Torr Of Helium Research Articles

Experimental study of the reactions of limonene with OH and OD radicals: kinetics and products.

The kinetics of the reactions of limonene with OH and OD radicals has been studied using a low-pressure flow tube reactor coupled with a quadrupole mass spectrometer: OH + C10H16 → products (1), OD + C10H16 → products (2). The rate constants of the title reactions were determined using four different approaches: either monitoring the kinetics of OH (OD) radicals or limonene consumption in excess of limonene or of the radicals, respectively (absolute method), and by the relative rate method using either the reaction OH (OD) + Br2 or OH (OD) + DMDS (dimethyl disulfide) as the reference one and following HOBr (DOBr) formation or DMDS and limonene consumption, respectively. As a result of the absolute and relative measurements, the overall rate coefficients, k1 = (3.0 ± 0.5) × 10(-11) exp((515 ± 50)/T) and k2 = (2.5 ± 0.6) × 10(-11) exp((575 ± 60)/T) cm(3) molecule(-1) s(-1), were determined at a pressure of 1 Torr of helium over the temperature ranges 220-360 and 233-353 K, respectively. k1 was found to be pressure independent over the range 0.5-5 Torr. There are two possible pathways for the reaction between OH (OD) and limonene: addition of the radical to one of the limonene double bonds (reactions 1a and 2a ) and abstraction of a hydrogen atom (reactions 1b and 2b ), resulting in the formation of H2O (HOD). Measurements of the HOD yield as a function of temperature led to the following branching ratio of the H atom abstraction channel: k2b/k2 = (0.07 ± 0.03) × exp((460 ± 140)/T) for T = (253-355) K.

Direct Measurement of the Equilibrium Constants of the Reaction of Formaldehyde and Acetaldehyde with HO2 Radicals

ABSTRACTRate and equilibrium constants for the reaction of HO2 with formaldehyde urn:x-wiley:05388066:kin20817:equation:kin20817-math-0001 and acetaldehyde urn:x-wiley:05388066:kin20817:equation:kin20817-math-0002 have been directly measured. The concentration of HO2 radicals was followed in a time‐resolved method by coupling cw‐CRDS (cavity ringdown spectroscopy) to laser photolysis. The reaction of HO2 with CH2O (R1) was measured at 50 Torr helium over the temperature range 292–306 K, whereas the reaction of CH3CHO with HO2 (R2) was measured in 50 Torr He but at only 294 K. The observed HO2 decay profiles were modeled to take into account secondary chemistry, especially that of the reaction products, hydroxyl‐peroxy radical adducts. The rate constants for forward and back reactions of (R1) at 297 K were found to be k1 = (3.3 ± 0.6) × 10−14 cm3 molecule−1 s−1 and k−1 = (55 ± 5) s−1, respectively, both roughly a factor of two slower than earlier measurements (possibly due to falloff effects), while the equilibrium constant was found to be K1 = (6.0 ± 1.8) × 10−16 molecule−1 cm3 at 297 K, in good agreement with earlier, more indirect determinations. The equilibrium constant of the reaction with CH3CHO was found to be K2 = (1.7 ± 0.5) × 10−17 molecule−1 cm3 at 294 K, with the forward rate constant k2 = (1.5 ± 0.75) × 10−14 cm3 molecule−1 s−1 and the rate constant for the back reaction k−2 = (900 ± 450) s−1.

