Photodissociation Of OH Research Articles

Near-threshold photodissociation of cool OH+ to O + H+ and O+ + H.

We have measured the spectrum of laser photodissociation of OH+ molecular ions to O + H+ and O+ + H fragments for photon energies of 38 100-40 900 cm-1. The OH+ ions were stored as a fast beam (5.50 MeV) in the storage ring TSR for several seconds to achieve rovibrational cooling into the lowest rotations N'' = 0-11 of the vibrational ground state X3Σ-(v'' = 0), close to room temperature (≈300 K). The many resonances in the spectra reveal the energies, widths, and O/O+ branching ratios of 44 predissociating quasibound levels (Feshbach resonances) that lie between the fine-structure states of the O fragment and belong to the last, near-threshold vibrational states v' = 9 and 10 of the A3Π electronic state. For the A3Π0,1 substates, isolated levels with v' = 11 are observed and attributed to double-well distortions of these curves due to nonadiabatic interactions. Another five isolated levels are assigned to the v' = 0 and 1 states of the shallow 15Σ- electronic state, borrowing oscillator strength from nearby A3Π levels. Together, the near-threshold levels deliver a new value D0 = 40 253.8(1.1) cm-1 for the dissociation energy of OH+. Through a two-step photodissociation process, 72 levels from the lower bound states A3Π(v' = 7-8) appear as well and are rotationally analyzed. The level energies are used to construct improved A3Π and 15Σ- Born-Oppenheimer potentials. The totality of the spectral data (energies, widths, intensities, and branching ratios) can provide tight constraints for the potentials and nonadiabatic interactions assumed in future coupled-channel calculations of OH+ photodissociation or of the related charge-exchange reaction O + H+ → O+ + H.

The benchmark halo giant HD 122563: CNO abundances revisited with three-dimensional hydrodynamic model stellar atmospheres

We present an abundance analysis of the low-metallicity benchmark red giant star HD 122563 based on realistic, state-of-the-art, high-resolution, three-dimensional (3D) model stellar atmospheres including non-grey radiative transfer through opacity binning with four, twelve, and 48 bins. The 48-bin 3D simulation reaches temperatures lower by ~ 300 - 500 K than the corresponding 1D model in the upper atmosphere. Small variations in the opacity binning, adopted line opacities, or chemical mixture can cool the photospheric layers by a further ~ 100 - 300 K and alter the effective temperature by ~ 100 K. A 3D local thermodynamic equilibrium (LTE) spectroscopic analysis of Fe I and Fe II lines gives discrepant results in terms of derived Fe abundance, which we ascribe to non-LTE effects and systematic errors on the stellar parameters. We also determine C, N, and O abundances by simultaneously fitting CH, OH, NH, and CN molecular bands and lines in the ultraviolet, visible, and infrared. We find a small positive 3D-1D abundance correction for carbon (+0.03 dex) and negative ones for nitrogen (-0.07 dex) and oxygen (-0.34 dex). From the analysis of the [O I] line at 6300.3 {\AA}, we derive a significantly higher oxygen abundance than from molecular lines (+0.46 dex in 3D and +0.15 dex in 1D). We rule out important OH photodissociation effects as possible explanation for the discrepancy and note that lowering the surface gravity would reduce the oxygen abundance difference between molecular and atomic indicators.

