Pulsed Laser Photolysis Research Articles

Pressure-Dependent Kinetics of the Reaction between CH3O2 and OH: TRIOX Formation.

Reaction of methyl peroxy radicals with hydroxyl radicals (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy at 296 K over the 1-100 bar pressure range (bath gas He): CH3O2 + OH → CH3O + HO2 (1a), CH3O2 + OH → CH2OO + H2O (1b), and CH3O2 + OH → CH3OOOH (TRIOX) (1d). Channel 1a is the dominant channel under ambient conditions, while the contribution of channel 1b is less than 5% at 1 bar and 296 K. Channel 1c is strongly pressure-dependent and becomes dominant at high pressures. The measured branching ratio of channel 1c is α1c = 0.87 ± 0.20 at 100 bar and 296 K. The chain termination channel 1c forming important product TRIOX is experimentally evaluated over an extended pressure range (1-100 bar) for the first time. The stabilization channel 1c might play a role at ambient pressures and low temperatures as well as high pressures at ambient and elevated temperatures.

Kinetics of the OH + NO2 reaction: rate coefficients (217–333 K, 16–1200 mbar) and fall-off parameters for N2 and O2 bath gases

Abstract. The radical terminating, termolecular reaction between OH and NO2 exerts great influence on the NOy∕NOx ratio and O3 formation in the atmosphere. Evaluation panels (IUPAC and NASA) recommend rate coefficients for this reaction that disagree by as much as a factor of 1.6 at low temperature and pressure. In this work, the title reaction was studied by pulsed laser photolysis and laser-induced fluorescence over the pressure range 16–1200 mbar and temperature range 217–333 K in N2 bath gas, with experiments at 295 K (67–333 mbar) for O2. In situ measurement of NO2 using two optical absorption set-ups enabled generation of highly precise, accurate rate coefficients in the fall-off pressure range, appropriate for atmospheric conditions. We found, in agreement with previous work, that O2 bath gas has a lower collision efficiency than N2 with a relative collision efficiency to N2 of 0.74. Using the Troe-type formulation for termolecular reactions we present a new set of parameters with k0(N2) = 2.6×10-30 cm6 molecule−2 s−1, k0(O2) = 2.0×10-30 cm6 molecule−2 s−1, m=3.6, k∞=6.3×10-11 cm3 molecule−1 s−1, and Fc=0.39 and compare our results to previous studies in N2 and O2 bath gases.

UV and infrared absorption spectra and 248 nm photolysis of maleic anhydride (C4H2O3)

Maleic anhydride (C2H2(CO)2O, Furan-2,5-dione) is emitted into or formed in the atmosphere during bio-mass burning events as well as being a photochemical degradation product of some aromatic volatile organic compounds. The photochemical fate of maleic anhydride, however, is presently not well enough characterized to enable modeling of its impact on the environment and human health. Maleic anhydride is a strong UV absorbing molecule with a continuous spectrum that extends well into the long-wavelength atmospheric actinic region, >295 nm. In this study, the room temperature UV and infrared absorption spectra of maleic anhydride were measured and quantified using several complementary calibration methods. In addition, photolysis quantum yields and stable end-product yields were measured following its 248 nm pulsed laser photolysis. Photoproducts were measured at room temperature using Fourier transform infrared spectroscopy detection under slow flow conditions at total pressures in the range 100–600 Torr (Syn. Air). The maleic anhydride photolysis quantum yield was determined to be 0.85 ± 0.20, independent of pressure, with CO, CO2, and acetylene (C2H2) photo-products formed with a 1:1:1 stoichiometric ratio and a yield of 1 ± 0.15, independent of total pressure.

