Torr Of Helium Research Articles

Direct studies on the decomposition of the tert-butoxy radical and its reaction with NO

The first laser induced fluorescence (LIF) spectrum for the tert-butoxy radical is reported following radical generation by excimer laser photolysis of tert-butyl nitrite. The laser flash photolysis-LIF technique is used to measure the temperature dependence of the rate coefficient for the reaction with NO (T=200–390 K): which can be represented in either Arrhenius or AT-n format: and the tert-butoxy decomposition. Whilst the former reaction shows no pressure dependence (p=70–500 Torr of helium), the latter reaction is in the fall-off regime over the entire range of experimental conditions (T=303–393 K, p=13–600 Torr helium). The fall-off data were fitted using an inverse Laplace transformation/master equation model to give the following Arrhenius parameters: These Arrhenius parameters are significantly lower than previous indirect measurements or calculations and the atmospheric and mechanistic implications are discussed.Finally, reaction (3) was also studied by monitoring the temporal dependence of the NO LIF signal following the photolysis of tert-butylnitrite with no additional NO. The results are in good agreement with the tert-butoxy monitoring and allow for an estimation of the rate parameters for the tert-butoxy self reaction.

Environmental effect on the electron emission from diamond surfaces

Electron emission from silicon field emitters with a thin coating of diamond were studied during exposure to varying pressures of Ne, He, H2, and D2. Introduction of Ne and He at pressures >10−4 Torr suppressed the emission current. Conditioning of field emitters in a 10−5 Torr helium ambient improved the emissivity. After hydrogen and deuterium exposure, a continuous emission current was measured below the initial threshold voltages for electron emission. The effects of deuterium were significantly greater than for hydrogen. We believe this phenomenon is due to the formation of a surface dipole layer of hydrogen or deuterium, bonded to the surface carbon atoms, which lowers the electron affinity of the diamond surface.

The gas-phase condensation method for the preparation of quantum-sized ZnS nanoparticles

Nanocrystalline zinc sulfide particles have been prepared by the gas-phase condensation method. By this method the material is evaporated in the presence of a few Torr of Helium; so that, by collisions with the inert gas atoms, the evaporated atoms lose kinetic energy and condense in the form of ultrafine particles of a few nanometers in diameter. The obtained material has been characterized by XRD (X-Ray Diffraction), TEM (Transmission Electron Microscopy) and UV–Vis absorption spectroscopy. The ZnS ultrafine powder shows an homogeneous particle size distribution with a mean crystallite diameter of 8 nm. The quantum size effect on the band-gap absorption energy, as measured by UV–Vis absorption spectroscopy, has been correlated with the particle size of the nanostructured material.

Low-Pressure Study of the Reaction of Cl Atoms with Isoprene

The kinetics of the reaction of chlorine atoms with isoprene (1) has been studied using the discharge-flow mass-spectrometric method: Cl + C5H8 (+M) → C5H8Cl (+M) (1a); Cl + C5H8 → C5H7 + HCl (1b). As a result of direct and relative measurements, the overall rate constant k1 = (6.7 ± 2.0) × 10-11 exp{(485 ± 85)/T} cm3 molecule-1 s-1 was obtained at a pressure of 1 Torr of helium over the temperature range 233−320 K. k1 was found to be pressure-independent (within 10%) at T = 298 K in the pressure range 0.25−3 Torr. Both HCl and C5H8Cl adduct were detected as products of reaction 1. The measurements of the HCl yield as a function of temperature led to the branching fraction for the H-atom abstraction channel (1b): k1b/k1 = (1.22 ± 0.4) exp{−(595 ± 90)/T} (P = 1 Torr, T = 233−320 K); k1b/k1 = 0.169 ± 0.022 at 298 K. The results are compared with those reported in a recent study4 carried out at 298 K. In addition, the rate coefficient has been measured for the reaction Cl + Br2 → BrCl + Br (2), which was u...

