Torr Total Pressure Research Articles

Formation pathways of HO 2 and OH changing as a function of temperature in photolytically initiated oxidation of dimethyl ether

The time resolved product formation in oxidation of dimethyl ether (DME) has been studied between 298–625 K and 20–90 torr total pressure. Near-infrared frequency modulation spectroscopy (FMS) with Herriott type multi pass optics and UV absorption spectroscopy (UV) were conducted in the same cell. The reaction was initiated by pulsed photolysis in a mixture of Cl 2, O 2, and DME via CH 3OCH 2 radical formation. The reaction process was investigated through FMS measurement of HO 2 and OH, and UV measurement of CH 3OCH 2O 2. The yields of HO 2 and OH are obtained by comparison with reference mixtures, Cl 2, O 2, and CH 3OH for HO 2, and Cl 2, O 2, CH 3OH, and NO for OH, which convert 100% of initial Cl to HO 2 and OH. The CH 3OCH 2O 2 yield is also obtained. It was found that the HO 2 yield increases sharply over 500 K mainly with a longer time constant than that of R + O 2 reaction, while a prompt component exists throughout the temperature range at a few percent yield. OH was found to be produced promptly at a yield considerably larger than that known for the simplest alkanes. The CH 3OCH 2O 2 profile has a prompt rise followed by a gradual decay whose rate is consistent with the slow HO 2 formation. The species profiles were successfully predicted with a model constructed by modifying the existing one to suit the reduced pressure condition. After modification, it was inferred that the HO 2 formation over 500 K is secondary from HCHO + OH and HCO + O 2 and a part of HCO is formed directly from the O 2 adduct, whereas the HO 2 formation below 500 K is governed by CH 3OCH 2O 2 chemistry. The HCO forming pathway via isomerization–decomposition of the O 2 adduct, which was not included in the former models, was supported by our quantum-chemical calculations.

Atmospheric HFEs Degradation in the Gas Phase: Reactions of HFE-7100 and HFE-7200 with Cl Atoms at Low Temperatures

Kinetic rate coefficients for the reactions of HFE-7100 (1) (C4F9OCH3) and HFE-7200 (2) (C4F9OC2H5) with Cl atoms have been measured using a discharge flow mass spectrometric technique (DFMS) at 1 Torr total pressure. The reactions have been studied under pseudo-first-order kinetic conditions in excess of HFEs over Cl atoms and the study has been extended from 333 down to 234 K to approach the tropospheric temperature profile. At room temperature the measured rate constants are k (1) = (1.43 +/- 0.28) x 10(-13) cm3molecule(-1)s(-1) and k (2) = (2.1 +/- 0.1) x 10(-12) cm3molecule(-1)s(-1). The Arrhenius expressions from our results are (units in cm3molecule(-1)s(-1)): k (1) = (2.3 +/- 1.4) x 10(-10) exp - (2254 +/- 177)/T(234-315 K) and k (2) = (3.7 +/- 0.5) x 10(-11) exp - (852 +/- 38)/T(234-333 K) (errors are sigma). The reactions proceed through the abstraction of an H atom to form HCl and the corresponding halo-alkyl radical. At 298 K and 1 Torr, yields on HCl of 0.88 +/- 0.09 and 0.95 +/- 0.10 (errors are 2sigma) were obtained for HFE-7100 and HFE-7200 reactions, respectively.

Kinetic Study of IO Radical with RO2 (R = CH3, C2H5, and CF3) Using Cavity Ring-Down Spectroscopy

The reactions of iodine monoxide radical, IO, with alkyl peroxide radicals, RO(2) (R = CH(3), C(2)H(5), and CF(3)), have been studied using cavity ring-down spectroscopy. The rate constant of the reaction of IO with CH(3)O(2) was determined to be (7.0 +/- 3.0) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K and 100 Torr of N(2) diluent. The quoted uncertainty is two standard deviations. No significant pressure dependence of the rate constant was observed at 30-130 Torr total pressure of N(2) diluent. The temperature dependence of the rate constants was also studied at 213-298 K. The upper limit of the branching ratio of OIO radical formation from IO + CH(3)O(2) was estimated to be <0.1. The reaction rate constants of IO + C(2)H(5)O(2) and IO + CF(3)O(2) were determined to be (14 +/- 6) x 10(-11) and (6.3 +/- 2.7) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, 100 Torr of N(2) diluent, respectively. The upper limit of the reaction rate constant of IO with CH(3)I was <4 x 10(-14) cm(3) molecule(-1) s(-1).

