrational distributions are found to be noninverted. Differential cross sections are almost symmetric in the forward and backward directions, with a slight preference for forward scattering. These features are consistent with the formation of an intermediate complex.

Influence of the Collision Energy on the O(1D) + RH → OH(X2Π) + R (RH = CH4, C2H6, C3H8) Reaction Dynamics: A Laser-Induced Fluorescence and Quasiclassical Trajectory Study

The influence of the collision energy (ET) on the O(1D) + RH → OH(X2Π) + R (RH = CH4, C2H6, and C3H8) reaction dynamics has been studied, using the N2O photodissociation at 193 nm as O(1D) precursor (〈ET〉 = 0.403 eV) and probing the OH v‘ ‘ = 0 and 1 levels by LIF. A triatomic QCT study of the reaction with CH4 on a fully ab initio based analytical PES has also been performed, and a quite good agreement with the experimental OH rovibrational distributions has been obtained. Our experimental results are similar to those obtained when the O3 photodissociation is used to produce O(1D) (〈ET〉 = 0.212 eV), as expected on the basis of the available energy in products and also from the QCT calculations. The P(v‘ ‘=0)/P(v‘ ‘=1) populations ratio values reported for C2H6 and C3H8 in a very recent work (Wada and Obi, J. Phys. Chem. A 1998, 102, 3481), where the N2O was also used to generate O(1D), are probably largely underestimated. The rotational distributions obtained are similar to those obtained in other experiments, and a quite good agreement has been obtained for the spin−orbit and Λ-doublet populations. The reaction takes place near exclusively through the insertion of the O(1D) atom into a C−H bond below 0.6 eV, and the mechanism may be direct or nondirect (mainly through short-lived (CH3)OH collision complexes) with about the same probability. The OH vibrational distribution arising from the direct mechanism is inverted, while the nondirect one leads to a noninverted distribution. At higher ET, the abstraction mechanism also contributes appreciably to reactivity.

Ab initio ground potential energy surface and quasiclassical trajectory study of the O(1D)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction dynamics

An ab initio study of the ground potential energy surface (PES) of the O(1D)+CH4→OH+CH3 reaction has been performed using the second and fourth order Mo/ller–Plesset methods with a large basis set. From the ab initio data a triatomic analytical ground PES with the methyl group treated as an atom of 15.0 amu has been derived. This PES has been employed to study the dynamics of the reaction by means of the quasiclassical trajectory (QCT) method. A good agreement between the experimental and QCT OH rovibrational distributions at a collision energy of 0.212 eV with the methane molecule at 298 K has been obtained. The analysis of the microscopic reaction mechanism shows that the reaction takes place almost exclusively through the insertion of the O(1D) atom into a C–H bond, due to the presence of the deep (CH3)OH minimum, and the resulting trajectories may be direct or nondirect (short-lived collision complexes mainly) with about the same probability. The OH vibrational distribution arising from the direct mechanism is inverted, while the nondirect mechanism leads to a noninverted one. There is some tendency to give broader OH rotational distributions peaking at higher N′ values, particularly for the vibrational levels v′=0–1, in the case of the nondirect trajectories. The PES derived here may be used in dynamics studies under conditions where the methyl group motions are not strongly coupled to the motions leading to reaction.

The altitude dependence of the OH(X 2Π) vibrational distribution in the nightglow: Some model expectations

A number of different OH Meinel band excitation models are used to investigate the extent to which quenching and vibrational deactivation processes could cause the nightglow OH vibrational distributions to vary with altitude. The various models, which are based upon those described by McDade and Llewellyn (1988, Planet. Space Sci. 36, 897), are used to calculate the steady state OH vibrational distributions throughout the 80–100 km region. The results of the calculation show that, irrespective of the basic model assumptions, the OH vibrational distributions should not be strongly altitude dependent and that the emission profiles of the Meinel bands from the υ′ levels 1 through 9 should not differ in altitude by more than one or two kilometers. The results of some recent laboratory studies pertaining to the Meinel band excitation mechanism are also discussed and it is shown that stepwise collisional vibrational deactivation is unlikely to play a major role in controlling the observed nightglow OH vibrational distribution.