First detection of [OI] 630 nm emission in the Enceladus torus

Observations of [OI] 630 nm emission in the Enceladus torus around Saturn have been made at the summit of Mt. Haleakala in Hawaii using a high‐dispersion echelle spectrograph coupled to a 40 cm telescope in the period of 13 May through 19 June 2011. A slit of the spectrograph was aligned perpendicular to the equatorial plane of Saturn and placed at a distance of 4 Saturn's radii (Rs) from the planetary center in the dawn side to put the Enceladus torus within the field of view. As a result, [OI] 630 nm torus emission was detected with S/N ~ 7 for summed exposure of 20 h during the observing period. The observed brightness has a maximum value of 4.1 ± 0.6 Rayleighs (R) near the equator, and it extends to north‐south (N‐S) direction with a full width at half maximum of 0.8 Rs. We made estimation to explain mechanism of the observed brightness taking into account an excitation of [OI] 630 nm by electron impact and photodissociation of water group molecule (OH and H2O). Densities of electron, O, OH, and H2O and electron temperature derived from data taken by Cassini and Hubble telescope were used for the estimation. The observed brightness is reasonably explained, taking into account an uncertainty of estimation depending on N‐S distributions of species and quiet solar activity conditions. The estimation also suggests that [OI] 630 nm emission is excited by photodissociation of OH and H2O and by electron impact of O with their contributions of 50%, 30%, and 20%, respectively, for quiet solar activity. We also note that the intensity due to photodissociation has considerable variability depending on the level of solar activity by a factor of 2.5.

EMISSION LINES FROM THE GAS DISK AROUND TW HYDRA AND THE ORIGIN OF THE INNER HOLE

We compare line emission calculated from theoretical disk models with optical to sub-millimeter wavelength observational data of the gas disk surrounding TW Hya and infer the spatial distribution of mass in the gas disk. The model disk that best matches observations has a gas mass ranging from $\sim10^{-4}-10^{-5}$\ms\ for $0.06{\rm AU} <r<3.5$AU and $\sim 0.06$\ms\ for $ 3.5 {\rm AU} <r<200$AU. We find that the inner dust hole ($r<3.5$AU) in the disk must be depleted of gas by $\sim 1-2$ orders of magnitude compared to the extrapolated surface density distribution of the outer disk. Grain growth alone is therefore not a viable explanation for the dust hole. CO vibrational emission arises within $r\sim 0.5$AU from thermal excitation of gas. [OI] 6300\AA\ and 5577\AA\ forbidden lines and OH mid-infrared emission are mainly due to prompt emission following UV photodissociation of OH and water at $r\lesssim0.1$AU and at $r\sim 4$AU. [NeII] emission is consistent with an origin in X-ray heated neutral gas at $r\lesssim 10$AU, and may not require the presence of a significant EUV ($h\nu>13.6$eV) flux from TW Hya. H$_2$ pure rotational line emission comes primarily from $r\sim 1-30$AU. [OI]63$\mu$m, HCO$^+$ and CO pure rotational lines all arise from the outer disk at $r\sim30-120$AU. We discuss planet formation and photoevaporation as causes for the decrease in surface density of gas and dust inside 4 AU. If a planet is present, our results suggest a planet mass $\sim 4-7$M$_J$ situated at $\sim 3$AU. Using our photoevaporation models and the best surface density profile match to observations, we estimate a current photoevaporative mass loss rate of $4\times10^{-9}$\ms\ yr$^{-1}$ and a remaining disk lifetime of $\sim 5$ million years.

Large‐Aperture [Oi] 6300 A Photometry of Comet Hale‐Bopp: Implications for the Photochemistry of OH