Kinetic study of the reactions Br + IBr→I + Br2 and I + Br2→Br + IBr

The kinetics of the title reactions have been studied using the discharge-flow mass spectrometic method at 296 K and 1 torr of helium. The rate constant obtained for the forward reaction Br+IBr→I+Br2 (1), using three different experimental approaches (kinetics of Br consumption in excess of IBr, IBr consumption in excess of Br, and I formation), is: k1=(2.7±0.4)×10−11 cm3 molecule−1s−1. The rate constant of the reverse reaction: I+Br2→Br+IBr (−1) has been obtained from the Br2 consumption rate (with an excess of I atoms) and the IBr formation rate: k−1=(1.65±0.2)×10−13 cm3molecule−1s−1. The equilibrium constant for the reactions (1,−1), resulting from these direct determinations of k1 and k−1 and, also, from the measurements of the equilibrium concentrations of Br, IBr, I, and Br2, is: K1=k1/k−1=161.2±19.7. These data have been used to determine the enthalpy of reaction (1), ΔH298°=−(3.6±0.1) kcal mol−1 and the heat of formation of the IBr molecule, ΔHf,298°(IBr)=(9.8±0.1) kcal mol−1. © 1998 John Wiley & sons, Inc. Int J Chem Kinet 30: 933–940, 1998

Kinetic study of the reactions Br + IBr?I + Br2 and I + Br2?Br + IBr

The kinetics of the title reactions have been studied using the discharge-flow mass spectrometic method at 296 K and 1 torr of helium. The rate constant obtained for the forward reaction Br+IBrI+Br2 (1), using three different experimental approaches (kinetics of Br consumption in excess of IBr, IBr consumption in excess of Br, and I formation), is: k1=(2.7±0.4)×10−11 cm3 molecule−1s−1. The rate constant of the reverse reaction: I+Br2Br+IBr (−1) has been obtained from the Br2 consumption rate (with an excess of I atoms) and the IBr formation rate: k−1=(1.65±0.2)×10−13 cm3molecule−1s−1. The equilibrium constant for the reactions (1,−1), resulting from these direct determinations of k1 and k−1 and, also, from the measurements of the equilibrium concentrations of Br, IBr, I, and Br2, is: K1=k1/k−1=161.2±19.7. These data have been used to determine the enthalpy of reaction (1), ΔH298°=−(3.6±0.1) kcal mol−1 and the heat of formation of the IBr molecule, ΔHf,298°(IBr)=(9.8±0.1) kcal mol−1. © 1998 John Wiley & sons, Inc. Int J Chem Kinet 30: 933–940, 1998

Temperature and Isotope Dependence of the Reaction of Methyl Radicals with Deuterium Atoms

The reactions of methyl isotopomers (CH3, CH2D, and CHD2) with excess deuterium atoms have been studied using discharge flow/mass spectrometry at 298 K and at pressures of ∼1 Torr (helium). At these low pressures the initially formed methane complex is not stabilized. However, zero-point energy differences between methyl isotopomers mean that ejection of H from energized methane is favored. In consequence, regeneration of the reactant isotopomer is inefficient and values of k1a-c may be extracted from the appropriate methyl radical decay. The experimental values can be used to calculate the high-pressure values for each isotopic reaction: (1a) CH3 + D → CH2D + H, = (2.3 ± 0.6) × 10-10 cm3 molecule-1 s-1; (1b) CH2D + D → CHD2 + H, = (2.1 ± 0.5) × 10-10 cm3 molecule-1 s-1; (1c) CHD2 + D → CD3 + H, = (1.9 ± 0.5) × 10-10 cm3 molecule-1 s-1. These, in turn, can be corrected for isotopic substitution and averaged to give a value of (2.9 ± 0.7) × 10-10 cm3 molecule-1 s-1 for the limiting high-pressure recombina...