Stargazin and \u03b34 slow the channel opening and closing rates of GluA4 AMPA receptors

As auxiliary subunits, transmembrane AMPA receptor regulatory proteins (TARPs) are known to enhance macroscopic current amplitude and alter kinetic properties of AMPA receptors on slow time scale, such as desensitization rate. Whether TARPs affect the rate of AMPA channel opening and closing, however, remains elusive. Using a laser-pulse photolysis technique, we investigated the effect of γ-2 (stargazin, a type 1a TARP) and γ-4 (a type 1b TARP) on the channel-opening and channel-closing rate constants (i.e., kop and kcl) of GluA4 homomeric channels. We found both TARPs slow the kop and kcl by 4-fold and 3-fold, respectively, without appreciable change of channel-opening probability, as compared with GluA4 channel alone. On the other hand, γ-4 has a stronger effect on slowing the channel desensitization rate than γ-2; yet, γ-2 causes a much more pronounced left shift of the dose-response relationship by increasing its affinity towards glutamate than γ-4. Our study shows that on the faster time scale, the major impact of TARP association with GluA4 is to lengthen the lifetime of the open channel, which is slow to form, to allow a larger charge transfer through the open channel that closes more slowly, without appreciable change of channel opening probability.

Kinetic Investigations of the CH (X2Π) Radical Reaction with Cyclopentadiene.

The reaction of the ground state methylidyne radical (CH (X2Π)) with cyclopentadiene (C5H6) is studied in a quasi-static reaction cell at pressures ranging from 2.7 to 9.7 Torr and temperatures ranging from 298 to 450 K. The CH radical is generated in the reaction cell by pulsed-laser photolysis (PLP) of gaseous bromoform at 266 nm, and its concentration monitored using laser-induced fluorescence (LIF) with an excitation wavelength of 430.8 nm. The reaction rate coefficient is measured to be 2.70(±1.34) × 10-10 cm3 molecule-1 s-1 at room temperature and 5.3 Torr and found to be independent of pressure or temperature over the studied experimental ranges. DFT and CBS-QB3 methods are used to calculate the reaction potential energy surface (PES) and to provide insight into the entrance channel of the reaction. The combination of the experimentally determined rate constants and computed PES supports a fast, barrierless entrance channel that is characteristic of CH radical reactions and could potentially lead to the formation of benzene isomers.

Temperature Dependence of the Reaction of Chlorine Atoms with CH3OH and CH3CHO.

Rate constants of the reactions Cl + CH3OH → CH2OH + HCl ( k1) and Cl + CH3CHO → CH3C(O) + HCl ( k3) were measured at 100 Torr over the temperature range 230.3-297.1 K. Radical chemistry was initiated by pulsed laser photolysis of Cl2 in mixtures of CH3OH and CH3CHO in a flow reactor. Heterodyne near-IR wavelength modulation spectroscopy was used to directly detect HO2 produced from the subsequent reaction of CH2OH with O2 in real time to determine the rate of reaction of Cl with CH3OH. The rate of Cl + CH3CHO was measured relative to that of the Cl + CH3OH reaction. Secondary chemistry, including that of the adducts HO2·CH3OH and HO2·CH3CHO, was taken into account. The Arrhenius expressions were found to be k1( T) = 5.02-1.5+1.8 × 10-11 exp[(20 ± 88)/ T] cm3 molecule-1 s-1 and k3( T) = 6.38-2.0+2.4 × 10-11 exp[(56 ± 90)/ T] cm3 molecule-1 s-1 (2σ uncertainties). The average values of the rate constants over this temperature range were k1 = (5.45 ± 0.37) × 10-11 cm3 molecule-1 s-1 and k3 = (8.00 ± 1.27) × 10-11 cm3 molecule-1 s-1 (2σ uncertainties), consistent with current literature values.

Temperature Dependence Study of the Kinetics and Product Yields of the HO2 + CH3C(O)O2 Reaction by Direct Detection of OH and HO2 Radicals Using 2f-IR Wavelength Modulation Spectroscopy.

