Torr Of He Research Articles

Reaction of Cl Atom with c-C5F8 and c-C5HF7: Relative and Absolute Measurements of Rate Coefficients and Identification of Degradation Products.

Partially and fully fluorinated olefins are a class of compounds with relatively short atmospheric lifetimes and low 100-year global warming potentials, compared to their saturated predecessors, which are used or considered as refrigerants, propellants, solvents, and other end-uses. The cyclic unsaturated compounds c-C5F8 and c-C5HF7 are currently under consideration as etching agents for the semiconductor industry. In this study, we expand on our previous work on the reaction of the OH radical with c-C5F8 and c-C5HF7 and report the rate coefficients, k, for the gas-phase reaction of the Cl atom with c-C5F8 and c-C5HF7 over a range of temperature (245-367 K) and pressure (100-200 Torr of He or N2 and 0 to 4.8 Torr O2) using a pulsed laser photolysis-resonance fluorescence (PLP-RF) technique. In addition, a relative rate (RR) technique, employing multiple reference compounds, was used to study the Cl atom reactions at 296 K, 100 and 630 Torr (N2 or air) total pressure. Reaction rate coefficients, k1, of the Cl atom reaction with c-C5F8 were found to be independent of pressure, over the pressure range used in this work, with k1(296 K), derived as an average of results from the PLP-RF and RR techniques being (1.07 ± 0.02) × 10-12 cm3 molecule-1 s-1 and k1(T) = (7.76 ± 0.73) × 10-13 × (exp[(98 ± 26)/T]) cm3 molecule-1 s-1, where the quoted error limits represent the 2σ data precision. Rate coefficients, k2, for the Cl atom + c-C5HF7 reaction were measured to be k2(296 K) = (4.61 ± 0.10) × 10-12 cm3 molecule-1 s-1 and k2(T) = (7.42 ± 0.89) × 10-13 × (exp[(540 ± 32)/T]) cm3 molecule-1 s-1. The Cl atom temporal profiles, observed with the PLP-RF technique, indicate that the Cl atom with c-C5F8 and c-C5HF7 reactions lead to adduct formation. The equilibrium constants for adduct formation were derived in this work, and a second-law analysis was used to obtain ΔH and ΔS values of -18.5 ± 0.4 kcal mol-1, -30.9 ± 1.2 cal K-1 mol-1, and -13.9 ± 0.5 kcal mol-1, -27.6 ± 1.1 cal K-1 mol-1 for the c-C5F8 and c-C5HF7 reactions, respectively. The Cl-initiated degradation of c-C5F8 and c-C5HF7 in the presence of O2 was studied and stable products were identified via infrared spectroscopy using experimental or theoretically derived spectra from our previous OH reaction work. For c-C5F8, FC(O)CF2CF2CF2C(O)F and FC(O)C(O)F were observed with molar yields of 0.80 and 0.10, respectively. For c-C5HF7, we observed the formation of HC(O)CF2CF2CF2C(O)F and HC(O)C(O)F with a combined molar yield of 0.72. Carbonyl difluoride, F2CO, was also a major product in the decomposition of c-C5F8 and c-C5HF7. The oxidation mechanism of the Cl-initiated degradation of c-C5F8 and c-C5HF7 is discussed. Based on the combined findings from this and our previous work, the atmospheric implications from the use of c-C5F8 and c-C5HF7 are presented.

194 nm microplasma lamps driven by excitation transfer: optical sources for the 199Hg ion atomic clock and photochemistry