Selected ion flow tube study of the reactions of O − and O 2− with CHC1 2F, CHC1F 2, CHF 3, CH 2C1F, CH 2F 2, CH 3F, CHF 2CHF 2, CH 2FCF 3, and CH 3CHF 2

In this paper we report the reactions of O − and O 2 − with several saturated hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) in a 0.5 Torr helium buffer gas at about 300 K using a selected ion flow tube. The reaction rate coefficients and branching ratios determined for the large number of reactions studied are presented. O − is found to react with all the molecules studied, and several distinct reaction processes are observed. These include nucleophilic attack on chlorine, nucleophilic attack on carbon, and hydrogen, proton, and H 2 + abstraction. Nucleophilic attack on carbon is found to dominate the O 2 − reactions with the HCFCs. Bimolecular reactions of O 2 − with the HFCs do not occur.

Gas-Phase DNA: Oligothymidine Ion Conformers

Gas-Phase DNA: Oligothymidine Ion Conformers

Kinetic study of the pressure dependence of the reaction CF3O+NO2at 298 K Rate constant measurements (P=0.5–9Torr) and competition between association (CF3ONO2)and disproportionation (CF2O+FNO2)channels

The kinetics of the reaction of the CF3O radical with NO2 has been studied at 298 K at low pressure (0.5–9 Torr of helium) by fast-flow–laser-induced fluorescence (LIF), pyrolysis of a dilute mixture of CF3OOCF3 and He being used as the source of the CF3O radical. In good agreement with the results of Zellner and co-workers (obtained in the pressure range 5–100 Torr), the rate constant has been found to be pressure dependent. All the experimental data were analysed by a multichannel RRKM procedure using the results of abinitio calculations as input data. This revealed that two reaction channels, the association CF3O + NO2 → CF3ONO2 (1a) and the disproportionation CF3O + NO2 → CF2O + FNO2 (1b), must be invoked and they proceed via a common energized adduct CF3ONO2*. The pressure dependence of the branching ratio was predicted by this calculation: the disproportionation channel would be negligible near atmospheric pressure and becomes the major channel at pressure below ca. 0.3 Torr, with a calculated second-order limiting low-pressure value of the rate equal to 3.2 × 10−12 cm3 molecule−1 s−1. The high-pressure limit rate constant obtained is k∞ = (1.65 ± 0.2) × 10−11 cm3 molecule−1 s−1. An analytical representation of the pressure dependence of the rate constant at 298 K is proposed using the conventional Troe expression with an added constant.

Critical behaviour of super-heated (1900 - 2000 K) vapours

We report on the experimental observation of a critical phenomenon (phase-transition-like) in super-heated vapours inside an alumina effusive oven in the temperature range of 1900 - 2000 K. This phenomenon is studied by modulated molecular beam mass spectrometry and the main observations are dramatic discontinuous decay of the flux by a factor of within and a simultaneous dramatic rise of a signal. Nearly complete thermal reversibility is demonstrated. Under flow conditions with 2 - 10 Torr helium, and are also observed. Both exhibit the same critical behaviour as . Tentative interpretations are in terms of heterogeneous gas - surface critical kinetic phenomena or vapour condensation leading to formation of liquid-like films followed by complete disintegration and oxidation.

Selected ion flow tube studies of the gas-phase reactions of O ·−, O 2·− and OH − with a variety of brominated compounds

The reactions of fourteen bromine-containing molecules, CH 3Br, CH 2Br 2, CHBrCl 2, CHBr 2Cl, C 2H 5Br, CH 2BrCH 2Br, CH 2BrCH 2Cl, CF 3Br, CF 2Br 2, CFBr 3, CBrClF 2, CFBr 2Cl, CCl 3Br and CF 2BrCF 2Br, with the anions O ·−, O 2 ·− and OH − in a 0.5 Torr helium buffer gas have been studied at 300 K using a selected ion flow tube. Reaction rate coefficients and branching ratios are presented. Several distinct reaction processes are observed among the large number of reactions studied, including nucleophilic attack, proton transfer, abstraction, and dissociative and non-dissociative electron transfer (within or out of a reaction complex). Of the forty-two bimolecular reactions, the rates of only two are significantly below the collisional rate and, for these two, competitive three-body processes are observed. Many of the reactions reported here have not previously been studied. This study, therefore, presents new data which provide additional insight into the reactions of anions with halogenated species.