Kinetics of Several Oxygenated Carbon-Centered Free Radical Reactions with NO2

Five oxygenated carbon-centered free radical reactions with nitrogen dioxide (NO2) have been studied in direct time-resolved measurements. Experiments were conducted in a temperature-controlled flow tube reactor coupled to a 193 nm exciplex laser photolysis and a resonance gas lamp photoionization mass spectrometer. Reactions were investigated under pseudofirst-order conditions, with the NO2 concentrations of the experiments in great excess over the initial radical concentrations ([R]0 << [NO2]). The study consists of the three isomeric C2H5O radicals (CH3CHOH, CH2CH2OH, and CH3OCH2), and the CH2OH and CH3CO radical reactions with NO2 and, hence, includes the three smallest hydroxyalkyl radical species (CH2OH, CH2CH2OH, and CH3CHOH). The obtained rate coefficients are high with the temperature-dependent rate coefficients given by a formula k(T) = k300K × (T/300 K)(-n) as (in units of cm(3) molecule(-1) s(-1)): k(CH2OH + NO2) = (8.95 ± 2.70) × 10(-11) × (T/300 K)(-0.54±0.27) (T = 298-363 K), k(CH2CH2OH + NO2) = (5.99 ± 1.80) × 10(-11) × (T/300 K)(-1.49±0.45)(T = 241-363 K), k(CH3CHOH + NO2) = (7.48 ± 2.24) × 10(-11) × (T/300 K)(-1.36±0.41) (T = 266-363 K), k(CH3OCH2 + NO2) = (7.85 ± 2.36) × 10(-11) × (T/300 K)(-0.93±0.28) (T = 243-363 K), and k(CH3CO + NO2) = (2.87 ± 0.57) × 10(-11) × (T/300 K)(-2.45±0.49) (T = 241-363 K), where the uncertainties refer to the estimated overall uncertainties of the values obtained. The determined rate coefficients show negative temperature dependence with no apparent bath gas pressure dependence under the current experimental conditions (241-363 K and about 1-3 Torr helium). This behavior is typical for a radical-radical addition mechanism with no potential energy barrier above the energy of the separated reactants in the entrance channel of the reaction. Unfortunately the absence of detected product signals prevented gaining deeper insight into the reaction mechanism.

An 80-W Laser Diode Array with 0.1 nm Linewidth for Rubidium Vapor Laser Pumping

The spectral linewidth of a 64-emitter laser-diode array is effectively suppressed by using a volume Bragg grating (VBG) based external cavity. At a maximal driven current of 90 A, the device produces a cw output of 80 W with 1.2 W/A slope efficiency and 0.1 nm spectral linewidth (FWHM) centered at 780nm. The power extraction efficiency reaches 90% as compared with the free running case. The central wavelength of the narrowed spectrum is tuned over a 0.3 nm range by adjusting the VBG's temperature. The absorption of 45% laser radiation by a 5-mm-long rubidium vapor cell with 150 Torr ethane and 450 Torr helium at 383 K is demonstrated.

ChemInform Abstract: Chloryl Nitrate: A Novel Product of the OClO + NO3 + M Recombination.

The products of the reaction of OClO with NO_3 were investigated between 220 and 298 K using a flow reactor and infrared, visible, and ultraviolet analysis. At temperatures below 250 K new infrared and ultraviolet absorption features were observed and assigned to the novel compound chloryl nitrate (O_2ClONO_2). Additionally, ClO and NO_2 were observed as reaction products, indicating the existence of a second reaction channel. O_2ClONO_2 formation predominates at temperatures below 230 K. The reaction rate constant at 220 K is estimated to be on the order of 10^(-14) cm^3 molecule^(-1) s^(-1) in 1-5 Torr of helium. These observations suggest that O_2ClONO_2 may exist in the terrestrial stratosphere.

ChemInform Abstract: Effect of Catalyst Structure and Carbon Deposition on Heptane Oxidation Over Supported Platinum and Palladium