Large-aperture photometric observations of comet Hale-Bopp (C/1995 O1) in the forbidden red line of neutral oxygen ([O I] 6300 angstroms) with the 150 mm dual-etalon Fabry-Perot spectrometer that comprises the Wisconsin H-alpha Mapper and a 50 mm dual-etalon Fabry-Perot spectrometer at the McMath-Pierce main telescope from 1997 late February to mid April yield a total metastable O((sup 1)D) production rate of (2.3-5.9) x 10(exp 30)/s. Applying the standard H2O and OH photodissociation branching ratios, we derive a water production rate, Q(H2O), of (2.6-6.1) x 10(exp 31)/s, which disagrees with Q(H2O = 1x10(exp 31)/s determined by independent H2O, OH, and H measurements. Furthermore, our own [O I] 6300 observations of the inner coma (< 30,000 km) using the 3.5 m Wisconsin-Indiana-Yale-NOAO telescope Hydra and Densepak multi-object spectrographs yield Q(H2O) = 1 x 10(exp 31)/s. Using our [O I] 6300 data, which cover spatial scales ranging from 2,000 to 1x10(exp 6) km, and a complementary set of wide-field ground-based OH images, we can constrain the sources of the apparent excess O((sup 1)D) emission to the outer coma, where photodissociation of OH is assumed to be the dominant O((sup 1)D) production mechanism. From production rates of other oxygen-bearing volatiles (e.g., CO and CO2), we can account for at most 30% of the observed excess O((sup 1)D) emission. Since even less O((sup 1)D) should be coming from other sources (e.g., electron excitation of neutral O and distributed nonnuclear sources of H2O), we hypothesize that the bulk of the excess O((sup 1)D) is likely coming from photodissociating OH. Using the experimental OH photo-dissociation cross section of Nee and Lee at Ly-alpha as a guide in modifying the theoretical OH cross sections of van Dishoeck and Dalgarno, we can account for approximately 60% of the observed O((sup 1)D) excess without requiring major modifications to the other OH branching ratios or the total OH photodissociation lifetime.

Optical Line Emission from Photodissociation Regions

We present a parameter study for the optical, low-ionization [O I] λ6300, λ6363, and λ5577 and the [S II] λ6731 and λ6716 lines emitted by photodissociation regions (PDRs). We assume in these models that the PDR has a plane-parallel, semi-infinite geometry and is illuminated from one side. The PDR models are characterized by the total hydrogen density 104 cm-3 ≤ n ≤ 107 cm-3 and the strength of the FUV field G0 in terms of the Habing field 103 < G0 < 107. Oxygen and singly ionized sulfur are excited thermally in the outer hot PDR layers due to collisions with electrons and atomic hydrogen. [O I] photons are also produced in the photodissociation of OH. We apply the results of the parameter study to the protoplanetary disks in Orion, layers between H II regions and molecular clouds, low-ionization nuclear emission-line regions, T Tauri stars, and planetary nebulae. We discuss the extent to which PDRs dominate or contribute to the observed [O I] and [S II] emission in these different objects.

Vector properties of O(3P) and O(1D) in OH photodissociation: Near-threshold resonance, neighboring resonance, and above-threshold behavior

Theoretical analysis is presented for the vector properties of the oxygen atom produced from the photodissociation of OH at energies from near the threshold to O(3P) to above the threshold to O(1D). The vector properties of O(3Pj, j=0,1,2) (angular distributions and alignment parameters) change rapidly across multichannel resonances near the dissociation threshold to O(3P). The vector properties near the close-lying Lorentzian resonances depend highly on the geometrical factors for the electronic transitions, corresponding to different final total angular momentum quantum numbers, and change rapidly between the resonances. At energies above the dissociation threshold to O(1D), quantum interference between the two direct dissociation pathways gives rise to highly oscillatory pattern of the vector properties of O(1D).

Control of vector properties in vibrationally mediated photodissociation near asymmetric resonances

Quantum mechanical analysis is presented on the control of the vector properties of the photoproducts by vibrationally mediated photodissociation of OH. The angular distributions and alignment of O(3P) fragments are calculated near isolated and overlapping asymmetric resonances. The vector properties depend very sensitively on the vibrational levels (νi=0–4) of the initial X 2Π state. The variations of the anisotropy parameters as a function of the excitation energy near the asymmetric resonances change markedly depending on νi. The widths of the variations tend to increase with increasing νi, which is very similar to the corresponding behaviors of the product branching ratios studied earlier [J. Chem. Phys. 104, 1912 (1996)], indicating that νi could be a useful tool for choosing the proper linewidths in the experiments for the control of the product branching ratios, angular distributions, and m distributions near asymmetric resonances. It is also found that the vector properties may exhibit splitting of the overlapping resonances for high νi, in contrast to scalar properties.