The HO2 + CH3C(O)O2 reaction consists of three product channels: CH3C(O)OOH + O2 (R1a), CH3C(O)OH + O3 (R1b), and OH + CH3C(O)O + O2 (R1c). The overall rate constant ( k1) and product yields (α1a, α1b, and α1c) were determined over the atmospherically relevant temperature range of 230-294 K at 100 Torr in N2. Time-resolved kinetics measurements were performed in a pulsed laser photolysis experiment in a slow flow cell by employing simultaneous infrared (IR) and ultraviolet (UV) absorption spectroscopy. HO2 and CH3C(O)O2 were formed by Cl-atom reactions with CH3OH and CH3CHO, respectively. Heterodyne near- and mid-infrared (NIR and MIR) wavelength modulation spectroscopy (WMS) was employed to selectively detect HO2 and OH radicals. Ultraviolet absorption at 225 and 250 nm was used to detect various peroxy radicals as well as ozone (O3). These experimental techniques enabled direct measurements of α1c and α1b via time-resolved spectroscopic detection in the MIR and the UV, respectively. At each temperature, experiments were performed at various ratios of initial HO2 and CH3C(O)O2 concentrations to quantify the secondary chemistry. The Arrhenius expression was found to be k1( T) = 1.38-0.63+1.17 × 10-12 exp[(730 ± 170)/ T] cm3 molecule-1 s-1. α1a was temperature-independent while α1b and α1c decreased and increased, respectively, with increasing temperatures. These trends are consistent with the current recommendation by the IUPAC data evaluation. Hydrogen-bonded adducts of HO2 with the precursors, HO2·CH3OH and HO2·CH3CHO, played a role at lower temperatures; as part of this work, rate enhancements of the HO2 self-reaction due to reactions of the adducts with HO2 were also measured.

Low temperature studies of the rate coefficients and branching ratios of reactive loss vs quenching for the reactions of 1CH2 with C2H6, C2H4, C2H2

The kinetics of the reactions of the first excited state of methylene, 1CH2, with C2H2, C2H4, and C2H6, has been measured over the temperature range 43–298 K by pulsed laser photolysis, monitoring 1CH2 removal by laser induced fluorescence. Low temperatures were obtained using a pulsed Laval expansion (43–134 K), while a slow flow reaction cell was used for temperatures of 160 K and above. The rate coefficients for the reactions with C2H2, C2H4, and C2H6, all showed a strong negative temperature dependence. In combination with other literature data, the coefficients can be parameterized as: kC2H2(43≤T/K≤298)=(3.22±0.15)×10−10×(T/298)(−0.394±0.066)kC2H4(43≤T/K≤298)=(2.16±0.14)×10−10×(T/298)(−0.612±0.089)kC2H6(43≤T/K≤298)=(1.78±0.10)×10−10×(T/298)(−0.545±0.078)Branching ratios for reactive removal of 1CH2 vs quenching to ground state were also determined for all three colliders and for H2 and CH4, at temperatures between 100 and 298 K. The values measured show that the dominant removal process of 1CH2 by H2, C2H2, and C2H4, changes from reactive removal to quenching to ground state 3CH2 as the temperature decreases from 298 K to 100 K, while for CH4 and C2H6, reactive removal drops from around 85% to around 55%. The impacts of the new measurements for Titan's atmosphere are examined using a 1D chemistry and transport model. A significant increase (∼25%) in the mixing ratio of benzene between 500 and 1550 km is calculated, due to the increased production of C3H3 from the reaction of 1CH2 with C2H2.

Heterobinuclear Light Absorber Coupled to Molecular Wire for Charge Transport across Ultrathin Silica Membrane for Artificial Photosynthesis.

Coupling of robust, all-inorganic heterobinuclear light absorbers to metal oxide catalysts for water oxidation across an ultrathin product-separating silica membrane requires charge transfer through organic molecular wires embedded in the silica. A synthetic approach for assembling the bimetallic units on the silica surface is introduced that is compatible with the presence of encapsulated organic molecules. Accurate selection and fine tuning of the concentration of embedded conducting wires are enabled by a two-step method consisting of surface attachment of a tripodal anchor, trimethoxysilyl aniline, followed by attachment of p-oligo(phenylene vinylene) through amide linkage. Each step of the assembly process was monitored and characterized by a combination of Fourier transform infrared, Fourier transform-Raman, and UV-vis spectroscopy techniques. Hole transfer was observed from transient CoIII, formed by TiIVOCoII → TiIIIOCoIII charge transfer excitation of the chromophore, to p-oligo(phenylene vinylene) molecule within the 8 ns width of the photolysis laser pulse by transient optical absorption spectroscopy of the wire radical cation. The rectifying property of the light absorber-wire assembly enabled by appropriate selection of redox potentials of metals and embedded wire obviates the need for a molecularly defined linkage between the components. Combined with the previously observed ultrafast hole injection from the embedded wires to Co oxide catalyst, the result implies visible-light-induced hole transfer from visible-light-excited binuclear light absorber to water oxidation catalyst across the silica separation membrane in a few nanoseconds or faster. Demonstration and understanding of this interfacial charge-transfer step is critical for developing nanoscale core-shell architectures for complete photosynthetic cycles.

