OH Transitions Research Articles

Wind-swept clouds and possible triggered star formation associated with the supernova remnant G357.7+0.3

We present evidence for interaction between the supernova remnant (SNR) G357.7+0.3 and nearby molecular clouds, leading to the formation of wind-swept structures and bright emission rims. These features are not observed at visual wavelengths, but are clearly visible in mid-infrared mapping undertaken using the Spitzer Space Telescope. Analysis of one of these clouds, the bright cometary structure G357.46+0.60, suggests that it contains strong polycyclic aromatic hydrocarbon emission features in the 5.8 and 8.0 μm photometric bands, and that these are highly variable over relatively small spatial scales. The source is also associated with strong variations in electron density; a far-infrared continuum peak associated with dust temperatures of ∼30 K; and has previously been observed in the 1720 MHz maser transition of OH, known to be associated with SNR shock excitation of interstellar clouds. This source also appears to contain a young stellar object (YSO) within the bright rim structure, with a steeply rising spectrum between 1.25 and 24 μm. If the formation of this star has been triggered recently by the SNR, then YSO modelling suggests a stellar mass ∼5–10 M⊙, and luminosity LYSO∼102–2 × 103 L⊙. Finally, it is noted that a further, conical emission region appears to be associated with the Mira V1139 Sco, and it is suggested that this may represent the case of a Mira outflow interacting with a SNR. If this is the case, however, then the distance to the SNR must be ∼half of that determined from CS J= 2–1 and 3–2 line radial velocities.

Characterizations of strip-line microwave micro atmospheric plasma and its application to neutralization

In this work, we estimate the plasma parameters of strip-line microwave micro atmospheric plasma (SMMAP) such as rotational temperature (Tr) both from OH and N2 rotational transitions (610–770 and 770–980 K in Ar, respectively), electron density (Ne) from Stark broadening (about 1013/cm3 in mixture of Ar and H2), and the distribution of electric field before ignition of SMMAP (5×104 V/m at maximum, and applied voltage less than 5 V). Since the lower applied voltage of SMMAP might enable us to conduct efficient processing without electrostatic damage (ESD), we applied jet-type SMMAP to neutralization. The result of neutralization showed that it can reduce surface charge from ±1000 to ±100 V for 0.2 s at 10 W with Ar gas flow within 4 V offset voltage, which provides efficient plasma processing without ESD.

Ultrahigh-frame-rate OH fluorescence imaging in turbulent flames using a burst-mode optical parametric oscillator

Burst-mode planar laser-induced fluorescence (PLIF) imaging of the OH radical is demonstrated in laminar and turbulent hydrogen-air diffusion flames with pulse repetition rates up to 50 kHz. Nearly 1 mJ/pulse at 313.526 nm is used to probe the OH P(2)(10) rotational transition in the (0,0) band of the A-X system. The UV radiation is generated by a high-speed-tunable, injection-seeded optical parametric oscillator pumped by a frequency-doubled megahertz-rate burst-mode Nd:YAG laser. Preliminary kilohertz-rate wavelength scanning of the temperature-broadened OH transition during PLIF imaging is also presented for the first time (to our knowledge), and possible strategies for spatiotemporally resolved planar OH spectroscopy are discussed.

Development of a two-line OH-laser-induced fluorescence thermometry diagnostics strategy for gas-phase temperature measurements in engines

This study aims at optimizing two-line OH thermometry strategies for in-cylinder measurement in internal combustion engines. Various aspects are investigated experimentally, such as the selection of suitable OH lines and the possibility of using a single calibration coefficient for variable mixture composition, temperature, and pressure conditions. Two kinds of experimental systems have been investigated. First, a laminar methane-air burner flame at atmospheric pressure, whose stability allowed the determination of OH-laser-induced fluorescence (LIF) intensity ratios from nonsimultaneous imaging. The temperature distribution in the flame is presented for OH-transition pairs with various temperature sensitivities. The burner flame was studied for equivalence ratios from phi=0.93 to 1.30 in order to check for the stability of calibration over various flame conditions. Additionally, OH LIF images were acquired in an optical engine for the chosen OH transitions yielding data about the effect of pressure on OH LIF signals under realistic experimental conditions.