Quantum mechanical analysis of photofragment alignment near asymmetric resonances

Quantum mechanical analysis is presented for the alignment of the oxygen atoms produced from the photodissociation of OH. The alignment parameters are predicted to be independent of energy across the isolated Lorentzian resonances, when only one channel contributes to indirect dissociation. When more than one channel interferes with one another, they may change very slowly. Across the asymmetric resonances, the alignments exhibit rapid variations due to the quantum interference between the indirect and the direct dissociation pathways. The alignments of O(3P2) and O(3P1) exhibit different variations, both of which are asymmetric across the asymmetric resonances. It is also shown that photoexcitation to repulsive states, coupled with bound electronic states, can give asymmetric resonances and sharp variations of the alignment, suggesting that the analysis of the dynamics of direct photodissociation by measurements of vector properties could be complicated by the effects of quantum interference.

Angular distributions near resonances: Effects of neighboring resonances and quantum interference.

Quantum-mechanical analysis is presented on the photodissociation dynamics near the resonances. It is shown that the product angular distributions exhibit very different variations depending on the characteristics of resonances. The fragment anisotropy parameters in OH photodissociation are predicted to be independent of energy along isolated Lorentzian resonances. Interferences between neighboring Lorentzian resonances give significant variations of the anisotropy parameters. Quantum interference between the direct and indirect dissociation pathways also contributes to very strong variations of the anisotropy parameters across asymmetric resonances. \textcopyright{} 1997 The American Physical Society.[S1050-2947(97)501412-6]

Selective production of photofragments by monitoring the shape of asymmetric resonances in OH photodissociation: Dependence on initial vibrational states

Quantum mechanical analysis is presented on the vibrational state dependence of the total dissociation cross sections and the branching ratios of O(3Pj, j=0,1,2) in the predissociation of OH. Two transformation matrices, each of which describes the relation between an atomic term limit and the correlating molecular states, are constructed and incorporated in the close coupling calculations. The branching ratios of O(3Pj, j=0,1,2) depend very sensitively on the vibrational levels (v=0–4) of the initial X 2Π state. The variations of the spin–orbit distributions as a function of the excitation energy near the asymmetric resonances change markedly depending on the vibrational levels. The variations are either redshifted or blueshifted from the resonance position, depending on the degree of asymmetry of the resonances. The widths of the variations tend to increase with increasing vibrational quantum number of the initial state, suggesting the possibility of choosing the proper linewidths in the experiments to selectively produce the photofragments in one-photon process. Discussion is presented on the applicability of the theoretical scheme to analyze the recent measurements by Neumark and co-workers [J. Chem. Phys. 103, 2495 (1995)] on the product fine structure distributions in the predissociation of O2.

Variation of fragment anisotropy parameters along asymmetric resonances in OH photodissociation. A quantum-mechanical study

Quantum mechanical analysis is presented for the anisotropy parameters of O(3Pj, j = 0, 1, 2) resulting from the photodissociation of OH. The angular distributions are predicted to be isotropic along Lorentzian resonances, because of the long lifetimes. The anisotropy parameters exhibit strong variations across asymmetric resonances due to the quantum interference between the binding A 2Σ+ and the dissociative 2Σ− states. It is also discussed that control of the photodissociation processes may be achieved simultaneously through selecting photofragments branching ratios and through angular resolution of the photofragments, simply by varying the excitation wavelength near the asymmetric resonances.