Experimental and computational studies of the kinetics of the reaction of hydrogen atoms with carbon disulfide

Pulsed laser photolysis at 193 nm of NH3/CS2 mixtures in an Ar bath gas combined with resonance fluorescence detection of atomic H yielded the rate constant for H + CS2 over 295–490 K. The reaction was found to be pressure dependent and in the fall-off region or near the low pressure limit. Fitting to the Troe formalism yielded k0 = 7.5 × 10−24 (T/300 K)−14.76 exp(−5180 K/T) cm6 molecule−2 s−1 over 295–490 K. At 490 K equilibration of adduct formation was observed which yielded a H-SCS bond dissociation enthalpy of 73 ± 10 kJ mol−1. These observations are consistent with W1 results for the potential energy surface and comparison suggests HSCS is the major addition product. Transition state theory yields an estimate for the high-pressure limit of addition over 290–300 K of 1.3 × 10−9 exp(−7.2 kJ mol−1/RT) cm3 molecule−1 s−1. Uncertainties are discussed in the text.

Vacuum ultraviolet photoionization cross section of the hydroxyl radical.

The absolute photoionization spectrum of the hydroxyl (OH) radical from 12.513 to 14.213 eV was measured by multiplexed photoionization mass spectrometry with time-resolved radical kinetics. Tunable vacuum ultraviolet (VUV) synchrotron radiation was generated at the Advanced Light Source. OH radicals were generated from the reaction of O(1D) + H2O in a flow reactor in He at 8 Torr. The initial O(1D) concentration, where the atom was formed by pulsed laser photolysis of ozone, was determined from the measured depletion of a known concentration of ozone. Concentrations of OH and O(3P) were obtained by fitting observed time traces with a kinetics model constructed with literature rate coefficients. The absolute cross section of OH was determined to be σ(13.436 eV) = 3.2 ± 1.0 Mb and σ(14.193 eV) = 4.7 ± 1.6 Mb relative to the known cross section for O(3P) at 14.193 eV. The absolute photoionization spectrum was obtained by recording a spectrum at a resolution of 8 meV (50 meV steps) and scaling to the single-energy cross sections. We computed the absolute VUV photoionization spectrum of OH and O(3P) using equation-of-motion coupled-cluster Dyson orbitals and a Coulomb photoelectron wave function and found good agreement with the observed absolute photoionization spectra.

Mid-infrared quantum cascade laser spectroscopy probing of the kinetics of an atmospherically significant radical reaction, $$\hbox {CH}_{3}\hbox {O}_{2}+\hbox {NO}_{2}+\hbox {M}\rightarrow \hbox {CH}_{3}\hbox {O}_{2}\hbox {NO}_{2}+\hbox {M}$$ CH 3 O 2 + NO 2 + M → CH 3 O 2 NO 2 + M , in the gas phase

