OH Molecules Research Articles

OH Formation from O and H Atoms Physisorbed on a Graphitic Surface through the Langmuir−Hinshelwood Mechanism: A Quasi-Classical Approach

We study the quasi-classical dynamics of OH formation on a graphitic surface through the Langmuir-Hinshelwood (LH) mechanism when both O and H ground-state atoms are initially physisorbed on the surface. The model proceeds from previous theoretical work on the LH formation of the H 2 molecule on graphite [Morisset, S.; Aguillon, F.; Sizun, M.; Sidis, V. J. Chem. Phys. 2004, 121, 6493; ibid 2005, 122, 194704]. The H-graphite system is first revisited with a view to get a tractable DFT-GGA computational prescription for the determination of atom physisorption onto graphitic surfaces. The DZP-RPBE combination is found to perform well; it is thereafter used along with MP2 calculations to determine the physisorption characteristics of atomic oxygen on graphitic surfaces. We also deal with chemisorption. In accordance with previous work, we find that O chemisorbs on graphite in a singlet spin state epoxy-like conformation. In the triplet state we find only "metastable" chemisorption with an activation barrier of 0.2 eV. The physisorption results are then used in the LH dynamics calculation. We show that in the [0.15 meV, 12 meV] relative collision energy range of the reacting O and H atoms on the surface, the OH molecule is produced with a large amount of internal energy ( approximately = 4eV) and a significant translation energy (>or=100 meV) relative to the surface.

Detonation diffraction in gases

We have experimentally investigated detonation diffraction out of a round tube into an unconfined half-space. The focus of our study is examining how the extent of detonation cellular instability influences the quantitative and qualitative features of diffraction. Detailed quantitative and qualitative measurements were obtained through simultaneous schlieren imaging, multiple-exposure chemiluminescence imaging, and planar laser-induced fluorescence imaging of OH molecules. Two types of stoichiometric mixtures, highly diluted H 2–O 2–Ar and H 2–N 2O, were studied in the sub-critical, critical and super-critical regime. These mixture types represent extreme cases in the classification of cellular instability with highly diluted H 2–O 2–Ar mixtures having very regular instability structures and H 2–N 2O having very irregular instability structures. The most striking differences between the mixtures occur in the sub-critical and critical regimes, for which the detonation fails to transition into the unconfined half-space. For the H 2–O 2–Ar mixture, the velocity on the center line was found to decay significantly slower than for the H 2–N 2O mixture. In case of the H 2–O 2–Ar mixture, it was evident from simultaneous schlieren-fluorescence images that the reaction front was coupled to the lead shock front up to 2.3 tube diameters from the exit plane. For the H 2–N 2O mixture, the reaction front velocity decreased to 60% of the corresponding Chapman–Jouguet value at 1.1 tube diameters from the tube exit plane. A geometric acoustic model showed that the observed differences in failure patterns are not caused by the differences in thermodynamic properties of the two mixtures but is linked to the larger effective activation energy and critical decay time in the H 2–N 2O mixture as compared to the H 2–O 2–Ar mixture. The re-initiation events appear similar for the two mixtures and are a consequence of local fluctuations at random locations within the region between the lead shock and decoupled reaction zone, resulting in strong transverse detonations sweeping through shocked but largely unreacted gas.

UV spectral measurements at moderately high resolution and of OH resonance scattering resolved by polarization during the MANTRA 2002–2004 stratospheric balloon flights

A moderately high-resolution (<0.1 nm) grating spectrometer designed to measure the solar radiation in the spectral range 295–315 nm was flown on the MANTRA stratospheric balloon payloads of 2002 and 2004. The instrument measures both the direct sunlight and the radiation scattered by the atmosphere. The latter can be observed in two orthogonal polarization directions, at 90° from the solar azimuth and at several elevations above the horizon. As the OH molecule is the principal resonant scatterer in this spectral region, this permits the inference of both ozone and OH column amounts as well as limited profile information. This paper describes the instrument and its in-flight characterization, the basic data processing and the influence of several aspects of the flight profile. The direct sun measurements are analyzed both to characterize the spectrometer responsivity to scattered radiation and to estimate the ozone abundance at the flight altitude and above. An example of a high-resolution solar spectrum at 37 km altitude is presented and compared with others in the literature. The measured OH and Rayleigh-scattered spectra are used to derive OH radiation intensity measurements (the OH airglow), which are compared with others in the literature.