A series of supported platinum and palladium catalysts were tested for heptane oxidation at 75 to 200°C, 20 Torr heptane, 245 Torr oxygen, 795 Torr helium, and conversions below 1%. At these low temperatures, carbon fouls the metal surface. The decline in the turnover frequency with time is accurately fitted with this function: TOFobs = TOFAexp(−kAt) + TOFbexp(−kBt). The turnover frequencies of the A and B sites depend on catalyst structure. At 140°C, the activities of the A sites are: 0.02 s−1 for 50 A platinum crystallites, 0.002 s−1 for 50 A palladium crystallites. The activity of the B sites is 10 times higher on platinum compared to palladium, and is 4 times higher on large crystallites compared to small ones. The amount of carbon on the catalyst depends on the half power of reaction time. These and other results indicate that carbon formed on the metal diffuses slowly to the support. The coke formation and the deactivation rate are much lower on 50 A metal crystallites. On large metal crystallites, the coke formation rate per exposed metal atom and the deactivation rate depend on the number of metal particles on the support and the support composition. Decreasing the number of particles sixfold increases the carbon deposition rate by 2.5 times, and decreases the deactivation rate by 5.3 times. Switching from zirconia (40 m2/g) to alumina (83 m2/g) at 5.0 wt% platinum increases the carbon deposition rate 5 times, and decreases the deactivation rate 5.5 times. Evidently, the faster the carbon migrates from the metal to the support, the slower the carbon fouls the metal surface.

ChemInform Abstract: Reactions of C60×+, C602+, and C60×3+ with C2H2 and C2H4 in the Gas Phase: Polymerization Initiated by C60×3+.

Abstract Reactions of the C60 cations C60•+, C602+ and C60•3+ have been investigated with C2H2 and C2H4 in the gas phase using the Selected-Ion Flow Tube (SIFT) technique at 295±2 K and 0.35±0.02 Torr of helium. Although C60•+ was found to be unreactive (k

Quantitative Deuterium Analysis of Titanium Samples in Ultraviolet Laser-Induced Low-Pressure Helium Plasma

An experimental study of ultraviolet (UV) laser-induced plasma spectroscopy (LIPS) on Ti samples with low-pressure surrounding He gas has been carried out to demonstrate its applicability to quantitative micro-analysis of deuterium impurities in titanium without the spectral interference from the ubiquitous surface water. This was achieved by adopting the optimal experimental condition ascertained in this study, which is specified by 5 mJ laser energy, 10 Torr helium pressure, and 1-50 mus measurement window, which resulted in consistent D emission enhancement and effective elimination of spectral interference from surface water. As a result, a linear calibration line exhibiting a zero intercept was obtained from Ti samples doped with various D impurity concentrations. An additional measurement also yielded a detection limit of about 40 ppm for D impurity, well below the acceptable threshold of damaging H concentration in Ti and its alloys. Each of these measurements was found to produce a crater size of only 25 mum in diameter, and they may therefore qualify as nondestructive measurements. The result of this study has therefore paved the way for conducting further experiments with hydrogen-doped Ti samples and the technical implementation of quantitative micro-analysis of detrimental hydrogen impurity in Ti metal and its alloys, which is the ultimate goal of this study.

Generation of Micro Inductively Coupled Plasma on a Chip

Inductively coupled plasma (ICP) sources are of great importance for a wide range of applications, including but not limited to micro and nanofabrication, chemical analysis, and plasma processing. Portable and battery-operated instruments can be developed using a miniaturized micro ICP chip considering the low-power and low-voltage requirements of micro ICPs. To the best of authors' knowledge, this is the first report for generation of a micro ICP on a chip with size of nearly 1 mm using a planar coil with diameter of 800 mum. This chip is capable of producing stable ICPs at the power of 1 W, frequency of 900 MHz, and pressure of 1 torr of helium.

Temperature Dependence of the HO2 + ClO Reaction. 2. Reaction Kinetics Using the Discharge-Flow Resonance-Fluorescence Technique