The kinetic parameters of an important atmospheric reaction, $$\hbox {CH}_{3}\hbox {O}_{2} + \hbox {NO}_{2} + \hbox {M}\rightarrow \hbox {CH}_{3}\hbox {O}_{2}\hbox {NO}_{2} + \hbox {M}$$ , have been recorded by monitoring directly the changes in concentrations of methylperoxy radicals $$(\hbox {CH}_{3}\hbox {O}_{2})$$ in the gas phase employing a new mid-infrared quantum cascade laser (QCL)-based apparatus. $$\hbox {CH}_{3}\hbox {O}_{2 }$$ radicals in our apparatus have been generated by pulsed UV laser (266 nm) photolysis of $$\hbox {CH}_{3}\hbox {I}$$ in a gaseous mixture with oxygen. The absorption band corresponding to the mid-infrared O-O stretching fundamental of the peroxy radical, within a narrow spectral range, 1070–1120 $$\hbox {cm}^{-1}$$ , has been recorded by tuning the wavelength of the QCL operated in CW mode. The kinetics of the aforementioned reaction of $$\hbox {CH}_{3}\hbox {O}_{2}$$ with $$\hbox {NO}_{2}$$ has been followed by analyzing the changes of the infrared (QCL) decay profile of $$\hbox {CH}_{3}\hbox {O}_{2}$$ at $$9.1~\upmu \hbox {m}$$ $$(1098.9~\hbox {cm}^{-1})$$ maintaining a pseudo first order reaction condition. We noticed that the rate constant of the reaction at 298 K varies in the range of (1.21–3.08) $$\times 10^{-12}$$ $$\hbox {cm}^{3 }~\hbox {molecule}^{-1 }\,\hbox {s}^{-1 }$$ for changing the total pressure in the range of 75–730 mbar. The absorption cross-section of $$\hbox {CH}_{3}\hbox {O}_{2}$$ at the probe wavelength $$(1098.9~\hbox {cm}^{-1})$$ , has been estimated for the first time to be $$8.3 \pm 0.4 \times 10^{-20}~\hbox {cm}^{2}$$ . Kinetics measurement of an atmospherically important gas phase reaction, $$\hbox {CH}_{3}\hbox {O}_{2}+\hbox {NO}_{2}+\hbox {M}\rightarrow \hbox {CH}_{3}\hbox {O}_{2}\hbox {NO}_{2}+\hbox {M}$$ , is studied using a newly developed apparatus based on pulsed UV laser photolysis time-resolved mid-infrared absorption spectroscopy. $$\hbox {CH}_{3}\hbox {O}_{2}$$ radicals are probed in the mid-infrared by a CW quantum cascade laser (QCL).

Rate Constants and H-Atom Product Yields for the Reactions of O(1D) Atoms with Ethane and Acetylene from 50 to 296 K

The gas phase reactions of atomic oxygen in its first excited state with ethane and acetylene have been investigated in a continuous supersonic flow reactor over the temperature range from 50 to 296 K. O(1D) atoms were produced by the pulsed laser photolysis of ozone at 266 nm. Two different types of experiments, kinetics measurements and H-atom product yield determinations, were performed by detecting O(1D) atoms and H(2S) atoms, respectively, by vacuum ultraviolet laser-induced fluorescence. The measured rate constants are in agreement with previous work at room temperature, and little or no temperature dependence was observed as the temperature was decreased to 50 K. H-atom yields were found to be independent of temperature for the reaction of O(1D) with ethane. These product yields are discussed in the context of earlier dynamics measurements at higher temperature. Due to the influence of secondary reactions, no H-atom yields could be obtained for the reaction of O(1D) with acetylene.

Kinetics of the Gas-Phase O(1D) + CO2 and C(1D) + CO2 Reactions over the 50-296 K Range.

The kinetics of the reactions of CO2 with atomic oxygen and atomic carbon in their first excited singlet states have been studied at room temperature and below using the Laval nozzle reactor method. O(1D) and C(1D) atoms were created in situ by the 266 nm pulsed laser photolysis of O3 and CBr4 precursor molecules, respectively. While O(1D) atoms were detected directly by vacuum ultraviolet laser-induced fluorescence at 115 nm, C(1D) atoms were followed indirectly through a chemical tracer method. The measured rate constants for the O(1D) + CO2 reaction are found to be in excellent agreement with earlier work and extend the available kinetic data for this process down to 50 K. The present work represents the first kinetics study of the C(1D) + CO2 reaction below room temperature. Both reactions display rate constants that increase as the temperature falls, with a more substantial rate increase observed for the O(1D) + CO2 reaction. This finding could be due to the increasing lifetimes of the intermediate species involved at lower temperatures.

Vibrationally Excited Ozone Relaxation by CO

Time profiles of ozone concentration after pulsed UV laser photolysis in the O2- O3-Ar-CO mixture, measured using time-resolved absorption spectroscopy, are presented. The experimental results show the dominance of the stabilization channel over the reactive one for the interaction between the vibrationally excited ozone molecule O3(υ) and carbon monoxide CO. The rate constant of the process O3(υ)+CO→O3 + CO, obtained by processing experimental data by the kinetic modeling method is (1.5 ± 0.2) · 10−13 cm3/s.