Correlation between electron field emission property and in situ plasma spectroscopy for carbon nanotubes synthesized by using MPECVD method

Carbon nanotubes (CNTs) were grown on Fe needle by a modified microwave plasma enhanced chemical vapor deposition (MPECVD) process, in which the plasma was induced in the vicinity of the tip of Fe needle through the interaction of microwave and Fe needles. The spectra of plasma were in situ examined, indicating that Ar / CH 4 ratio and total pressure remarkably affect the characteristics of the plasma spectra, which, in turn, correlated intimatedly with the electron field emission properties of CNTs. The presence of singly ionized carbons (C II) in the plasma is presumed to be the main factor facilitating the growth of CNTs, possessing good electron field emission properties. The maximum electron field emission current density is 8 mA/cm 2 for carbon nanotubes grown at 10% CH 4 and 300 Torr total pressure. The corresponding turn-on field ( E o) is 20 V/μm. Restated, the electron field emission properties of CNTs can be optimized by controlling the spectral characteristics of plasma.

Kinetic Study of the ClOO + NO Reaction Using Cavity Ring-Down Spectroscopy

Cavity ring-down spectroscopy was used to study the reaction of ClOO with NO in 50-150 Torr total pressure of O2/N2 diluent at 205-243 K. A value of k(ClOO+NO) = (4.5 +/- 0.9) x 10(-11) cm3 molecule(-1) s(-1) at 213 K was determined (quoted uncertainties are two standard deviations). The yield of NO(2) in the ClOO + NO reaction was 0.18 +/- 0.02 at 213 K and 0.15 +/- 0.02 at 223 K. An upper limit of k(ClOO+Cl2) < 3.5 x 10(-14) cm3 molecule(-1) s(-1) was established at 213 K. Results are discussed with respect to the atmospheric chemistry of ClOO and other peroxy radicals.

An FT-IR study of the isomerization of 1-butoxy radicals under atmospheric conditions

Relative rate-studies of the reactions of 1-butoxy radicals have been carried out using a 47 L static reactor with detection of end products by FT-IR spectroscopy. Experiments were performed at 700 torr total pressure and over the temperature range 253–295 K. The chemistry of 1-butoxy is characterized by a competition between reaction with oxygen CH 3CH 2CH 2CH 2O + O 2 → n-C 3H 7CHO + HO 2 (R2), which yields butanal and isomerization CH 3CH 2CH 2CH 2O → CH 2CH 2CH 2CH 2OH (R3), to form a hydroxylated carbonyl-product. A reference spectrum attributed to the product of 1-butoxy isomerization was obtained and used to determine the competition between 1-butoxy isomerization versus reaction with oxygen. The results indicate that isomerization is the dominant fate of 1-butoxy radicals at ambient temperature and pressure and that its importance decreases with decreasing temperature. The rate-coefficient ratio k 3/ k 2 (molecule cm −3) = 5.5 × 10 23 exp[(−25.1 ± 0.9 kJmol −1)/RT] was obtained. This agrees with other estimates based on methods without monitoring of the isomerization product.

Direct Observation of Adduct Formation of Alkyl and Aromatic Iodides with Cl Atoms Using Cavity Ring-Down Spectroscopy

The reactions of Cl atoms with RI (R = n-C3H7, n-C4H9, cyclo-C6H11, C6H5, C6F5, and p-CH3C6H4) have been studied using cavity ring-down spectroscopy at a temperature range of 233-313 K and at 100 Torr total pressure of N2 diluent. Visible absorption spectra of the RI-Cl adducts were recorded at 440-520 nm at 263 K. The yields of the adducts were temperature-dependent. There was no discernible reaction of the adducts in the presence of 100 Torr of O2 at 263 K. Theoretical calculations were performed for C4H9I-Cl and C6H5I-Cl for quantitative explanation of the absorption spectra and the strength of the I-Cl bonds in the charge-transfer complexes. Evidence for the adduct formation following the reaction of Cl with C6H5Br was sought but not found at 440 and 520 nm.