Reactions of C 60•+, C 602+ and C 60•3+ with C 2H 2 and C 2H 4 in the gas phase: Polymerization initiated by C 60•3+

Reactions of the C 60 cations C 60 •+, C 60 2+ and C 60 •3+ have been investigated with C 2H 2 and C 2H 4 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 C 60 •+ was found to be unreactive (k < 1×10 −12 cm 3 molecule −1 s −1), and C 60 2+ reacted only slowly (k ≤ 2×10 −12 cm 3 molecule −1 s −1) by adduct formation, C 60 •3+ reacted rapidly with C 2H 2 by adduct formation (k = 6.6×10 −11 cm 3 molecule −1 s −1) and with C 2H 4 by adduct formation (30%) and electron transfer (70%) (k = 1.7×10 −9 cm 3 molecule −1 s −1). C 58 •3+, which was present as an impurity ion, was observed to react 10 times faster than C 60 •3+ with C 2H 2, k = 6.0×10 −10 cm 3 molecule −1 s −1 and this was attributed to a greater sp 3 character of the reacting C site. Higher-order addition reactions were observed with C 60 •3+ and C 58 •3+ and these were interpreted as the first example of cation-induced “ball-and-chain” polymerization by triply-charged fullerene cations. Mechanisms have been proposed for this polymerization without and with an intramolecular hydride shift in the derivatized fullerene cation. Also, C 60C 2H 4 •3+ was observed to undergo proton transfer with C 2H 4 and this is the first example of such a reaction with a triply-charged C 60 derivative.

Collision-induced dissociation evidence for charge separation and “ball-and-chain” propagation in the addition of 1-butene to C 602+

The collision-induced dissociation of the adduct ions C60(C4H8) 2 (2+) and C60(C4H8) 3 (2+) formed by sequential reactions of C 60 (2+) with 1-butene has been investigated by using a selected-ion flow tube (SIFT) apparatus. Experiments at 295 ± 2 K in 0.35 ± 0.02 torr of helium indicated that C 60 (2+) adds at least five molecules of 1-butene in a sequential fashion with rates that decrease with the number of molecules added. Collision-induced dissociation experiments in which the downstream sampling nose cone of the SIFT was biased with respect to the flow tube indicated that the adduct ions C60(C4H8) 2 (2+) and C60(C4H8) 3 (2+) dissociate into C 60 (·+) and (C4H8) 2 (·+) and (C4H8) 3 (·+) , respectively. These observations provide evidence for the occurrence of charge separation in the derivatization of C60 dications and support the "ball-and-chain" mechanism first proposed by Wang et al. in 1992 for the sequential multiple addition of 1,3-butadiene to C 60 (2+) and C 70 (2+) .

Use of infrared multiphoton photodissociation with SWIFT for electrospray ionization and laser desorption applications in a quadrupole ion trap mass spectrometer.

Infrared multiphoton photodissociation (IRMPD) is combined with stored wave form inverse Fourier transforms (SWIFT) to effect dissociation and ion ejection in a quadrupole ion trap mass spectrometer. The application of IRMPD to the structural characterization of biochemical ions generated by chemical ionization and electrospray ionization and the feasibility of utilizing infrared photons for the activation of laser-desorbed metal ion-crown ether complexes was examined. The effect of helium pressure on the dissociation efficiency and relative dissociation rate constants for systems with well-known thermochemistry was evaluated. The helium pressure is not detrimental to the IRMPD experiment when nominal pressures lower than 2 x 10(-5) Torr are used. At pressures close to nominally 8 x 10(-5) Torr of helium, collisonal deactivation dominates. Results show conventional CAD is a more selective dissociation technique; however, the amount of fragment ion information generated depends highly on the qz value. IRMPD, on the other hand, is independent of the value of qz such that low rf storage values can be utilized during the irradiation period. Thus, under these conditions, informative lower mass fragment ions are trapped and detected. A larger number of structurally informative fragments is generated upon irradiation with infrared photons relative to the CAD method because of the further excitation of primary fragment ions upon photoabsorption. SWIFT wave forms are successfully utilized to determine the extent of excitation of primary fragment ions as well as prove/disprove dissociation pathways of a variety of ions such as macrolide antibiotics and hydrogen-bonded complexes.