The total rate coefficient, k3, for the reaction HO2 + ClO --> products has been determined over the temperature range of 220-336 K at a total pressure of approximately 1.5 Torr of helium using the discharge-flow resonance-fluorescence technique. Pseudo-first-order conditions were used with both ClO and HO2 as excess reagents using four different combinations of precursor molecules. HO2 molecules were formed by using either the termolecular association of H atoms in an excess of O2 or via the reaction of F atoms with an excess of H(2)O(2). ClO molecules were formed by using the reaction of Cl atoms with an excess of O3 or via the reaction of Cl atoms with Cl(2)O. Neither HO2 nor ClO were directly observed during the course of the experiments, but these species were converted to OH or Cl radicals, respectively, via reaction with NO prior to their observation. OH fluorescence was observed at 308 nm, whereas Cl fluorescence was observed at approximately 138 nm. Numerical simulations show that under the experimental conditions used secondary reactions did not interfere with the measurements; however, some HO2 was lost on conversion to OH for experiments in excess HO2. These results were corrected to compensate for the simulated loss. At 296 K, the rate coefficient was determined to be (6.4 +/- 1.6) x 10(-12) cm3 molecule(-1) s(-1). The temperature dependence expressed in Arrhenius form is (1.75 +/- 0.52) x 10-12 exp[(368 +/- 78)/T] cm3 molecule(-1) s(-1). The Arrhenius expression is derived from a fit weighted by the reciprocal of the measurement errors of the individual data points. The uncertainties are cited at the level of two standard deviations and contain contributions from statistical errors from the data analysis in addition to estimates of the systematic experimental errors and possible errors from the applied model correction.

Daytime tropospheric loss of hexanal and &amp;lt;I&amp;gt;trans&amp;lt;/I&amp;gt;-2-hexenal: OH kinetics and UV photolysis

Abstract. The ultraviolet (λ=250–370 nm) photolysis and the OH-initiated oxidation of hexanal and trans-2-hexenal, which are relevant atmospheric processes, have been investigated at room temperature and as a function of temperature (T=263–353 K), respectively. This kinetic study as a function of temperature is reported here for the first time. Absolute absorption cross sections (σλ) were obtained using a recently built system operating in the UV region. The obtained σλ allowed the estimation of the photolysis rates (J) across the troposphere. Kinetic measurements of the gas-phase reaction of hydroxyl radicals (OH) with hexanal and trans-2-hexenal were performed by using the laser pulsed photolysis/laser-induced fluorescence technique. Rate coefficients kOH for both aldehydes were determined at temperatures between 263 and 353 K at 50 Torr in helium or argon bath gases. The temperature dependence of kOH for both aldehydes was found to be slightly negative. The tropospheric lifetime of hexanal and trans-2-hexenal due to the chemical removal by OH radicals has been estimated across the troposphere. The loss rate due to the OH chemical removal was compared with the estimated photolysis rates. Our results show that OH-reaction is the main loss process for these aldehydes in the troposphere, although photolysis is not negligible for hexanal.

Measured laser-beam evolution during high-order harmonic generation in a semi-infinite gas cell

We report on direct measurements of self-guiding of 800 nm, 30 fs, 5 mJ laser pulses used to generate high-order harmonics in 80 torr helium. We track the spatial evolution of the laser pulses as they propagate several centimeters near the focus under conditions suitable for harmonic generation. The laser is observed to focus, diverge, and refocus. This behavior is accompanied by a flattop beam profile. Both of these features are absent when the laser is focused in vacuum. We also observed a 4 nm spectral blue shift in the center of the laser beam near the focus in contrast with no spectral shift at wider radii.

Direct Measurement and Theoretical Calculation of the Rate Coefficient for Cl + CH3 in the Range from T = 202−298 K

The rate coefficient has been measured under pseudo-first-order conditions for the Cl+CH3 association reaction at T=202, 250, and 298 K and P=0.3-2.0 Torr helium using the technique of discharge-flow mass spectrometry with low-energy (12-eV) electron-impact ionization and collision-free sampling. Cl and CH3 were generated rapidly and simultaneously by reaction of F with HCl and CH4, respectively. Fluorine atoms were produced by microwave discharge in an approximately 1% mixture of F2 in He. The decay of CH3 was monitored under pseudo-first-order conditions with the Cl-atom concentration in large excess over the CH3 concentration ([Cl]0/[CH3]0=9-67). Small corrections were made for both axial and radial diffusion and minor secondary chemistry. The rate coefficient was found to be in the falloff regime over the range of pressures studied. For example, at T=202 K, the rate coefficient increases from 8.4x10(-12) at P=0.30 Torr He to 1.8x10(-11) at P=2.00 Torr He, both in units of cm3 molecule-1 s-1. A combination of ab initio quantum chemistry, variational transition-state theory, and master-equation simulations was employed in developing a theoretical model for the temperature and pressure dependence of the rate coefficient. Reasonable empirical representations of energy transfer and of the effect of spin-orbit interactions yield a temperature- and pressure-dependent rate coefficient that is in excellent agreement with the present experimental results. The high-pressure limiting rate coefficient from the RRKM calculations is k2=6.0x10(-11) cm3 molecule-1 s-1, independent of temperature in the range from 200 to 300 K.