Theoretical study of the kinetics and mechanism of the decomposition of trifluoromethanol, trichloromethanol, and tribromomethanol in the gas phase

Ab initio calculations at the G2 level were used in a theoretical analysis of the kinetics of unimolecular and water-accelerated decomposition of the halogenated alcohols CX(3)OH (X = F, Cl, and Br) into CX(2)O and HX. The calculations show that reactions of the unimolecular decomposition of CX(3)OH are of no importance under atmospheric conditions. A considerably lower energy pathway for the decomposition of CX(3)OH is accessible by homogenous reactions between CX(3)OH and water. It is shown that CX(3)OH + H(2)O reactions proceed via the formation of intermediate complexes. The mechanism of the reactions appears to be complex and consists of three consecutive elementary processes. The calculated values of the second-order rate constants are of 2.5 x 10(-21), 2.1 x 10(-19), and 1.2 x 10(-17) cm(3)molecule(-1)s(-1) at 300 K for CF(3)OH + H(2)O, CCl(3)OH + H(2)O, and CBr(3)OH + H(2)O, respectively. The theoretically derived atmospheric lifetimes of the CX(3)OH molecules indicate that the water-mediated decomposition reactions CX(3)OH + H(2)O may be the most efficient process of CF(3)OH, CCl(3)OH, and CBr(3)OH loss in the atmosphere.

Theoretical determination of rate constants for vibrational relaxation and reaction of OH(XΠ2,v=1) with O(P3) atoms

Collisions of the vibrationally excited OH(v = 1) molecule with atomic oxygen are investigated theoretically using a coupled-states, statistical capture (CS-ST) model. Vibrational relaxation can occur by inelastic scattering, and the vibrationally excited molecule can also be removed by reaction to form O(2) in both the ground (X (3)Sigma(g)(-)) and first excited (a (1)Delta(g)) state. In the former case, reaction occurs on the lowest potential energy surface of (2)A(") symmetry, and, in the latter case, by reaction on the lowest potential energy surface of (2)A(') symmetry. We report new ab initio potential energy surfaces for both these states in the product and reactant regions necessary for application of the coupled-states, statistical method. Comparison with exact, reactive scattering calculations within the J-shifting approximation indicate that the CS-ST rate constants for removal of OH(v = 1) can be expected to be reasonably accurate. Our calculated rate constants at 300 K agree well with the experimental results of Khachatrian and Dagdigian [Chem. Phys. Lett. 415, 1 (2005)]. Reaction to yield O(2) (X (3)Sigma(g)(-)) is the dominant removal pathway. At subthermal temperatures, the rate constants for the various vibrational quenching processes all increase down to T approximately = 60 K and then decrease at lower temperature.

Ab Initio Molecular Dynamics Study on the Initial Chemical Events in Nitramines: Thermal Decomposition of CL-20

CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane or HNIW) is a high-energy nitramine explosive. To improve atomistic understanding of the thermal decomposition of CL-20 gas and solid phases, we performed a series of ab initio molecular dynamics simulations. We found that during unimolecular decomposition, unlike other nitramines (e.g., RDX, HMX), CL-20 has only one distinct initial reaction channelhomolysis of the N-NO2 bond. We did not observe any HONO elimination reaction during unimolecular decomposition, whereas the ring-breaking reaction was followed by NO 2 fission. Therefore, in spite of limited sampling, that provides a mostly qualitative picture, we proposed here a scheme of unimolecular decomposition of CL-20. The averaged product population over all trajectories was estimated at four HCN, two to four NO2, two to four NO, one CO, and one OH molecule per one CL-20 molecule. Our simulations provide a detailed description of the chemical processes in the initial stages of thermal decomposition of condensed CL-20, allowing elucidation of key features of such processes as composition of primary reaction products, reaction timing, and Arrhenius behavior of the system. The primary reactions leading to NO2, NO, N 2O, and N2 occur at very early stages. We also estimated potential activation barriers for the formation of NO2, which essentially determines overall decomposition kinetics and effective rate constants for NO2 and N2. The calculated solid-phase decomposition pathways correlate with available condensed-phase experimental data.