The Kinetics of the Reactions of C2 (a3Πu) with Alcohols

The reactions of C2 (a 3pi(u)) radicals with a series of alcohols have been studied at about 6.5 Torr total pressure and room temperature using the pulsed laser photolysis/laser-induced fluorescence technique. The relative concentration of C2 (a 3pi(u)) radicals, which are generated via the photolysis of C2Cl4 with the focused output from the fourth harmonic of a Nd:YAG laser (266 nm), was monitored by laser-induced fluorescence (LIF) in the (0, 0) band of the C2 (d 3pi(g)<--a 3pi(u)) transition at 516.5 nm. Under pseudo-first-order conditions, we measured the time evolution of C2 (a 3pi(u)) and determined the rate constants for reactions of C2 (a 3pi(u)) with alcohols. The rate constants increase linearly with the number of C atoms in the alcohols. All of them are larger than those for reactions of C2 (a 3pi(u)) with alkanes (C1-C5). Based on the bond dissociation energy and linear free energy correlations, we believe the reactions of C2 (a 3pi(u)) with alcohols proceed via the mechanism of hydrogen abstraction. The experimental results show that the H-atom on the C-H bonds is activated at the presence of the OH substituent group in the alcohol molecule. The theoretical calculations for the reaction of C2 (a 3pi(u)) with methanol also support these hypotheses.

Laboratory Measurements of NiH by Intracavity Laser Absorption Spectroscopy

The visible electronic spectrum of the NiH B 2Δ5/2-X 2Δ5/2 transition has been recorded with rotational resolution by intracavity laser absorption spectroscopy. The gas-phase NiH molecules were produced in an electric discharge using a nickel cathode in a pure hydrogen atmosphere at 1-3 torr total pressure. Peak positions for the NiH isotopomers are presented. 58NiH and 60NiH have been observed in sunspots, and attempts to observe NiH in stellar spectra are underway.

Observation of Adducts in the Reaction of Cl Atoms with XCH2I (X = H, CH3, Cl, Br, I) Using Cavity Ring-Down Spectroscopy

The reactions of Cl atoms with XCH2I (X = H, CH3, Cl, Br, I) have been studied using cavity ring-down spectroscopy in 25-125 Torr total pressure of N2 diluent at 250 K. Formation of the XCH2I-Cl adduct is the dominant channel in all reactions. The visible absorption spectrum of the XCH2I-Cl adduct was recorded at 405-632 nm. Absorption cross-sections at 435 nm are as follows (in units of 10(-18) cm2 molecule(-1)): 12 for CH3I, 21 for CH3CH2I, 3.7 for CH2ICl, 7.1 for CH2IBr, and 3.7 for CH2I2. Rate constants for the reaction of Cl with CH3I were determined from rise profiles of the CH3I-Cl adduct. k(Cl + CH3I) increases from (0.4 +/- 0.1) x 10(-11) at 25 Torr to (2.0 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) at 125 Torr of N2 diluent. There is no discernible reaction of the CH3I-Cl adduct with 5-10 Torr of O2. Evidence for the formation of an adduct following the reaction of Cl atoms with CF3I and CH3Br was sought but not found. Absorption attributable to the formation of the XCH2I-Cl adduct following the reaction of Cl atoms with XCH2I (X = H, CH3, Br, I) was measured as a function of temperature over the range 250-320 K.

Kinetics and product studies of the reaction of Br, Cl, and NO with ClOOCl using discharge-flow mass spectrometry

The kinetics of the reactions Cl + ClOOCl --> products, Br + ClOOCl --> products (7), and NO + ClOOCl --> products have been studied as a function of temperature at 1 Torr total pressure using a discharge-flow mass spectrometric technique. The measured rate coefficients, expressed in Arrhenius form, are k5 = (7.60 +/- 0.56) x 10(-11)exp((65.4 +/- 17.9)/T) cm3 s(-1) for 217 K < T < 298 K, and k7 = (5.88 +/- 0.50) x 10(-12) exp(-173 +/- 20/T) cm3 s(-1) for 223 K < T< 298 K. The observed temperature dependencies of these rate coefficients indicate a common direct halogen atom abstraction mechanism. Mass spectral product studies indicate that BrCl is the only major Br-containing product from reaction. Extensive product studies were used to estimate that the peroxide form of the ClO-dimer is formed in > 90% yield from the ClO self-reaction. No evidence for the formation of ClOClO and ClClO2 was observed. No reaction between NO and ClOOCl was observed and an upper limit of k8 < 1 x 10(-15) cm3 s(-1) for 220 K < T < 298 K was determined.