Kinetic and mechanistic study of the reactions of OH with IBr and HOI

The kinetics and mechanism of the reactions of OH radicals with IBr and HOI have been studied using the mass spectrometric discharge-flow method at 320 K and at a total pressure of 1 Torr of Helium. The rate constant of the reaction OH + IBr → products (1) was measured under pseudo-first order conditions either in excess of IBr or in excess of OH radicals: k 1 = (1.4 ± 0.4) × 10 −10 cm 3 molecule −1 s −1. Both HOI and HOBr were detected as products of reaction (1) and the branching ratios 0.84 ± 0.07 and 0.14 ± 0.05, respectively, were found for the channels forming these species. For the reaction OH + HOI → products (2) the total rate constant was determined from the kinetics of HOI consumption in excess of OH radicals: k 2 = (5.0 ± 1.2) × 10 −12 cm 3 molecule −1 s −1.

BIOLOGICALLY PROGRAMMED SILICON NANOPARTICLES ASSEMBLY

Silicon nanoparticles have been assembled to a large extent ≈ 600 nm using a DNA network template. Nanoparticles have been synthesized by thermal vapor deposition technique in 200 torr helium ambient. Heating effect of DNA mixed silicon nanoparticles has been observed and it was found that heating of the solution made the alignment to a very large extent, i.e., of several micrometers.

A Kinetic and Product Study of the Cl + HO2 Reaction

Absolute rate data and product branching ratios for the reactions Cl + HO2 --> HCl + O2 (k1a) and Cl + HO2 --> OH + ClO (k1b) have been measured from 226 to 336 K at a total pressure of 1 Torr of helium using the discharge flow resonance fluorescence technique coupled with infrared diode laser spectroscopy. For kinetic measurements, pseudo-first-order conditions were used with both reagents in excess in separate experiments. HO2 was produced by two methods: through the termolecular reaction of H atoms with O2 and also by the reaction of F atoms with H2O2. Cl atoms were produced by a microwave discharge of Cl2 in He. HO2 radicals were converted to OH radicals prior to detection by resonance fluorescence at 308 nm. Cl atoms were detected directly at 138 nm also by resonance fluorescence. Measurement of the consumption of HO2 in excess Cl yielded k1a and measurement of the consumption of Cl in excess HO2 yielded the total rate coefficient, k1. Values of k1a and k1 derived from kinetic experiments expressed in Arrhenius form are (1.6 +/- 0.2) x 10(-11) exp[(249 +/- 34)/T] and (2.8 +/- 0.1) x 10(-11) exp[(123 +/- 15)/T] cm3 molecule(-1) s(-1), respectively. As the expression for k1 is only weakly temperature dependent, we report a temperature-independent value of k1 = (4.5 +/- 0.4) x 10(-11) cm3 molecule(-1) s(-1). Additionally, an Arrhenius expression for k1b can also be derived: k1b = (7.7 +/- 0.8) x 10(-11) exp[-(708 +/- 29)/T] cm3 molecule(-1) s(-1). These expressions for k1a and k1b are valid for 226 K < or = T < or = 336 and 256 K < or = T < or = 296 K, respectively. The cited errors are at the level of a single standard deviation. For the product measurements, an excess of Cl was added to known concentrations of HO2 and the reaction was allowed to reach completion. HCl product concentrations were determined by IR absorption yielding the ratio k1a/k1 over the temperature range 236 K < or = T < or = 296 K. OH product concentrations were determined by resonance fluorescence giving rise to the ratio k1b/k1 over the temperature range 226 K < or = T < or = 336 K. Both of these ratios were subsequently converted to absolute numbers. Values of k1a and k1b from the product experiments expressed in Arrhenius form are (1.5 +/- 0.1) x 10(-11) exp[(222 +/- 17)/T] and (10.6 +/- 1.5) x 10(-11) exp[-(733 +/- 41)/T] cm3 molecule(-1) s(-1), respectively. These expressions for k1a and k1b are valid for 256 K < or = T < or = 296 and 226 K < or = T < or = 336 K, respectively. A combination of the kinetic and product data results in the following Arrhenius expressions for k1a and k1b of (1.4 +/- 0.3) x 10(-11) exp[(269 +/- 58)/T] and (12.7 +/- 4.1) x 10(-11) exp[-(801 +/- 94)/T] cm3 molecule(-1) s(-1), respectively. Numerical simulations were used to check for interferences from secondary chemistry in both the kinetic and product experiments and also to quantify the losses incurred during the conversion process HO2 --> OH for detection purposes.