Competing Pathways in the Photochemistry of Ru(H)2(CO)(PPh3)3

The photochemistry of Ru(H)2(CO)(PPh3)3 (1) has been reinvestigated employing laser and conventional light sources in conjunction with NMR spectroscopy and IR spectroscopy. The sensitivity of NMR experiments was enhanced by use of p-H2-induced polarization (PHIP), and a series of unexpected reactions were observed. The photoinduced reductive elimination of H2 was demonstrated (a) via NMR spectroscopy by the observation of hyperpolarized 1 on pulsed laser photolysis in the presence of p-H2 and (b) via nanosecond time-resolved infrared (TRIR) spectroscopy studies of the transient [Ru(CO)(PPh3)3]. Elimination of H2 competes with photoinduced loss of PPh3, as demonstrated by formation of dihydrogen, triphenylarsine, and pyridine substitution products which are detected by NMR spectroscopy. The corresponding coordinatively unsaturated 16-electron intermediate [Ru(H)2(CO)(PPh3)2] exists in two isomeric forms according to TRIR spectroscopy that react with H2 and with pyridine on a nanosecond time scale. These tw...

Gas phase kinetics of the OH + CH3CH2OH reaction at temperatures of the interstellar medium (T = 21-107 K).

Ethanol, CH3CH2OH, has been unveiled in the interstellar medium (ISM) by radioastronomy and it is thought to be released into the gas phase after the warm-up phase of the grain surface, where it is formed. Once in the gas phase, it can be destroyed by different reactions with atomic and radical species, such as hydroxyl (OH) radicals. The knowledge of the rate coefficients of all these processes at temperatures of the ISM is essential in the accurate interpretation of the observed abundances. In this work, we have determined the rate coefficient for the reaction of OH with CH3CH2OH (k(T)) between 21 and 107 K by employing the pulsed and continuous CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme, which means Reaction Kinetics in a Uniform Supersonic Flow) technique. The pulsed laser photolysis technique was used for generating OH radicals, whose time evolution was monitored by laser induced fluorescence. An increase of approximately 4 times was observed for k(21 K) with respect to k(107 K). With respect to k(300 K), the OH-reactivity at 21 K is enhanced by two orders of magnitude. The obtained T-expression in the investigated temperature range is k(T) = (2.1 ± 0.5) × 10-11 (T/300 K)-(0.71±0.10) cm3 molecule-1 s-1. In addition, the pressure dependence of k(T) has been investigated at several temperatures between 21 K and 90 K. No pressure dependence of k(T) was observed in the investigated ranges. This may imply that this reaction is purely bimolecular or that the high-pressure limit is reached at the lowest total pressure experimentally accessible in our system. From our results, k(T) at usual IS temperatures (∼10-100 K) is confirmed to be very fast. Typical rate coefficients can be considered to range within about 4 × 10-11 cm3 molecule-1 s-1 at 100 K and around 1 × 10-10 cm3 molecule-1 s-1 at 20 K. The extrapolation of k at the lowest temperatures of the dense molecular clouds of ISM is also discussed in this paper.

Low temperature studies of the removal reactions of 1CH2 with particular relevance to the atmosphere of Titan