Dynamics of Inelastic Scattering of OH Radicals from Reactive and Inert Liquid Surfaces

The inelastic scattering of gas-phase OH radicals from a liquid hydrocarbon and a liquid perfluorinated polyether (PFPE) has been investigated. The surfaces examined were the potentially reactive, branched hydrocarbon squalane (C30H62, 2,6,10,15,19,23-hexamethyltetracosane) and the inert PFPE Krytox 1506 (F−[CF(CF3)CF2O]14ave−CF2CF3). Superthermal OH was formed by 355-nm laser photolysis of a low pressure of HONO above the liquid surface. Laser-induced fluorescence (LIF) was used to determine the relative yields and nascent translational and rotational distributions of OH (v′ = 0). The time-of-flight profiles from both liquids can be resolved, at least empirically, into two components. The dominant, faster component is consistent with direct, inelastic scattering. It has a higher average translational energy from PFPE than from squalane. This faster OH also has a higher Boltzmann-like rotational temperature for PFPE (655 ± 45 K) than for squalane (473 ± 27 K), in both cases considerably hotter than the incoming OH. For both liquids, there is also a slower component, with characteristics consistent with a thermalized, trapping-desorption mechanism. This is a higher proportion for squalane (0.22 ± 0.02) than for PFPE (0.09 ± 0.01). These results are consistent with squalane being the “softer” surface, exhibiting more efficient momentum transfer than PFPE, and more able to temporarily trap OH. Relative to PFPE, around half (0.49 ± 0.04) of the OH molecules that collide with squalane are lost, presumably due to reaction forming H2O. These results are compared with previous studies of the scattering of inert gas species from both squalane and PFPE. The reactive branching fraction of OH on squalane is discussed in the context of previous observations of enhanced reactivity at the gas−liquid interface.

Low-temperature low-damage sterilization based on UV radiation through plasma immersion

This paper introduces a new type of high-frequency (HF) sustained discharge where the HF field applicator is a planar transmission line that allows us to fill with plasma a long chamber of rectangular cross-section (typically 1 m × 15 cm × 5 cm). Peculiar interesting features of this plasma source are a low gas temperature (typically below 40 °C in the 1 Torr range in argon), broadband impedance matching with no need for retuning, stability and reproducibility of the discharge (non-resonant behaviour). This type of plasma source could be useful for web processing; nonetheless, it is applied here to plasma sterilization, taking advantage of its low gas temperature to inactivate microorganisms on polymer-made medical devices to avoid damaging them. The predominant biocide species are the UV photons emitted by the discharge whereas most plasma sterilization techniques call for reactive species such as O atoms and OH molecules, which induce significant erosion damage on polymers. Polystyrene microspheres are actually observed to be erosion-free under the current plasma sterilization conditions (scanning electron micrographs have been examined). Moreover, inactivation is quite fast: 106 B. atrophaeus spores deposited on a Petri dish are inactivated in less than 1 min. Correlation of the UV radiation with the spore inactivation rate is examined by (i) considering the emitted light intensity integrated over the 112–180 nm vacuum UV (VUV) range with a photomultiplier; (ii) looking with an optical spectrometer at the emission spectrum over the 200–400 nm UV range; (iii) using absorption spectroscopy to determine the role of the VUV argon resonant lines (105 and 107 nm) on spore inactivation. It is found that the test-reference spores are mainly inactivated by VUV photons (112–180 nm) that are primarily emitted by impurities present in the argon plasma.