A series of miniature, microcavity plasma lamps emitting predominantly at 194 nm has been successfully developed and tested as the optical pump for the microwave 199Hg-ion atomic clock (40.507 GHz), replacing low pressure, RF-powered Ar/Hg discharges. Intense fluorescence on the 6p 2P1/2 → 6s 2S1/2 transition of the singly-charged 202Hg ion at 194.23 nm has been generated in arrays of cylindrical microplasmas through electron-impact excitation of He, followed by three-body formation of He2(a 3 ) and Penning ionization of Hg. Emission spectroscopy and kinetic modeling of He/Hg vapor plasmas demonstrate that the population of the Hg+(62P1/2) radiating state (16.82 eV), produced by direct or two-step electron impact processes, is 1% of that generated by excitation transfer to Hg by H. Flat, fused silica lamps having emitting areas as small as 4 mm2 and containing several mg of 202Hg and 50–800 Torr of He have been fabricated and serve as optical drivers for the Hg+ atomic clock cycle. Based on small arrays of 500 μm–1 mm diameter microcavities, these lamps produce peak and average intensities at 194 nm greater than those associated with the Hg resonance transition at ∼ 254 nm, despite the factor of 3 difference between the energies of the 6p 3P1 and 6p 2P1/2 states of neutral Hg and Hg+, respectively. These lamps are unique in the sense that the desired radiating species is an excited ion and the background He gas pressure can reach 1 atm, both of which contribute to a dense glow plasma placing severe demands on E/N, power conditioning, and materials selection. Nevertheless, with proper attention given to design, vacuum processing, and preparation of the lamps, lifetimes above 1500 h have been realized to date. When these lamps drive Jet Propulsion Laboratory Hg+ clocks, the stability floor has been measured to be −14. The implications of this lamp for gas-phase and solid-state photochemistry are also discussed.

Efficient cascade lasing on over 17 wavelengths from two-photon excitation of the cesium [formula omitted]D states

A broad survey is taken to assess the agility of the cesium 62D states in the generation of cascade lasing from 455 to 3650 nm. The experiment utilized a heat pipe with 0 to 10 Torr of He, cesium densities of 1014 to 1017 atoms per cm3, and it is pumped by a pulsed dye laser with 1 to 30 mJ/pulse. Cascade lasing, generated by amplified spontaneous emission with no aid of a cavity, is observed on over 17 transitions—nearly every dipole allowed transition from energy levels below 62D. Many lasing lines possess 100 to 229 μJ of energy, slope efficiencies of 4 to 7%, low thresholds favorable for diode pumping, and together have combined optical conversion efficiencies as high as 10%±3%. This agile system could have applications in infrared counter measures, target illuminator lasers, beacon lasers, underwater communication, and other tactical laser applications.

Rate Constants for Reactions of HCCO and HCCCO Radicals with O2 over the Temperature Range 243-423 K.

A pulsed laser photolysis-photoionization mass spectrometer system has been employed to measure the rate constants of HCCO + O2 and HCCCO + O2 over the temperature range 243-423 K in 1.2-8.4 Torr of He or N2. Radicals of HCCO and HCCCO were produced by 193 nm ArF laser photolysis of ethyl ethynyl ether and methyl propiolate, respectively. HCCO was photoionized by a Kr resonance lamp with a CaF2 window (10.03 eV), and HCCCO was ionized by a Xe lamp with a sapphire window (8.44 eV). Both ions were detected as parent ions in a quadrupole mass spectrometer. From analysis of the time profiles of the ion signals for various O2 concentrations, the overall rate constants at 298 K are represented by the values k2 = (6.3 ± 1.0) × 10-13 for HCCO + O2 and k5 = (5.7 ± 0.6) × 10-12 for HCCCO + O2 in the units cm3 molecule-1 s-1. The rate coefficients for the two reactions can be described by k2(T) = (1.5-0.7+1.5) × 10-12 exp[-(225 ± 220)/T] and k5(T) = (1.8-0.9+1.9) × 10-12 exp[(343 ± 228)/T] in the units cm3 molecule-1 s-1 over the temperature range 243-423 K.

Temperature-Dependent Kinetic Study of the Reaction of Hydroxyl Radicals with Hydroxyacetone

The kinetics of the reaction of OH radicals with hydroxyacetone has been investigated as a function of temperature at (2.0 - 2.1) Torr total pressure of helium and over an extended temperature range, T = 250 - 830 K, and as a function of pressure at T = 301 K in the pressure range 1.0-10.4 Torr. The rate constant of the reaction OH + CH3C(O)CH2OH  products (1) was measured using both absolute (from the kinetics of OH consumption in excess of hydroxyacetone) and relative rate method, k1 = 4.7×10-22×T3.25exp(1410/T) cm3molecule-1s-1 at T = 250-830 K. The present data combined with selected previous temperature dependent studies of reaction (1) yield k1 = 4.4×10-20×T2.63exp(1110/T) cm3molecule-1s-1, which is recommended from the present work at T = 230-830K (with conservative uncertainty of 20% at all temperatures). k1 was found to be pressure independent in the pressure range from 1.0 to 10.4 Torr of He at T = 301 K. The present results are compared with previous experimental and theoretical data.