Kinetic Study of the Reactions of C2H5O and C2H5O2 with NO3 at 298 K

A discharge flow reactor coupled to laser-induced fluorescence and mass spectrometry, for the detection of ethoxy and nitrate radicals, respectively, has been used to study the kinetics of the reactions of C2H5O and C2H5O2 with NO3 at ca. 1 Torr of helium and 298 K. From the analysis of the C2H5O concentration−time profiles in two sets of experiments, the reactions C2H5O + NO3 → C2H5O2 + NO2 (1) and C2H5O2 + NO3 → C2H5O + NO2 + O2 (2) were observed to be strongly coupled. The rate constants were derived from modeling calculations, k1 = (3.3 ± 0.9) × 10-12 cm3 molecule-1 s-1 and k2 = (2.3 ± 0.5) × 10-12 cm3 molecule-1 s-1. These data are compared with other very recent literature data, and their atmospheric significance for nighttime chemistry is briefly discussed.

Gas-phase reactions of the buckminsterfullerene cations C ·+60, C 2+60, and C ·3+60 with aldehydes, ketones, carboxylic acids, and esters

The reactions of the fullerene ions C ·+ 60, C 2+ 60, and C ·+ 60 with the carbonyl and carboxyl-containing molecules RCHO (R = H, CH 3, CH 2H 5), CH 3COR (R  CH 3, C 2H, C 2H 5, n-C 3H 7), C 2H 5CO C 2H 5, c-C 5H 8O, R COOH (R = H, CH 3), and RCOOCH 3 (R = H, CH 3), have been studied using a Selected-Ion Flow Tube (SIFT) at 294 ± 2K and 0.35 ± 0.01 Torr of helium. A wide range in reactivity was observed. The monocation C ·+ 60 was unreactive with all of the neutrals surveyed. The dication C 2+ 60 was observed to add to most of the neutrals: the efficiency of adduct formation was seen to be strongly dependent upon the functional group, and was also observed to increase with increasing size of the alkyl substituents. The adducts of ketones, aldehydes and carboxylic acids were observed to undergo proton transfer to the reactant neutral, as a secondary process; in contrast, proton transfer did not occur from the adducts of esters. Structural factors can account for these differences in reactivity. Proton transfer from carbonyl-containing adducts is presumed to involve deprotonation from an α carbon atom, as indicated by the failure of the dicationic adducts of benzaldehyde to undergo deprotonation. Proton transfer from the carboxylic acid adducts involves loss of the carboyxl proton rather than α CH proton loss, since the ester adducts did not display proton transfer. The reaction chemistry of C ·3+ 60 with these neutrals was dominated by charge transfer, although addition to some neutrals with an ionization energy, IE > 9.9eV was noted also. General features of the secondary chemistry of the triply-charged adducts were similar to the features of the dication adduct chemistry: similar structural factors appear to influence the dication and trication adduct chemistry, although a greater tendency towards charge separation reactions was apparent in the chemistry of the tricationic adducts. Notably, a methyl cation transfer channel was observed in the reactions of the tricationic ester adducts with the parent neutrals.