Theory of diatomic photodissociation involving several atomic term limits: Quantum interference and product control in OH photodissociation

A quantum mechanical theory is described for diatomic photodissociation processes to atomic fine structure states for which more than one atomic term limit must be explicitly considered. The theory is employed to treat the photodissociation of OH molecule. Two frame transformation matrices are constructed and incorporated, for the first time, to describe the correlations between two oxygen terms [O(3P) and O(1D)] and the adiabatic Born–Oppenheimer states. We find that very interesting dynamics results from the quantum interferences between electronic states. Near the dissociation threshold to O(3P), asymptotic interactions between asymptotically degenerate states correlating to O(3P) term are shown to manifest as multichannel resonances. At energies between thresholds to O(3P) and O(1D) terms, quantum interferences between A 2Σ+ and 2Σ− states are predicted to result in asymmetric resonances. Partial cross sections to the triplet oxygen fine structure states O(3Pj, j=0,1,2) exhibit different degrees of asymmetry due to the combined effects of the quantum interference between A 2Σ+ and 2Σ− states crossing in the Franck–Condon region, and the asymptotic interactions among 4Σ−, 2Σ−, and 4Π states correlating to O(3P). Consequently, the branching ratios of O(3Pj, j=0,1,2) exhibit strong variations across the asymmetric resonances, suggesting the possibility of controlling the product distributions by tuning at the excitation wavelengths across a single asymmetric resonance in one-photon process. Interference between the dissociative 2Δ and 2 2Π states, correlating with O(1D) term, are shown to give rise to highly oscillatory variations of the dissociation cross sections to O(1D) at energies above the threshold to O(1D).

The survival of 1612-MHz OH masers in asymptotic giant branch plus white dwarf binaries

We have constructed a model of the OH distribution in an oxygen-rich outflow from an asymptotic giant branch star possessing a white dwarf companion. We assess the probability of 1612-MHz OH maser emission occurring despite the presence of a luminous accretion disc around the white dwarf. We find that the OH is resistant to photodissociation in all but the least massive winds, as a result of shielding by the dust component of the AGB outflow and self-shielding by H2O and OH molecules. This conclusion only partially supports the suggestion of Lewis that a ~ 40 per cent non-detection rate of OH 1612-MHz masers in potential OH/IR stars is due to photodissociation of OH. Our results are more compatible with the more tightly directed survey by Lewis & Engels of oxygen-rich OH/IR-star colour ‘mimics’, which shows a 1612-MHz detection rate of at least 75 per cent. We also postulate that other effects, such as the disruption of the velocity coherence of the wind, may be as important as photodissociation in inhibiting maser formation in AGB + WD binary systems.

Photodissociation of the hydroxyl radical (OH) at 157 nm

The photodissociation of the OH radical was studied at 157 nm via the detection of the product H atoms with the resonance fluorescence technique. OH radicals were produced in a fast flow cell from the reaction between H and NO2 and subsequently photodissociated by an excimer laser operating on the F2 emission. The quantum yield for photodissociation of OH was measured to be 1.10±0.28. The photodissociation cross section was calculated to be 6.6×10−18 cm2 (±25%) which is in good agreement with the work of Nee and Lee (Ref. 8).

Chemistry in Interstellar Hydroxyl Maser Regions

The pumping of the upper levels of the 2π3/2 J=3/2 state of OH at sufficient rates to account for the observed strengths of interstellar hydroxyl masers requires that OH contains a substantial fraction of the oxygen. Shocks have been suggested as the sites of the OH production, but a better understanding of OH photodissociation and observations of hydroxyl maser kinematics have led to the realization that the original hydroxyl maser shock model must be revised. the possibility that interstellar hydroxyl masers occur in dense molecular photodissociation zones, heated by photoabsorption and perhaps by ion-neutral friction in the magnetic precursor of a shock, is considered. Possible infrared line emission from interstellar hydroxyl maser regions is mentioned.