Kinetics of the Reaction of CH3O2 + NO: A Temperature and Pressure Dependence Study with Chemical Ionization Mass Spectrometry

The overall rate coefficient (k4) for the reaction CH3O2 + NO → products (4) has been measured by using the turbulent flow technique with chemical ionization mass spectrometry (CIMS) for the detection of reactants and products. The temperature dependence of the rate coefficient was investigated between 193 and 300 K. Across the temperature range the experimentally determined rate coefficients were fitted by using an Arrhenius type analysis to yield the expression k4 = ( ) × 10-12 exp[(435 ± 35)/T] cm3 molecule-1 s-1. Experiments were carried out at 100 and 200 Torr total pressure within the stated temperature range, where the rate coefficients were shown to be invariant with pressure. The branching ratio of the reaction was also assessed as a function of temperature and was found to proceed 100 ± 10% via the channel forming CH3O + NO2, there being no discernible increase in the yield of CH3ONO2 at low temperatures. This work represents the first study of the branching ratio as a function of temperature an...

Mechanistic and Kinetic Study of the Gas-Phase Reaction of Atomic Chlorine with Cyclohexanone using an Absolute and a Relative Technique; Influence of Temperature

The reaction of Cl with cyclohexanone (1) was investigated, for the first time, as a function of temperature (273–333 K) and at a low total pressure (1 Torr) with helium as a carrier gas using a discharge flow-mass spectrometry technique (DF-MS). The resulting Arrhenius expression is proposed, k1= (7.7 ± 4.1) × 10−10 exp[–(540 ± 169)/T]. We also report a mechanistic study with the quantitative determination of the products of the reaction of Cl with cyclohexanone. The absolute rate constant derived from this study at 1 Torr of total pressure and room temperature is (1.3 ± 0.2) × 10−10 cm3 molecule−1 s−1. A yield of 0.94 ± 0.10 was found for the H-abstraction channel giving HCl. In relative studies, using a newly constructed relative rate system, the decay of cyclohexanone was followed by gas chromatography coupled with flame-ionisation detection. These relative measurements were performed at atmospheric pressure with synthetic air and room temperature. Rate constant measured using the relative method for reaction (1) is: (1.7 ± 0.3) × 10−10 cm3 molecule−1 s−1. Finally, results and atmospheric implications are discussed and compared with the reactivity with OH radicals.

Formation of Iodine Monoxide Radical from the Reaction of CH2I with O2

The rate constants of IO radical formation from the reaction of CH 2 I with O 2 were determined in the pressure range of 5-80 Torr with N 2 diluent at 278-313 K, using cavity ring-down spectroscopy. The room temperature rate constant is (4.0 ′ 0.4) x 10 - 1 3 cm 3 molecule - 1 s - 1 at 30 Torr total pressure. No significant dependences on temperature and total pressure were observed. The yield of IO from CH 2 I + O 2 was estimated to be unity in 100 Torr total pressure of N 2 diluent.

Kinetics of C 2 (a [formula omitted]) reactions with sulfur containing molecules

Rate constants for the reactions of C 2 (a 3 Π u ) with H 2S, SO 2 and CS 2 were measured under pseudo-first-order conditions at 298 K and 6.5 Torr total pressure. C 2 (a 3 Π u ) radicals were generated by photolysis of C 2Cl 4 with the focused output from the fourth harmonic of a Nd:YAG laser at 266 nm. The relative concentration of C 2 (a 3 Π u ) radicals was monitored in the (0, 0) band of the C 2 (d 3 Π g ← a 3 Π u ) transition at 516.5 nm by laser-induced fluorescence (LIF). The rate constants obtained (cm 3 molecule −1 s −1) are k H 2S =(8.90±0.51)×10 −12, k SO 2 =(2.33±0.17)×10 −12 and k CS 2 =(1.59±0.08)×10 −10, respectively. The errors given are 1 σ. The obtained rate constant for the reaction of C 2 (a 3Π u) with CS 2 is quite large and near the theoretical gas kinetic collision frequency. Ab initio calculations were performed in order to gain an insight into the mechanism. Theoretical calculation shows that the primary step of the reaction proceeds via addition without a barrier, which is consistent with experimental results.