Reflection of OH molecules from magnetic mirrors

We have reflected a Stark-decelerated beam of OH molecules under normal incidence from mirrors consisting of permanent magnets. Two different types of magnetic mirrors have been demonstrated. A long-range flat mirror made from a large disc magnet has been used to spatially focus the reflected beam in the longitudinal direction (‘bunching’). A short-range curved mirror composed of an array of small cube magnets allows for transverse focusing of the reflected beam.

Ab Initio Molecular Dynamics Study of the Solvated OHCl− Complex: Implications for the Atmospheric Oxidation of Chloride Anion to Molecular Chlorine

We have studied the OHCl(-) complex in a six-water cluster and in bulk liquid water by means of Born−Oppenheimer molecular dynamics based on generalized gradient-corrected BLYP density functional theory. Self-interaction-corrected results, which predict a hydrogen-bonded OH···Cl(-) complex, are compared to the uncorrected results, which predict a hemibonded (HO-Cl)(-). A second-order Møller−Plesset potential energy landscape of the gas-phase complex in its ground-state was computed to determine which of the two configurations represents the true nature of the complex. Because no evidence of a local minimum was found in the vicinity of the geometry corresponding to (HO-Cl)(-), we conclude that the self-interaction-corrected results are more accurate and, therefore, that the complex is held together by a hydrogen-bond-like interaction in both an asymmetric solvation environment, as represented by the cluster, and a symmetric solvation environment, as represented by the bulk system. We postulate that the mechanism that governs the atmospheric oxidation of Cl(-)(aq) to Cl(2)(g) on the surface of marine aerosols is initiated by the formation of a H-bonded OH···Cl(-) complex. Furthermore, because no evidence of charge transfer from Cl(-) to OH was found, in either the liquid or the cluster environment, we propose that the second step of the oxidation of Cl(-) is the reaction of the complex with a second Cl(-), resulting in the formation of the species Cl(2)(-) and OH(-). Cl(2)(g) could then be formed via an electron-transfer reaction with an impinging OH molecule.

Response to “Comment on ‘Anharmonic properties of the vibrational quantum computer’ ” [J. Chem. Phys. 128, 167101 (2008)

A benchmark study of the approximate approach to the optimal control of vibrational qubits, which was used in the original paper [J. Chem. Phys.126, 204102 (2007)], is presented. Two simplified assumptions are used in this method: A linear approximation for the dipole moment function and a harmonic approximation for the vibrational wave functions. Both assumptions are often used in spectroscopy and are known to work well for many molecules when the vibrational excitation is low. Here, we show that our method works well for the OH molecule, which exhibits a very anharmonic spectrum of vibrational eigenstates and a complicated dipole moment function.

The structure of protoplanetary disks surrounding three young intermediate mass stars

We present high spectral resolution optical spectra of three young intermediate mass stars, in all of which we spectrally resolve the 6300 Angstrom [OI] emission line. Two of these have a double peaked line profile. We fit these data with a simple model of the [OI] emission caused by photo-dissociation of OH molecules in the upper layer of a circumstellar disk by stellar UV radiation and thus translate the Doppler broadened [OI] emission profile into an amount of emission as a function of distance from the central star. The resulting spectra are in agreement with the expected disk shapes as derived from their spectral energy distribution. We find evidence for shadowing by an inner rim in the disk surrounding HD101412 and see a flaring disk structure in HD179218 while the [OI] spectrum of HD135344 is more complex. The [OI] emission starts for all three targets at velocities corresponding to their dust sublimation radius and extends up to radii of 10 -- 90 AU. This shows that this method can be a valuable tool in the future investigation of circumstellar disks.