Reorientation of hydroxide ions in water

Connecting the macroscopic properties of aqueous systems with the underlying molecular motion of water molecules remains a challenge in the quest to understand the unique role played by water in living systems. The structural and dynamical properties of the H + and OH − ions in water are of interest, both as the fundamental acid-base pair and as an example of the unique ability of water to hydrate and transport ions, in the bulk and at interfaces. However, in spite of considerable efforts an understanding of the properties of OH −(aq) remain elusive as a result of conflicting theoretical and experimental results. In this work we use femtosecond spectroscopy of the charge transfer to solvent transition of OH − to measure the reorientation time of the individual OH − ions in water as function of temperature. The experimental data reveal a drastic temperature dependence caused by increased structural integrity of the hydration sphere at low temperatures.

Detection of 6.7 GHz methanol absorption towards hot corinos

Context. Methanol masers at 6.7 GHz have been found exclusively towards high-mass star forming regions. Recently, some Class 0 protostars have been found to display conditions similar to what are found in hot cores that are associated with massive star formation. These hot corino sources have densities, gas temperatures, and methanol abundances that are adequate for exciting strong 6.7 GHz maser emission. Aims. This raises the question of whether 6.7 GHz methanol masers can be found in both hot corinos and massive star forming regions, and if not, whether thermal methanol emission can be detected. Methods. We searched for the 6.7 GHz methanol line towards five hot corino sources in the Perseus region using the Arecibo radio telescope. To constrain the excitation conditions of methanol, we observed thermal submillimeter lines of methanol in the NGC 1333-IRAS 4 region with the APEX telescope. Results. We did not detect 6.7 GHz emission in any of the sources, but found absorption against the cosmic microwave background in NGC 1333-IRAS 4A and NGC 1333-IRAS 4B. Using a large velocity gradient analysis, we modeled the excitation of methanol over a wide range of physical parameters, and verify that the 6.7 GHz line is indeed strongly anti-inverted for densities lower than 10 6 cm -3 . We used the submillimeter observations of methanol to verify the predictions of our model for IRAS 4A by comparison with other CH 3 OH transitions. Our results indicate that the methanol observations from the APEX and Arecibo telescopes are consistent with dense ( n ~ 10 6 cm -3 ), cold ( T ~ 15-30 K) gas. Conclusions. The lack of maser emission in hot corinos and low-mass protostellar objects in general may be due to densities that are much higher than the quenching density in the region where the radiation field is conducive to maser pumping.

Laser-Induced Fluorescence Image of OH Radicals for Atmospheric-Pressure Nonequilibrium Dry Air Gas DC Pulse Plasma Jet

OH radicals were successfully measured for the first time ever in an atmospheric-pressure nonequilibrium DC pulse discharge plasma jet in dry air gas using the laser-induced fluorescence method and OH transition [A2Sigma+(v'=1)larrX2Pi(v=0)] at 282 nm to monitor ground-state radicals.

IR and Raman spectra of liquid water: Theory and interpretation

IR and Raman (parallel- and perpendicular-polarized) spectra in the OH stretch region for liquid water were measured some years ago, but their interpretation is still controversial. In part, this is because theoretical calculation of such spectra for a neat liquid presents a formidable challenge due to the coupling between vibrational chromophores and the effects of motional narrowing. Recently we proposed an electronic structure/molecular dynamics method for calculating spectra of dilute HOD in liquid D(2)O, which relied on ab initio calculations on clusters to provide a map from nuclear coordinates of the molecules in the liquid to OH stretch frequencies, transition dipoles, and polarizabilities. Here we extend this approach to the calculation of couplings between chromophores. From the trajectories of the fluctuating local-mode frequencies, transition moments, and couplings, we use our recently developed time-averaging approximation to calculate the line shapes. Our results are in good agreement with experiment for the IR and Raman line shapes, and capture the significant differences among them. Our analysis shows that while the coupling between chromophores is relatively modest, it nevertheless produces delocalization of the vibrational eigenstates over up to 12 chromophores, which has a profound effect on the spectroscopy. In particular, our results demonstrate that the peak in the parallel-polarized Raman spectrum at about 3250 wavenumbers is collective in nature.

Discovery of OH in Circumstellar Disks around Young Intermediate-Mass Stars

We detect emission from multiple low-excitation ro-vibrational transitions of OH from the two Herbig Ae stars AB Aurigae and MWC 758 in the 3.0 - 3.7 micron wavelength range (L-band), using the NIRSPEC instrument on Keck II. The inner radius for the emitting region in both stars is close to 1 AU. We compare an optically thin LTE model and a thin-wedge fluorescence model, finding rotational temperatures of 650 - 800 K and OH abundances of 10^42 - 10^45 molecules for the two stars. Comparisons with current chemical models support the fluorescence excitation model for AB Aurigae and possibly MWC 758, but further observations and detailed modeling are necessary to improve constraints on OH emission in different disk environments.