LIBS measurement of trace tantalum and rhenium in tungsten for in-situ diagnostic of nuclear transmutation

Calibration-free laser-induced breakdown spectroscopy (CF-LIBS) was applied to tungsten (W)- tantalum (Ta) alloy and W-rhenium (Re) alloy. In the W-Ta alloy measurement, an apparent Ta I line at 648.5 nm was found to be isolated from W I lines and used for the concentration determination. Accurate concentration measurements were achieved for a W - 2 at.% Ta specimen at the ambient gas pressure between 0.1 and 10 Torr of He, Ne and Ar gas. Within these ambient gas species, the highest line intensities were obtained by Ar gas. In the W-Re alloy measurement, the detection limit of trace Re in W-Re was investigated. CF-LIBS results agree well with the nominal concentration of Re down to W-0.5 at.% Re specimen which is better than the measurement accuracy of EDX (energy-dispersive X-ray micro-analysis). Further improvements are required in order to measure 0.2 at.% Re in W expected at ITER.

Demonstration of a CW diode-pumped Ar metastable laser operating at 4 W.

Optically pumped rare gas lasers are being investigated as potential high-energy, high beam quality systems. The lasing medium consists of rare gas atoms (Rg=Ne, Ar, Kr, or Xe) that have been electric discharge excited to the metastable np5(n+1)s P32 state. Following optical excitation, helium (He) at pressures of 200-1000Torr is used as the energy transfer agent to create a population inversion. The primary technical difficulty for this scheme is the discharge production of sufficient Rg* metastables in the presence of >200 Torr of He. In this Letter, we describe a pulsed discharge that yields >1013 cm-3Ar* in the presence of He at total pressures up to 750Torr. Using this discharge, a diode-pumped Ar* laser providing 4.1W has been demonstrated.

Mechanistic and kinetic study on the reaction of methylperoxyl radical with atomic hydrogen

Singlet and triplet potential energy surfaces for the CH3O2 with H reaction have been investigated computationally to propose the reaction mechanisms and possible products. On the singlet PES, addition-dissociation was located. At 3.65Torr with He as bath gas, the formation of CH2O+H2O channel is dominated at the whole temperature range. Furthermore, the predicted rate constants for kCH2O+H2O at 298K 3.65Torr of He agree well with the available experimental values. The pathways on the triplet PES will not compete with the pathways on the singlet PES in kinetically.

Effect of nature and pressure of ambient environments on the surface morphology, plasma parameters, hardness, and corrosion resistance of laser-irradiated Mg-alloy

AbstractThe influence of nature and pressure of ambient environment on the surface modification, plasma parameters, hardness, and corrosion resistance of Mg-alloy has been investigated. Nd: YAG laser (1064 nm, 10 ns, 25 mJ) at a fluence of 1.3 J cm−2 has been employed as an irradiation source. Targets of Mg-alloy were exposed in the ambient environments of argon (Ar), neon (Ne), and helium (He) at pressures ranging from 5 to 760 Torr. Scanning electron microscope has been employed to investigate the surface morphology of the irradiated targets. It reveals the formation of cavities, cones, droplets, ripples, and islands on the surface of the irradiated sample. Laser-induced breakdown spectroscopy technique was employed to measure electron temperature (Te) and electron number density (Ne) of Mg-alloy. The value of electron temperature ranges from 6628 to 12,855 K, whereas the value of electron number density varies from 5.4 × 1017 to 19.2 × 1017 cm−3. The maximum Te and Ne are observed in Ar and minimum in case of He. It was also revealed that both the surface morphology and plasma parameters are strongly dependent upon nature and pressure of environmental gases. The maxima of Te is achieved at a pressure of 10 Torr for all the three ambient environments that is, Ar, Ne, and He; whereas maxima of Ne is achieved at different pressures, that is, at 760 Torr for Ar, at 200 Torr for Ne, and at 50 Torr for He. The hardness and corrosion resistance of irradiated Mg-alloy have been explored using Vickers Micro-hardness tester and Potentio-dynamic polarization technique, respectively. It was investigated that as compared with un-irradiated target, the hardness as well as corrosion resistance of the laser-irradiated target has been increased significantly in all environments. Plasma parameters, mechanical, and electrical properties of laser-irradiated Mg-alloy have been correlated with induced surface modifications and are strongly influenced by environmental conditions.