Real-Time Single-Molecule Imaging of the Spatial and Temporal Distribution of Reactive Oxygen Species with Fluorescent Probes: Applications to TiO2 Photocatalysts

The single-molecule detection of airborne reactive oxygen species (ROS), such as singlet oxygen (1O2) and hydroxyl radical (•OH), diffused from the photoirradiated TiO2 surface, was successfully demonstrated using single-molecule fluorescence spectroscopy. Airborne single 1O2 and •OH molecules were selectively detected by the fluorescent probes, terrylenediimide (TDI) and 3‘-(p-hydroxyphenyl) fluorescein (HPF), respectively. Generation of the airborne 1O2 and •OH from the TiO2 surface has been investigated under various conditions, such as the excitation wavelengths (UV or visible) and the types of TiO2 (pure or nitrogen (N)-doped). Upon UV excitation, 1O2 and •OH were detected from both the pure and N-doped TiO2 samples, while 1O2 was exclusively detected only from the N-doped TiO2 upon visible excitation. Furthermore, the spatial and temporal distribution of the airborne •OH molecules diffused from the photoirradiated TiO2 surface was investigated by the real-time single-molecule imaging technique. The ...

Quantum-based models of charge-dependent potential energy surfaces: Three-state models

Quantum-based models of how potential energies depend on charge are developed from a three-state model, at the level of neglecting state-to-state overlap. The energy as a function of charge is defined as proposed previously (Valone and Atlas, J Chem Phys 2004, 120, 7262). With this definition, addition of a third state smooths the derivatives of the energy model with respect to charge at integer values of charge that are in the interior of the allowed charge range. These derivatives are related to the chemical potential. At the dissociation limit, this model converges to established limits. Another dependence is proposed that uses two different charges simultaneously. The concepts are illustrated, with calculations on an OH molecule. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

PRECISION MEASUREMENT BASED ON ULTRACOLD ATOMS AND COLD MOLECULES

Ultracold atoms and molecules provide ideal stages for precision tests of fundamental physics. With microkelvin neutral strontium atoms confined in an optical lattice, we have achieved a fractional resolution of 4 × 10-15 on the 1S0–3P0 doubly forbidden 87 Sr clock transition at 698 nm. Measurements of the clock line shifts as a function of experimental parameters indicate systematic errors below the 10-15 level. The ultrahigh spectral resolution permits resolving the nuclear spin states of the clock transition at small magnetic fields, leading to measurements of the 3P0 magnetic moment and metastable lifetime. In addition, photoassociation spectroscopy performed on the narrow 1S0–3P1 transition of 88 Sr shows promise for efficient optical tuning of the ground state scattering length and production of ultracold ground state molecules. Lattice-confined Sr 2 molecules are suitable for constraining the time variation of the proton–electron mass ratio. In a separate experiment, cold, stable, ground state polar molecules are produced from Stark decelerators. These cold samples have enabled an order-of-magnitude improvement in the measurement precision of ground state, Λ doublet microwave transitions in the OH molecule. Comparing the laboratory results to those from OH megamasers in interstellar space will allow a sensitivity of 10-6 for measuring the potential time variation of the fundamental fine structure constant Δα/α over 1010 years. These results have also led to improved understandings of the molecular structure. The study of the low magnetic field behavior of OH in its 2Π3/2 ro-vibronic ground state precisely determines a differential Landé g factor between opposite parity components of the Λ doublet.

Humidity effects on Pd/Au-based all-optical hydrogen sensors

Abstract The hydrogen sensing performance of Pd (60 at%)/Au (40 at%) thin films has been examined under room temperature conditions and at variable humidity levels using either air or nitrogen as a carrier gas. Two humidity effects on the sensing properties of these films have been observed: (1) the baseline optical transmittance of the Pd/Au thin film progressively and reversibly increases with an increase in humidity for both air and N 2 carrier gases; (2) the transmittance signal change becomes reduced and eventually inverts with an increase in humidity while using an air carrier gas for the hydrogen exposure cycles. Optical microscopy and H 2 /H 2 O vapor exposure experiments reveal that the baseline signal increase is induced by the blistering of the Pd/Au thin film due to the adsorption of H 2 O. The transmittance signal change reduction/inversion during H 2 /humidified air exposures is caused by Pd surface-catalyzed reactions followed by H 2 O desorption. Specifically, the sensing characteristics as a function of carrier gas and humidity levels can be explained by the following surface reactions: H 2 O is dissociatively adsorbed on the film surface forming OH molecules with assistance of surface chemisorbed atomic oxygen. Once H 2 is admitted and dissociatively adsorbed as atomic H, a ready reaction between H and OH dominates the consumption of H 2 into the formation of H 2 O, thereby reducing the formation of PdH x . The overall desorption of the surface formed H 2 O causes a decrease in the optical transmittance observed for a given hydrogen exposure level, thus desensitizing the Pd/Au thin film towards the detection of hydrogen. The addition of Ru or YSZ as a buffer layer between the glass substrate and the Pd/Au thin film demonstrates a reduced or a completely removed baseline shifting, but H 2 desensitization in the presence of humidified air is still present.