On the mechanism of formation of helium molecules in a low-temperature plasma

The glow of He2 molecules in a low-pressure (P ≤ 2 Torr) helium plasma excited by a monochromatic electron beam is detected and studied. The experimental data point to the existence of two basic mechanisms of formation of an excited He2 molecule, namely, the collision of an excited helium atom with two unexcited atoms and the recombination of a molecular ion.

Experimental study of the interaction of HO2radicals with soot surface

The reaction of HO2 with toluene and kerosene flame soot was studied over the temperature range 240-350 K and at P = 0.5-5 Torr of helium using a discharge flow reactor coupled to a modulated molecular beam mass spectrometer. A flat-flame burner was used for the preparation and deposition of soot samples from premixed flames of liquid fuels under well controlled and adjustable combustion conditions. The independent of temperature in the range 240-350 K value of gamma = (7.5 +/- 1.5) x 10(-2) (calculated with geometric surface area) was found for the uptake coefficient of HO2 on kerosene and toluene soot. No significant deactivation of soot surface during its reaction with HO2 was observed. Experiments on soot ageing under ambient conditions showed that the reactivity of aged soot is similar to that of freshly prepared soot samples. The results show that the HO2 + soot reaction could be a significant loss process for HOx in the urban atmosphere with a potential impact on photochemical ozone formation. In contrast this process will be negligible in the upper troposphere even in flight corridors.

Ion Mobility Spectrometer with Radial Collisional Focusing

An ion mobility spectrometer that has its mobility cell as a 20-segment quadrupole and functionally the q2 of a triple-quadrupole mass spectrometer has been assembled and tested. The combination of high cell pressure (maximum of 4 Torr of helium) and low axial field (20-160 V per 20.2 cm) results in negligible internal excitation of the ions despite applications of rf and axial fields. The presence of collisional focusing ensures efficient ion transmission and good sensitivity. Collision cross sections of atomic, cluster, peptide, and protein ions were measured and found comparable to literature and calculated cross sections.

A selected ion flow tube study of the reactions of OH− with a number of fully and partially halogenated methanes

In this paper we report the reactions of OH− with the halogenated methanes CCl4, CCl3F, CCl2F2, CClF3, CF4, CHCl3, CHCl2F, CHClF2, CHF3, CH2Cl2, CH2ClF, CH2F2, CH3Cl, and CH3F in a 0.5Torr helium buffer gas at 300K using a selected ion flow tube. Reaction rate coefficients were measured and branching ratios determined for the large number of reactions studied. The results are discussed in terms of the thermochemistry of the reactions and barriers that exist to different reaction pathways. It is demonstrated that there is no barrier to proton and Cl+ abstraction, while significant barriers exist for nucleophilic substitution.