Atmospheric Implications of the Emission ofCF<sub>3</sub>CF<sub>2</sub>CH=CH<sub>2</sub> (HFC 1345fz) as a Consequence of its Use as Foam Blowing Agents

HFC-1345fz (3,3,4,4,4-pentafluoro-1-butene, CF3CF2CH=CH2) is currently considered as an acceptable alternative to high global warming potential (GWP) HFCs as foam blowing agents. The tropospheric photooxidation of HFC-1345fz initiated by hydroxyl (OH) radicals is expected to be faster than the saturated HFCs because of the presence of the C-C double bond. To evaluate the tropospheric persistence of HFC-1345fz it is necessary to determine the rate coefficients of the main removal pathway in the gas-phase, i.e. its reaction with OH (kOH). In this paper, we present the first absolute measurement of kOH at 298 K as a function of total pressure (31 - 624 Torr of He). The determination of kOH has been performed by the pulsed laser photolysis – laser induced fluorescence technique using H2O2 and HNO3 as OHprecursor. A global tropospheric lifetime of HFC-1345fz due to the reaction with OH radicals (τOH) of around 9 days was estimated from kOH and assuming a 24-h averaged OH concentration of 1×106 cm-3. Degradation route of HFC-1345fz initiated by OH radicals is compared with other atmospheric removal processes. The IR absorption cross sections, σv , of HFC-1345fz are also determined in the wavenumber range of 500-4000 cm-1 to quantify its radiative properties, such as the radiative efficiency (RE). The lifetime corrected RE for HFC-1345fz is calculated to be 0.022 W m-2 ppbv-1, which implies a negligible global warming potential. Therefore, from the kinetic and atmospheric degradation points of view, the studied hydrofluoroolefin seems to be suitable for replacing saturated HFCs as foam blowing agents, since they are shortlived species that do not contribute to the Earth’s global warming.

Hydroxyl radical reaction rate coefficients as a function of temperature and IR absorption cross sections for CF3CH=CH2 (HFO-1243zf), potential replacement of CF3CH2F (HFC-134a).

CF3CH=CH2 (hydrofluoroolefin, HFO-1243zf) is a potential replacement of high global-warming potential (GWP) hydrofluorocarbon (HFC-134a, CF3CFH2). Both the atmospheric lifetime and the radiative efficiency of HFO-1243zf are parameters needed for estimating the GWP of this species. Therefore, the aim of this work is (i) to estimate the atmospheric lifetime of HFO-1243zf from the reported OH rate coefficients, k OH, determined under tropospheric conditions and (ii) to calculate its radiative efficiency from the reported IR absorption cross sections. The OH rate coefficient at 298 K also allows the estimation of the photochemical ozone creation potential (ε(POCP)). The pulsed laser photolysis coupled to a laser-induced fluorescence technique was used to determine k OH for the reaction of OH radicals with HFO-1243zf as a function of pressure (50-650 Torr of He) and temperature (263-358 K). Gas-phase IR spectra of HFO-1243zf were recorded at room temperature using a Fourier transform IR spectrometer between 500 and 4,000 cm(-1). At all temperatures, k OH did not depend on bath gas concentration (i.e., on the total pressure between 50 and 650 Torr of He). A slight but noticeable T dependence of k OH was observed in the temperature range investigated. The observed behavior is well described by the following Arrhenius expression: k OH(T) = (7.65 ± 0.26) × 10(-13) exp [(165 ± 10) / T] cm(3) molecule(-1) s(-1). Negligible IR absorption of HFO-1243zf was observed at wavenumbers greater than 1,700 cm(-1). Therefore, IR absorption cross sections, [Formula: see text], were determined in the 500-1,700 cm(-1) range. Integrated [Formula: see text] were determined between 650 and 1,800 cm(-1) for comparison purposes. The main diurnal removal pathway for HFO-1243zf is the reaction with OH radicals, which accounts for 64% of the overall loss by homogeneous reactions at 298 K. Globally, the lifetime due to OH reaction (τ OH) was estimated to be 8.7 days under the assumption of a well-mixed atmosphere. Assuming other removal pathways, the atmospheric lifetime (τ) was estimated to be ∼6 days. Considering the estimated τ OH and the measured IR absorption cross sections of HFO-1243zf in the atmospheric window (720-1,250 cm(-1)), its lifetime corrected radiative efficiency was calculated to be 0.019 W m(-2) ppbv(-1). GWP100 years for the HFO investigated, 0.29, is negligible compared to that of HFC-134a, the HFC to be potentially replaced (GWP100 years = 1,300, Hodnebrog et al. (Rev Geophys 51:300-378, 2013)). ε POCP for HFO-1243zf was estimated to be around 1 order of magnitude lower than that for ethylene. In conclusion, HFO-1243zf is fast degraded in the atmosphere, and it does not appreciably contribute to global warming and local/regional air pollution. Therefore, HFO-1243zf can be a suitable replacement for HFC-134a in air conditioning units.