Detection of Extended Hot Water in the Outflow from NGC 2071

We report the results of spectroscopic mapping observations carried out toward a ~1 min x 1 min region within the northern lobe of the outflow from NGC 2071 using the Infrared Spectrograph (IRS) of the Spitzer Space Telescope. These observations covered the 5.2-37 um spectral region and have led to the detection of a number of ionic, atomic, and molecular lines, including fine-structure emission of Si+, Fe+, S++, S, the S(0)-S(7) pure rotational lines of H2, the R(3) and R(4) transitions of HD, and at least 11 transitions of H2O. In addition, the 6.2, 7.4, 7.6, 7.9, 8.6 and 11.3 um PAH emission bands were also observed and several transitions of OH were tentatively detected. Most of the detected line transitions were strong enough to map including, for the first time, three transitions of hot H2O. We find that: (1) the water emission is extended; (2) the extended emission is aligned with the outflow; and, (3) the spatial distribution of the water emission generally follows that observed for H2. Based on the measured line intensities, we derive an HD abundance relative to H2 of 1.1-1.8 10^-5 and an H2O number density of 12-2 cm^3. The H2 density in the water-emitting region is not well constrained by our observations, but is likely between 3 10^4 and 10^6 cm^3, yielding an H2O abundance relative to H2 of between 2 10^-5 and 6 10^-4. Future observations planned for the Herschel Space Observatory should greatly improve the density estimate, and thus our knowledge of the H2O abundance, for the water-emitting regions reported here. Finally, we note a possible departure from the H2O ortho-to-para ratio of 3:1 expected for water formed in hot post-shocked gas, suggesting that a significant fraction of the water vapor we detect may arise from H2O sputtered from cold dust grains.

Dramatic Enhancement of OH(A2Σ+) Density in a Recombining Hydrogen Plasma

Optical emission spectra in the wavelength range of molecular hydrogen were observed in both ionizing and recombining plasma modes of hydrogen discharges. The optical emission spectrum from the ionizing plasma was dominated by the Fulcher-α system (aΣg − d3Π−u ) of molecular hydrogen. On the other hand, the optical emission spectrum from the recombining plasma was composed of many lines, and was completely different from the spectrum of the ionizing plasma. The many peaks observed from the recombining plasma were assigned to the X2Π − A2Σ+ transition of OH. The OH(A2Σ+) is thought to originate from the chemical etching of a glass discharge tube by atomic hydrogen. This is analogous to the interaction between a detached divertor plasma and a graphite plate, and a possible process for the enhancement of OH(A2Σ+) density in a recombining plasma is discussed with this analogy in mind.

Double-Trap Kinetic Equation for the Oxygen Reduction Reaction on Pt(111) in Acidic Media

We derived an intrinsic kinetic equation for the four-electron oxygen reduction reaction (ORR) in acidic media using free energies of activation and adsorption as the kinetic parameters. Our kinetic model consists of four essential elementary reactions: a dissociative adsorption (DA) and a reductive adsorption (RA), which yield two reaction intermediates, O and OH; a reductive transition (RT) from O to OH; and a reductive desorption (RD) of OH. Analytic expressions were found for the O and OH adsorption isotherms by solving the steady-state rate equations. For the ORR on Pt(111) in 0.1 M HClO4 solution, we analyzed the measured polarization curves, thereby deducing activation free energies that are consistent with the values from theoretical calculations. The reductive adsorption (DeltaG*0RA=0.46 eV) is not the rate-determining step (RDS) for the ORR on Pt because dissociative adsorption (DeltaG*0DA=0.26 eV) offers a more favorable pathway at high potentials. It, however, generates strongly adsorbed O. The high activation barriers for the O to OH transition (DeltaG*0RT=0.50 eV) and OH desorption (DeltaG*0RD=0.45 eV) cause a large potential loss for the desorption-limited ORR. As the OH coverage increases to a constant value with decreasing potential, the Tafel slope increases to the value determined by a symmetric electron-transfer coefficient. We discuss the role of adsorption isotherm in kinetic analysis and, via activity-and-barrier plots, illustrate why the RDS may vary with reaction conditions or may not exist. Recognizing such features of electrocatalytic reactions can facilitate reaching the long-standing goal of quantitative descriptions and predictions of electrocatalysts' activities.