Polymer-based microsensor for soil moisture measurement

A moisture microsensor based on poly(3,4-ethylenedioxythiophene–poly(styrene-sulfonate) (PEDOT–PSS) conductive polymer is developed and presented in this paper. The change in electrical characteristics of the PEDOT–PSS polymer film is used to determine its sensitivity and working mechanism when exposed to different levels of moisture content. The output characteristics, the change in electrical sheet resistance of the PEDOT–PSS film versus the percentage change in relative humidity (%RH), show that the conductivity of the film decreases when it is exposed to increasing levels of moisture content. The moisture sensors thus fabricated based on the PEDOT–PSS thin film were used to detect the gravimetric water content present in highly plastic (CH) soil samples (Buckshot Clay) for geological and geotechnical engineering applications. The Fourier transform infrared spectroscopy (FT-IR) study of the PEDOT–PSS film on a glass substrate showed the incorporation of OH molecules in the film when it was exposed to moisture environment. This incorporation of OH molecules caused the change in resistance of the PEDOT–PSS film when exposed to moisture content. The change in the output resistance of the sensor device was observed to be from 2.5 to 4.0 M ohm when exposed to soil samples with 15–35% change in gravimetric water content.

Theoretical Study of Above-Threshold Dissociation on Diatomic Molecules by Using Nonresonant Intense Laser Pulses

The above-threshold dissociation of the ground state of a OH molecule under intense nonresonant laser pulses has been studied using the time-dependent Schrödinger equation with discrete variable representation. The applied field is assumed as a two-color mixed nonresonant laser pulses which has the nonresonant frequency omega and the overtone 2omega. After modulating the relative phase factor between the omega and 2omega pulse, we extracted a three-photon absorption peak or a five-photon absorption peak in the ATD spectrum.

Cavity Cooling of Internal Molecular Motion

We predict that it is possible to cool rotational, vibrational, and translational degrees of freedom of molecules by coupling a molecular dipole transition to an optical cavity. The dynamics is numerically simulated for a realistic set of experimental parameters using OH molecules. The results show that the translational motion is cooled to a few muK and the internal state is prepared in one of the two ground states of the two decoupled rotational ladders in a few seconds. Shorter cooling times are expected for molecules with larger polarizability.

Dynamics of interfacial reactions between O(3P) atoms and long-chain liquid hydrocarbons

Recent progress that has been made towards understanding the dynamics of collisions at the gas–liquid interface is summarized briefly. We describe in this context a promising new approach to the experimental study of gas–liquid interfacial reactions that we have introduced. This is based on laser-photolytic production of reactive gas-phase atoms above the liquid surface and laser-spectroscopic probing of the resulting nascent products. This technique is illustrated for reaction of O(3P) atoms at the surface of the long-chain liquid hydrocarbon squalane (2,6,10,15,19,23-hexamethyltetracosane). Laser-induced fluorescence detection of the nascent OH has revealed mechanistically diagnostic correlations between its internal and translational energy distributions. Vibrationally excited OH molecules are able to escape the surface. At least two contributions to the product rotational distributions are identified, confirming and extending previous hypotheses of the participation of both direct and trapping-desorption mechanisms. We speculate briefly on future experimental and theoretical developments that might be necessary to address the many currently unanswered mechanistic questions for this, and other, classes of gas–liquid interfacial reaction.