Low-Voltage Field Ionization of Gases Up to Torr-Level Pressures Using Massive Arrays of Self-Aligned Gated Nanoscale Tips

Large arrays of self-aligned gated nanoscale tips (320000 tips in 32 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with average radii <;5 nm) are reported for application as gas ionizers in portable analytical instruments. The fabricated devices are capable of field ionization (FI) of gases at voltages <; 150 V even for helium with the highest ionization potential among molecules. Reliable operation of the devices was demonstrated at pressures as high as 1 Torr in He and 10 Torr in N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . FI at higher pressures was only prohibited by gas discharge rather than device failure. Ion currents as high as 0.35 nA in N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and 0.1 nA in He were generated at a tip-to-gate bias of 200 V. Continuous operation for 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s at 10 Torr confirmed excellent stability of the device for FI of gases. The device characteristics slowly degraded during continuous operation possibly due to particle adsorption on the tip surface. The device characteristics can be restored by operating the device as a field emitter at pressures <;10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> Torr.

Rate Coefficients for Vibrational Relaxation of OH(X2Π, v = 1–4) by He

The vibrational relaxation of OH(X(2)Π) by collisions with rare gases is very slow due to small molecular interactions. No measurement of the rate coefficients has been made for relaxation of relatively low vibrational levels v ≤ 4 of OH by He, and there is only one report of the upper limit for v = 2, <1 × 10(-14) cm(3) molecule(-1) s(-1). In this article, we have studied vibrational relaxation of the levels v = 1-4 of OH(X(2)Π) by collisions with He. A gaseous mixture of O3 and H2 in a carrier gas at 70-130 Torr of He was irradiated at 266 nm, and OH(X(2)Π, v ≤ 4) was generated in the reaction O((1)D) + H2. A single vibrational level of OH was detected with laser-induced fluorescence (LIF) via the A(2)Σ(+)-X(2)Π transition. Time-resolved LIF intensities of OH(v) were recorded, and kinetic analysis was made by an originally developed integrated profiles method (IPM). On the basis of the evaluation of the pressure-dependent rate coefficients of diffusion loss and the effect of impurities on the kinetics, the rate coefficients of vibrational relaxation for OH(X(2)Π, v = 1-4) by He have been determined to be (2.9 ± 1.5) × 10(-17), (1.4 ± 0.4) × 10(-16), (5.2 ± 0.5) × 10(-16), and (1.6 ± 0.2) × 10(-15) cm(3) molecule(-1) s(-1) for v = 1, 2, 3, and 4, respectively (the confidence limits are 2σ). The rate coefficients are larger at higher vibrational levels and smoothly correlate to those reported previously for v = 10-12.

Atmospheric Degradation of 4-Hydroxy 4-Methyl 2-Pentanone with OH in the Gas Phase at 297 K

The reaction of 4-hydroxy-4-methyl-2-pentanone (4H4M2P) with OH radicals was investigated in the gas-phase using an absolute rate method at room temperature and over the pressure range of 10 to 745 Torr in He and air as diluent gases. The rate coefficients were measured with the Pulsed Laser Photolysis method (PLP) to produce OH and the Laser Induced Fluorescence technique (LIF) to detect the fluorescence of OH radicals. An average value of (4.5±1.5)×10-12 cm3molecule-1s-1 was obtained. This study is the first absolute determination of the rate coefficient of 4H4M2P with OH at room temperature as a function of pressure and the nature of diluent gases. No pressure dependence of the rate coefficient was observed. The obtained results are compared with those of other hydroxyketones in terms of the structure reactivity relationship. The calculated tropospheric lifetime obtained in this work suggests that once emitted into the atmosphere, 4H4M2P may contribute to the photochemical pollution in a local or regional scale.