Effelsberg Observations of Excited-State (6.0 GHz) OH in Supernova Remnants and W3(OH)

While masers in the 1720 MHz transition of OH are detected toward many supernova remnants (SNRs), no other OH transition is seen as a maser in SNRs. We present a search for masers at 6049 MHz, which has recently been predicted to produce masers by pure collisional excitation at conditions similar to that required for 1720 MHz masing. The Effelsberg 100 m telescope was used to observe the excited-state 6016, 6030, 6035, and 6049 MHz lines of OH toward selected SNRs, most of which have previously-detected bright 1720 MHz masers. No excited-state masers are found toward SNRs, consistent with previous observations of the 6049 MHz and other excited-state transitions. We do not see clear evidence of absorption toward SNR target positions, although we do see evidence of absorption in the molecular cloud at +50 km/s near Sgr A East. Weak absorption is detected at 6016 MHz toward W3(OH), while stronger, narrower emission is seen at 6049 MHz, suggesting that the 6049 MHz emission is a low-gain maser. We conclude that conditions in SNRs are not conducive to excited-state maser emission, especially in excited-state satellite lines.

Excited-State OH Main-Line Masers in AU Geminorum and NML Cygni

Excited-state OH maser emission has previously been reported in the circumstellar envelopes of only two evolved stars: the Mira star AU Geminorum and the hypergiant NML Cygni. We present Very Large Array (VLA) observations of the 1665, 1667, and excited-state 4750 MHz main-line OH transitions in AU Gem and Expanded Very Large Array (EVLA) observations of the excited-state 6030 and 6035 MHz OH main-line transitions in NML Cyg. We detect masers in both main-line transitions in AU Gem but no excited-state emission in either star. We conclude that the excited-state OH emission in AU Gem is either a transient phenomenon (such as for NML Cyg, outlined below) or possibly an artifact in the data and that the excited-state OH emission in NML Cyg was generated by an episode of enhanced shock between the stellar mass loss and an outflow of the Cyg OB2 association. With these single exceptions, it therefore appears that excited-state OH emission indeed should not be predicted nor observable in evolved stars as part of their normal structure or evolution.

Variation of the transition dipole moment across the OH stretching band of water

Hydrogen bonding increases the magnitude of the OH stretching transition dipole moment. We investigate the significance of this non-Condon effect on the linear and nonlinear infrared spectroscopy of the OH stretching vibration of HOD in D 2O. The frequency dependence of the transition dipole across the line shape is tested by comparing temperature-dependent amplitudes of IR and Raman spectra and IR photon echoes. Since the Raman polarizability is largely independent of hydrogen bonding, differences between IR and Raman spectra reflect the changes in the IR transition dipole moment induced by hydrogen bonding. Increasing temperature shifts the OH density of states to higher frequencies consistent with a weakening of hydrogen bond strength and therefore a decrease in the transition dipole. Photon echo intensity is found to decrease significantly with temperature. Using an empirical transition dipole variation determined from the square root of the ratio of the IR and Raman intensity, we are able to model the IR spectra and the IR photon echo across a broad temperature range. The variation with frequency of this empirical transition dipole is nonlinear, changes by a factor of 1.7 across the OH line shape, and on resonance has a slope similar to recent theoretical predictions. The role of water dynamics and motional narrowing on the non-Condon effect is evaluated with the help of molecular dynamics simulations of the fluctuating OH frequency and transition dipole moment. Simulated linear and two-dimensional infrared line shapes that use the empirical model are red shifted compared to line shapes calculated within the Condon approximation, and give slightly better agreement with measured data.

OH Masers in G11.904-0.141

The massive star-forming region G11.904-0.141 is one of only 11 sources to show maser emission in the highly-excited 13441 MHz transition of OH. VLBA observations of the 1665, 1667, 4765, and 13441 MHz transitions of OH toward G11.904-0.141 are presented. Masers are detected at 1665, 1667, and 4765 MHz, but the 13441 MHz masers are not detected. Consistent magnetic field strengths of approximately +3.5 mG are detected in the ground-state masers, in contrast with a possible -3.0 mG magnetic field previously detected at 13441 MHz. The variable 13441 MHz masers may be associated with an outflow.