Study of the kinetics and equilibrium of the benzyl‐radical association reaction with molecular oxygen

AbstractThe forward rate constant, k1, and the equilibrium constant, Kp, for the association reaction of the benzyl radical with oxygen have been determined. The rate constant k1 was measured as a function of temperature (between 298 and 398 K) and pressure (at 20 and 760 torr of N2) by two different techniques, argon‐lamp flash photolysis and excimer‐laser flash photolysis, both of which employed UV absorption spectroscopy (at 253 nm and 305 nm, respectively) to monitor the benzyl radical concentration. Over the range of conditions studied, we find that the reaction is independent of pressure and is almost independent of temperature, which is in accord with two early studies of the reaction but in apparent disagreement with more recent work. For our results in 760 torr of N2 and for 298 &lt; T/K &lt; 398, a linear least‐squares fitting of the data yield the expression: k1 = (7.6 ± 2.4) × 10−13 exp[(190 ± 160)K/T] cm3 molecule−1 s−1. With the flash‐photolysis technique, we determined Kp over the temperature range 398–525 K. Experimental values were analyzed alone and combined with theoretically determined entropy values of the benzyl and benzylperoxy radicals to determine the enthalpy of reaction: ΔH = (−91.4 ± 4) kJ mol−1. Previous work on the benzyl radical enthalpy of formation allows us to calculate ΔH°f 298 (Benzylperoxy) = (117 ± 6) kJ mol−1. In addition, we carried out an RRKM calculation of k1 using as constraints the thermodynamic information gained by the study of Kp. We find that all the studies of the association reaction are in good agreement once a fall‐off effect is taken into account for the most recent work conducted at pressures near 1 torr of helium. © 1994 John Wiley &amp; Sons, Inc.

Investigation into the kinetics and mechanism of the reaction of NO3 with CH3O2 at 298 K and 2.5 Torr: a potential source of OH in the night-time troposphere?

The kinetics of the reaction CH3O2+ NO3→ CH3O + NO2+ O2(1) have been studied at 298 K and at pressures between 2 and 3 Torr of helium using the discharge-flow technique combined with laser-induced fluorescence detection of the methoxyl radical and measurements of the NO3 radical using visible absorption. Numerical modelling of the concentration–time profile of CH3O with or without NO3 and NO as a titrant has allowed us to show that CH3O is a product of reaction (1) and to derive a rate constant k1=(1.0 ± 0.6)× 10–12 cm3 molecule–1 s–1, at 95% confidence limits. A comparison of the reactivities of NO3 and NO2 towards the species R, RO and RO2, where R = H or CH3, is given. The implication of reaction (1) in the possible production of OH in the atmosphere at night is discussed.

Study of the atomization of boron in electrothermal atomic absorption spectrometry and hollow cathode furnace atomic non-thermal excitation spectrometry

Coating a total pyrolytic graphite tube with tungsten carbide or lanthanum carbide increased the optimum pyrolysis temperature of B from 850 to > 2200 °C. Addition of a calcium–magnesium modifier to the boron solutions increased the pyrolysis temperature to 1200 °C, whereas a titanium-ascorbic acid modifier had no significant effect. The lowest characteristic mass of B, 0.8 ng, was obtained with the calcium–magnesium modifier. The low temperature loss of boron, without a modifier, was investigated using dynamic secondary-ion mass spectrometry, which confirmed that vaporization of boron species occurs above 900 °C. Boron atomic emission signals were obtained at <800 °C by hollow cathode furnace atomic non-thermal extraction spectrometry (HC-FANES) with a 30 Torr helium plasma. Molecular dissociation and boron atom excitation apparently occurred through a one-step collisional process. The detection limit of B by He HC-FANES, under non-optimum conditions, was 71 pg (no modifier). The study suggests that atomization of B in electrothermal atomic absorption spectrometry (ETAAS) is likely to occur through molecular dissociation rather than solely by sublimation of B. The modifiers probably prevent low temperature dissociative desorption of B2O3 occurring at active carbon sites and so increased the optimum pyrolysis temperature to >850 °C. The poor detection limit of B in ETAAS is due to the inefficient thermal dissociation of the B-containing species (probably oxides and carbides) produced by dissociative desorption of B2O3. Also, once formed, B atoms apparently undergo a series of condensation–vaporization steps, which cause a persistant plateau in the tail of the AAS signal, and result in severe memory effects.