First Experimental Determination of the Absolute Gas-Phase Rate Coefficient for the Reaction of OH with 4-Hydroxy-2-Butanone (4H2B) at 294 K by Vapor Pressure Measurements of 4H2B

The reaction of the OH radicals with 4-hydroxy-2-butanone was investigated in the gas phase using an absolute rate method at room temperature and over the pressure range 10-330 Torr in He and air as diluent gases. The rate coefficients were measured using pulsed laser photolysis (PLP) of H(2)O(2) to produce OH and laser induced fluorescence (LIF) to measure the OH temporal profile. An average value of (4.8 ± 1.2) × 10(-12) cm(3) molecule(-1) s(-1) was obtained. The OH quantum yield following the 266 nm pulsed laser photolysis of 4-hydroxy-2-butanone was measured for the first time and found to be about 0.3%. The investigated kinetic study required accurate measurements of the vapor pressure of 4-hydroxy-2-butanone, which was measured using a static apparatus. The vapor pressure was found to range from 0.056 to 7.11 Torr between 254 and 323 K. This work provides the first absolute rate coefficients for the reaction of 4-hydroxy-2-butanone with OH and the first experimental saturated vapor pressures of the studied compound below 311 K. The obtained results are compared to those of the literature and the effects of the experimental conditions on the reactivity are examined. The calculated tropospheric lifetime obtained in this work suggests that once emitted into the atmosphere, 4H2B may contribute to the photochemical pollution in a local or regional scale.

An experimental and theoretical study on the reaction of Cl with CF3CF2CH2OH

An absolute kinetic study of the reaction of Cl atoms with CF3CF2CH2OH is reported as a function of temperature (T = 268–377 K) and at a total pressure of 100 Torr by the Pulsed Laser Photolysis - Resonance Fluorescence (PLP-RF) technique. No pressure dependence of the rate coefficient for the title reaction, k, was observed between 50 and 200 Torr of He at 298 K. The derived Arrhenius expression in that T-range was k(T) = (2.18 ± 0.24) × 10−12 exp(–(333 ± 34)/T) cm3 molecule−1 s−1, where the uncertainties are ±2σ. From these results, the average tropospheric lifetime of CF3CF2CH2OH due to the reaction with Cl was estimated to be 50 years, considering a global Cl concentration of 103 atom cm−3 and an average temperature of 272 K. Additionally, a theoretical study of the Cl + CF3CF2CH2OH reaction has been carried out by ab initio Möller–Plesset second-order perturbation treatment with 6-311G** basis set to investigate the reaction mechanism. Molecular energies of the different critical points of the potential energy surface have been calculated at QCISD(T) level. The theoretical study shows that the H-atom abstraction from the –CH2– group is the most favourable reaction pathway.

Temperature-Dependent Kinetics Study of the Reactions of OH with C2H5I, n-C3H7I, and iso-C3H7I

Flash photolysis (FP) coupled with resonance fluorescence (RF) was used to measure the absolute rate coefficients for the reactions of OH(X(2)Π) radicals with C(2)H(5)I (k(1)), n-C(3)H(7)I (k(2)), and iso-C(3)H(7)I (k(3)) at temperatures between 297 and 372 K in 188 Torr of He; this represents the first temperature-dependent kinetics studies for the title reactions. The experiments involved time-resolved RF detection of the OH (A(2)Σ(+) → X(2)Π transition at λ = 308 nm) radicals following FP of H(2)O/C(2)H(5)I/He, H(2)O/n-C(3)H(7)I/He, and H(2)O/iso-C(3)H(7)I/He mixtures. The OH(X(2)Π) radicals were produced by FP of H(2)O in vacuum-UV at wavelengths λ > 120 nm. Decays of OH radicals in the presence of C(2)H(5)I, n-C(3)H(7)I, and iso-C(3)H(7)I were observed to be exponential, and the decay rates were found to be linearly dependent on the C(2)H(5)I, n-C(3)H(7)I, and iso-C(3)H(7)I concentrations. The results are described by the following Arrhenius expressions (units of cm(3) molecule(-1) s(-1)): k(1)(297-372 K) = (5.55 ± 3.20) × 10(-12) exp[-(830 ± 90) K/T], k(2)(300-370 K) = (1.65 ± 0.90) × 10(-11) exp[-(780 ± 90) K/T] and k(3)(299-369 K) = (7.58 ± 3.70) × 10(-12) exp[-(530 ± 80) K/T]. Reported errors in E/R and in the pre-exponential factors are 2σ random errors, returned by the weighted (by 1/σ(2)) least-squares fits to the kinetic data. The implications of the reported kinetic results for understanding both atmospheric and nuclear safety interests of C(2)H(5)I, n-C(3)H(7)I, and iso-C(3)H(7)I are discussed.