Theoretical studies on the reactions of formaldehyde with OH and OH −

The titled reactions are theoretically studied at MP4 (SDTQ) level with 6-311++G(3df,3pd) basis set. All the possible reaction channels are considered for the first time. The most favorable reaction path in H 2CO + OH is the channel of producing H 2O, which is exothermic by −32.02 kcal/mol, in well agreement with reported experimental result. For H 2CO + OH −, the most favorable reaction channel is for H 2 molecule formation through several paths. The reaction channels for producing H 2 molecule in the titled systems are proposed for the first time. H atom formation in H 2CO + OH and hydrogen symmetric transition in H 2CO + OH − is also evaluated at higher theory level than literatures.

OH(AΣ+2) and rare gas-deuteride (NeD, ArD) excimers generated in microcavity plasmas: Ultraviolet emission spectra and formation kinetics

Emission in the ultraviolet from the AΣ+2 electronic excited states of OH, NeD, and ArD, and the formation kinetics of these excited heteronuclear diatomics, have been investigated in microcavity plasmas generated in rare gas/H2O or D2 gas mixtures. Excitation transfer from the aΣu+3(1u,0u−) Rydberg state of Ar2 appears to be the dominant pathway to OH(AΣ+2) formation in Ar∕H2O vapor mixtures with total pressures of 400–800Torr and H2O partial pressures of 100mTorr–3Torr. Maximum emission on the (v′,v″)=(0,0) vibrational band of the OH(A→X) transition is observed in a 25μm, 45nl microcavity for 600–800Torr Ar∕0.5Torr H2O mixtures. Comparisons of experimental and simulated fluorescence spectra show the OH[AΣ+2(v′=0)] state rotational temperature to be 425K for 600Torr Ar∕100mTorr H2O mixtures but to rise linearly with the H2O partial pressure and exhibit a slope of 170K∕Torr H2O for 100mTorr⩽pH2O⩽3Torr. Excitation of Ne or Ar∕D2 gas mixtures in 50×50 arrays of Si microplasma devices generates broadband spectra, peaking in the mid-ultraviolet (λ∼280–320nm), which are attributed to the A→X transition of the ArD or NeD excimers. The optimal D2 concentration is observed to be ∼0.5% and the primary kinetic formation mechanism for the deuterides involves D atom transfer in collisions between Ar(4sP3) and D2.

High-resolution laser spectroscopy and photodissociation dynamics of OH+

The paper reports high-resolution spectra of the c1Π–b1Σ+ and c1Π–a1Δ electronic transitions of OH+ obtained by detecting O and O+ photofragments, which arise from predissociation of the c1Π electronic state. Five vibrational bands of the c1Π–b1Σ+ system and three of the c1Π–a1Δ system of 16OH+ were recorded and molecular parameters derived from the analysis. Spectra of the 16OD+ isotopic modification were also taken. A study of perturbations between the A3Π and b1Σ+ states leads to improved determination of the spin–orbit interaction between these states and their relative energies. A selective predissociation mechanism operates with laser excitation to 1Π (e) levels strongly weighted towards formation of O + H+, whereas 1Π (f) levels yield O+ + H exclusively. The predissociation decay rates are also dependent on the c 1Π state vibrational and rotational quantum numbers. The measured linewidths for v′ = 2 and 3 are in good agreement with both a theoretical approach based on spectroscopic parameters and the results of high-quality ab initio calculations of Yarkony.

Far-Infrared Detection of H 2 D + toward Sgr B2

We report on the first far-IR detection of H2D+, using the Infrared Space Observatory, in the line of sight toward Sgr B2 in the galactic center. The transition at 126.853 um connecting the ground level of o-H2D+, 1_1,1 with the the 2_1,2 level at 113 K, is observed in absorption against the continuum emission of the cold dust of the source. The line is broad, with a total absorption covering 350 km s^-1, i.e., similar to that observed in the fundamental transitions of H2O, OH and CH at ~179, 119 and 149 um respectively. For the physical conditions of the different absorbing clouds the H2D+ column density ranges from 2 to 5x10^13 cm^-2, i.e., near an order of magnitude below the upper limits obtained from ground based submillimeter telescopes. The derived H2D+ abundance is of a few 10^-10, which agrees with chemical models predictions for a gas at a kinetic temperature of ~20K.