Resonances in the photodissociation of OH by absorption into coupled 2Π states: Adiabatic and diabatic formulations

The bound 3 2Π and repulsive 2 2Π states of OH are strongly coupled by the action of the nuclear kinetic energy operator. The process of photodissociation by absorption into the coupled 2Π states is studied theoretically. The adiabatic electronic eigenfunctions and potential energy curves of the 2 2Π and 3 2Π states are calculated using large configuration-interaction (CI) representations and the nuclear radial coupling matrix elements are obtained by numerical differentiation. The coupled equations for the nuclear wave functions of the two states are set up in an adiabatic and in a diabatic formulation and are solved by numerical integration. The electric dipole transition moments connecting the ground X 2Π state to the 2 2Π and 3 2Π states are computed from the CI wave functions and the resulting photodissociation cross sections of OH arising from absorption into the coupled 2 2Π and 3 2Π states are obtained. Two alternative sets of potential curves, coupling matrix elements, and transition moments are employed to provide an assessment of the accuracy of the results. The photodissociation cross section shows a series of resonances superimposed on a broad continuous background. The resonances are located near to the vibrational levels of the uncoupled bound diabatic potential curve. They have asymmetric Beutler–Fano profiles and vary in width from 50 cm−1 for the lowest levels to 2 cm−1 for the higher levels. The accuracy of adiabatic and diabatic approximations, carried to first order in the coupling, is explored and it is demonstrated that the diabatic approximation provides a more satisfactory representation of the photodissociation process. The discrete-continuum configuration interaction theory of Fano is applied in the diabatic formulation and the resonance structures are calculated. The discrete-continuum interaction theory yields profile parameters and level shifts which agree well with the accurate values obtained by solving the coupled equations but its application is considerably more laborious. The contribution of absorption into the coupled 2Π states to the interstellar photodissociation rate is evaluated. In the optically thin limit, the rate is1.5×10−10 s−1. The total rate is 4×10−10 s−1.

The dissociation of OH and OD in comets by solar radiation

The radiative lifetimes of cometary OH are calculated as a function of the heliocentric velocity of the comet and the velocity distributions of the product atoms are determined. At a distance of 1 AU from the Sun, the lifetimes vary between 1.2×10 5 and 1.9×10 5 sec at solar minimum and between 1.0×10 5 and 1.4×10 5 sec at solar maximum, depending upon velocity. Continuous absorption into the repulsive 1 2Σ - state is major destruction path. The calculated lifetimes are generally consistent with the lifetimes inferred from observations, but suggest some elaboration of the models is necessary. Photodissociation of OH produces a low-velocity component of hydrogen atoms at 8 km sec −1 relative to the parent OH molecule and a high-velocity component between 17 and 27 km sec −1. Photodissociation of OH leads to metastable O( 1D) and O( 1S) and is an additional source of the red and green line emission of atomic oxygen. The lifetime of OD is estimated to be about 4.3× 10 5 sec at solar minimum and 2.6×10 5 sec at solar maximum so that the OD/OH ratio in comets is enhanced relative to the HDO/H 2O production ratio by a factor between 2 and 3. Photodissociation of OD produces only high-velocity D atoms with a mean value of 17 km sec −1.

Photodissociation of OH in Interstellar Clouds

view Abstract Citations (69) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photodissociation of OH in Interstellar Clouds van Dishoeck, E. F. ; Dalgarno, A. Abstract Calculations are presented of the lifetime of OH against photodissociation by the interstellar radiation field as a function of depth into interstellar clouds containing grains of various scattering properties. The effectiveness of the different photodissociation channels changes with depth into a cloud. Absorption into the repulsive 1 2Σ- state is usually the main dissociation channel, but with forward-scattering grains in clouds of small optical depths, absorption into the 3 2Π state is more effective. Publication: The Astrophysical Journal Pub Date: February 1984 DOI: 10.1086/161729 Bibcode: 1984ApJ...277..576V full text sources ADS |

Hydrogen transport process in OH − doped KCl and KBr

It was investigated the kinectics of formation and destruction of hydrogen related defects originated from the randon aggregation of hydrogen atoms in their ground state that were allowed to diffuse through the crystal after the photodissociation of OH − defects. Formation and activation energies for the several processes involved in three main steps were obtained utilizing 1st. order kinectics. A model for a new U 2x center is proposed.