State-Specific Reactions of Cu+(1S, 3D) with CH3X and CF3X (X = Cl, Br, I): Exploring the Influence of Dipole Orientation on Association and C–X Bond Activation

The reactions of gas-phase Cu(+)((1)S) and Cu(+)((3)D) with CF(3)X and CH(3)X (X = Cl, Br, and I) have been examined experimentally using the drift cell technique at 3.5 Torr in He at room temperature. State-specific product channels and overall bimolecular rate constants for depletion of the two Cu(+) states were determined using electronic state chromatography. The results showed that Cu(+)((1)S) participates exclusively in association with all of these neutrals, whereas, depending on the neutral, Cu(+)((3)D) initiates up to three bimolecular processes, resulting in the formation of CuX(+), CuC(H/F)(3)(+), and C(H/F)(3)X(+). Possible structures for the singlet association products were explored using density functional methods. These calculations indicated that Cu(+) preferentially associates with the labile halogen (Cl, Br, I) with all neutrals except CF(3)Cl, for which a "backside" geometry occurs in which Cu(+)((1)S) is weakly bound to the -CF(3) end of the molecule. All products observed on the triplet reaction surface can be understood in terms of either known or calculated thermochemical requirements. Product distributions and overall reaction efficiencies for C-X bond activation (X = Br, I) through Cu(+)((3)D) suggest that the orientation of the neutral dipole has little or no effect in controlling access to specific product channels. Likewise, second-order rate constants for reactions with X = Br and I indicate efficient depletion of Cu(+)((3)D) and do not exhibit the dramatic variations in reaction efficiency previously observed with CH(3)Cl and CF(3)Cl. These results suggest that C-X bond activation proceeds through a bond-insertion mechanism as opposed to direct abstraction.

3D hierarchical cauliflower-like carbon nanotubes/platinum–tin nanostructure and its electrocatalytic activity for ethanoloxidation

We report the synthesis of an advanced carbon paper (CP) macro-substrate (current collector)/carbon nanotubes (catalyst support)/Pt75Sn25 (nanocatalyst) architecture. The Pt75Sn25 was synthesized onto carbon nanotubes by mean of the cross-beam laser deposition (CBLD) technique. By varying the background gas pressure in the deposition chamber, smooth to porous structures Pt75Sn25 perfectly coating the carbon nanotubes were obtained. Pt75Sn25 deposited under 2 Torr of He resulted in a unique cauliflower-like nanostructure. This nanostructure when used as an electrode exhibits an enhanced electrocatalytic activity and high stability towards ethanol oxidation, a reaction of technological importance to direct ethanol fuel cells.

Kinetic study of the reaction of OH with CH3I revisited

AbstractA flash photolysis resonance fluorescence technique has been employed to investigate the kinetics and mechanism of the reaction of OH(X2Π) radicals with CH3I over the temperature and pressure ranges 295–390 K and 82–303 Torr of He, respectively. The experiments involved time‐resolved RF detection of the OH (A2Σ+ → X2Π transition at λ = 308 nm) following FP of H2O/CH3I/He mixtures. The OH(X2Π) radicals were produced by FP of H2O in the vacuum‐UV at wavelengths λ &gt; 115 nm using a commercial Perkin‐Elmer Xe flash lamp. Decays of OH in the presence of CH3I are observed to be exponential, and the decay rates are found to be linearly dependent on the CH3I concentration. The measured rate coefficients for the reaction of OH with CH3I are described by the Arrhenius expression kOH+CH3I = (4.1 ± 2.2) × 10−12 exp [(−1240 ± 200)K/T] cm3 molecule−1s−1. The implications of the reported kinetic results for understanding the CH3I chemistry of both atmospheric and nuclear industry interests are discussed. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 547–556, 2011