Trajectory studies of OH vibrational excitation propensities in the reaction of O(1D) with H2

The reaction of excited oxygen atoms with hydrogen molecules has been explored using classical trajectory methods, with particular emphasis on those features of the dynamics responsible for the OH vibrational distribution. Plots of final OH vibrational action vs. initial H2 phase are analyzed. A Monte Carlo ensemble of 6300 trajectories is also studied. Two methods are used to classify trajectories: (1) the amount of exponential separation of nearby trajectories and (2) the number of minimum bond length exchanges during the course of the trajectory. Trajectories which undergo different numbers of minimum exchanges are found to have markedly different propensities for producing OH in a given vibrational state. Direct insertions are found to play an important role in the reaction; two very direct types of trajectories are identified, one preferentially populating OH in v=0; the other leading to highly vibrationally excited OH (v=4 or 5). Slightly less direct trajectories are found to result preferentially in moderately excited OH(v=2). The disposition of energy in OH vibration is found to be highly correlated with the manner in which the complex dissociates; the most important patterns of dissociation are identified and described.

OH vibrational distribution in the reaction O( 1D)+CH 4

Vibrationally excited OH radicals were generated by 266 nm laser photolysis of ozone in the presence of methane. Temporal profiles of OH (ν=0, 1, 2) concentrations were directly monitored by standard LIF techniques at various CH 4 pressures. The total vibrational distribution of OH product in the reaction O ( 1D) + CH 4 was found to be much hotter than the statistical distribution P(ν=0): P(ν=1): P(ν=2): P(ν=3): P(ν=4) = 0.18: 0.29: 0.37: 0.15: 0.01. The rate constants for vibrational relaxation of OH(ν=1, 2, 3) in collisions with CH 4 were measured to be (5.1±1.0) × 10 −13, (1.5±0.4) × 10 −12 and (6.8±2.0) × 10 −12 cm 3/s, respectively.

Kinetic parameters related to sources and sinks of vibrationally excited OH in the nightglow

Kinetic parameters related to vibrational deactivation and chemical removal of vibrationally excited OH radicals in the mesosphere are deduced from ground‐based measurements of the mean vibrational distribution of the OH Meinel bands in the nightglow. The derived parameters, which rely on a laboratory measured rate coefficient for the removal of OH(υ = 9) by O2 (Finlayson‐Pitts and Kleindienst, 1981) and a set of relative Meinel band transition probabilities (Murphy, 1971), have been obtained for two limiting Meinel band excitation models that differ in the extent to which single‐quantum vibrational deactivation and “sudden death” collisional removal processes determine the OH vibrational distribution. It is shown that the OH Meinel band emission can be adequately explained with the deduced parameters and the H + O3→OH + O2 reaction as the only chemical source of vibrationally excited OH. Evidence is presented which suggests that the reaction HO2 + O ↔ HO + O2 may perhaps be involved as a sink of vibrationally excited OH rather than as a potential source. The deduced kinetic parameters should be particularly useful in future Meinel band studies as they have been obtained from an analysis for which there is no assumption about the very uncertain OH radiative lifetimes.

Energy partitioning in O(1D2) reactions. II. O(1D2)+CH4 → OH(v′)+CH3

The complete OH vibrational distribution created by the reaction of O(1D2) with CH4 has been recorded directly, using low pressure infrared emission spectroscopy. The result shows that the energy is partitioned nonstatistically into this degree of freedom. The population of OH(v′=2) is approximately equal to or slightly greater than that of OH(v′=1) under the conditions of this experiment and substantial populations are recorded from all levels up to OH(v′=4), the maximum allowed by the exoergicity of the reaction.