Methylene, CH2, is one of the major photolysis products of methane by Lyman-α radiation and is involved in the photochemistry of the atmospheres of Titan and the giant planets. The kinetics of the reactions of the first excited state of methylene, 1CH2, with He, N2, O2, H2 and CH4 have been measured over the temperature range 43–160 K by pulsed laser photolysis, monitoring 1CH2 removal by laser induced fluorescence. Low temperatures were obtained with either a pulsed Laval expansion (43–134 K) or a, slow flow reaction cell (160 K). The rate coefficients for the reactions with N2, O2, H2 and CH4 all showed a strong negative temperature dependence. In combination with other literature data, the rate coefficients can be parameterised as:kHe(43 < T/K < 800) = (1.90 ± 0.23) × 10−12 × (T/298)1.74±0.16 × exp((88±23)/T)kN2(43<T/K<800) = (2.29 ± 1.12) × 10−12 × (T/298)−2.15±1.38 × exp((-74±96)/T) + (3.91 ± 0.78) × 10−11 × exp((-469±114)/T)kO2(43<T/K<300) = (6.16 ± 1.09) × 10−11 × (T/298)−0.65±0.14kH2(43<T/K<800) = (1.10 ± 0.04) × 10−10 × (T/298)−0.40±0.06 × exp((11.1±6.9)/T)kCH4(43<T/K<475) = (8.20 ± 0.46) × 10−11 × (T/298)−0.93±0.10 × exp((-20.5±12.8)/T)For the reactions of 1CH2 with H2 and CH4, the branching ratio for quenching to ground state, 3CH2, vs chemical reaction was also determined at 160 and 73 K. The values measured (H2: 0.39 ± 0.10 at 160 K, 0.78 ± 0.15 at 73 K; CH4: 0.49 ± 0.09 at 160 K, 0.64 ± 0.19 at 73 K) confirm trends of an increased proportion of reactive loss with increasing temperature determined at higher temperatures. The impacts of the new measurements for Titan's atmosphere have been ascertained using a 1D chemistry and transport model. A significant decrease (∼40%) in the mixing ratio of ethane between 800 and 1550 km is calculated due to the decrease contribution of methyl production from the reaction of 1CH2 with CH4, with smaller increases in the concentrations of ethene and acetylene. Ethene production is enhanced by more methylene being converted to methylidene, CH, and the subsequent reaction of CH with CH4 to generate ethene. Photolysis of ethene is the major route to acetylene formation.

Collisional relaxation of O2(a1Δ, υ = 1, 2, 3) by CO2

Rate coefficients for the vibrational relaxation of O2(a1Δ, υ) by CO2 have been determined from the decay of fluorescence of the a1Δg → X3Σg- (υ–υ′) transitions with υ = υ′ = 1, 2 and 3. O2(a1Δ, υ) molecules were produced by pulsed laser photolysis of ozone at a wavelength of 266 nm. The rate coefficients were measured to be (1.9 ± 0.2) × 10−14 cm3/s, (2.4 ± 0.2) × 10−14 cm3/s and (2.7 ± 0.3) × 10−14 cm3/s for υ = 1, 2 and 3 respectively.

Atmospheric Chemistry of E- and Z-CF3CH═CHF (HFO-1234ze): OH Reaction Kinetics as a Function of Temperature and UV and IR Absorption Cross Sections.

We report here the rate coefficients for the OH reactions (kOH) with E-CF3CH═CHF and Z-CF3CH═CHF, potential substitutes of HFC-134a, as a function of temperature (263-358 K) and pressure (45-300 Torr) by pulsed laser photolysis coupled to laser-induced fluorescence techniques. For the E-isomer, the existing discrepancy among previous results on the T dependence of kOH needs to be elucidated. For the Z-isomer, this work constitutes the first absolute determination of kOH. No pressure dependence of kOH was observed, while kOH exhibits a non-Arrhenius behavior: kOH(E) = [Formula: see text] and kOH(Z) = [Formula: see text] cm3 molecule-1 s-1, where uncertainties are 2σ. UV absorption cross sections, σλ, are reported for the first time. From σλ and considering a photolysis quantum yield of 1, an upper limit for the photolysis rate coefficients and lifetimes due to this process in the troposphere are estimated: 3 × 10-8 s-1 and >1 year for the E-isomer and 2 × 10-7 s-1 and >2 months for Z-CF3CH═CHF, respectively. Under these conditions, the overall estimated tropospheric lifetimes are 15 days (for the E-isomer) and 8 days (for the Z-isomer), the major degradation pathway being the OH reaction, with a contribution of the photolytic pathway of less than 3% (for E) and 13% (for Z). IR absorption cross sections were determined both experimentally (500-4000 cm-1) and theoretically (0-2000 cm-1). From the theoretical IR measurements, it is concluded that the contribution of the 0-500 cm-1 region to the total integrated cross sections is appreciable for the E-isomer (9%) but almost negligible for the Z-isomer (0.5%). Nevertheless, the impact on their radiative efficiency and global warming potential is negligible.