Kinetics of C2(a3Piu) radical reactions with alkanes by LIF.

The reactions of C2(a3Piu) radicals with a series of alkanes have been studied at room temperature and 6.5 torr total pressure using the pulsed laser photolysis/laser-induced fluorescence technique. C2(a3Piu) radicals were generated by photolysis of C2Cl4 with the focused output from the fourth harmonic of a Nd: YAG laser at 266 nm. The relative concentration of C2(a3Piu) radicals was monitored on the (0,0) band of the C2(d3Pig <-- a3Piu) transition at 516.5 nm by laser-induced fluorescence. From the analysis of the relative concentration-time behavior of C2(a3Piu) under pseudofirst-order conditions, the rate constants for the reactions of C2(a3Piu) with alkanes (C1-C8) were determined. The rate constant increases linearly with the increasing of the number of CH2 groups in the alkanes. The experimental results indicate that the reaction of C2(a3Piu) with small alkanes (C1-C8) follows the typical hydrogen abstraction process. Based on the correlation of the experimental results with the bond dissociation energy of the alkanes, the reactions of C2(a3Piu) with small alkanes likely proceed via the mechanism of hydrogen abstraction.

Growth and Metrology of Silicon Oxides on Silicon Carbide

AbstractThermal oxidation of SiC by the afterglow method has opened new pathways of opportunity to address both thin film growth and defects that hinder electronic device development with this important semiconductor material. Oxide growth, with rates up to 700Å per hour, on SiC has been demonstrated using this technique over a temperature range from 400°C to 1100°C at 1 Torr total pressure. Electrical and physical properties of oxide films grown by conventional means or by the afterglow method were obtained with a novel, non-contact charge-voltage (Q-V) metrology approach. This instrument employs a combination of incremental contact potential difference values obtained in response to applied corona charge generated from air. The slope of the Q-V characteristic within a bias range corresponding to accumulation of the semiconductor provides an effective dielectric permittivity value for the grown film. Effective permittivity values for afterglow oxides grown on SiC approach that of SiO2 grown on silicon substrates whereas the values for oxides grown on SiC in an atmospheric steam oxidation process are always depressed relative to SiO2 on silicon, indicating that the latter process always produces low-k oxides. A mechanistic discussion regarding these observed differences between the two oxidation methods is presented along with suggestions for an integrated process and metrology approach to reduce defects in oxide films on SiC.

Kinetics of CCN Radical Reactions with a Series of Normal Alkanes

The reactions of CCN(X̃2Πi) radicals with normal alkanes have been studied at about 10 Torr total pressure and room temperature using the pulse laser photolysis/laser-induced fluorescence technique. CCN (X̃2Πi) radicals are generated by the fourth harmonic of a Nd:YAG laser photolysis of Cl3CCN at 266 nm. The relative concentration of CCN (X̃2Πi) radicals was monitored in the (0−0) band of the CCN (Ã2Δ ← X̃2Πi) transition at 470.9 nm by laser induced fluorescence (LIF). From the analysis of the relative concentration−time behavior of CCN (X̃2Πi) under pseudo-first-order conditions, the rate constants for the reaction of CCN (X̃2Πi) with a series of normal alkanes (C1−C8) were determined for the first time. The new data establish that the gas-phase reactivity of small alkanes (C1−C8) towards the CCN radicals follows the linear free energy relationship typical of hydrogen abstraction. A comparison with the corresponding reactions of CN and OH radicals with a series of normal alkanes, leads us to suggest that the reaction of CCN (X̃2Πi) with small normal alkanes proceeds via the mechanism of hydrogen abstraction. The plausibility of the suggested reaction mechanism is strengthened by bond dissociation energy (BDE) correlations and linear free energy correlations.

Measurements of the kinetics of the OH‐initiated oxidation of methyl vinyl ketone and methacrolein

AbstractThe mechanisms of the OH‐initiated oxidation of methyl vinyl ketone and methacrolein have been studied at 300 K and 100 Torr total pressure, using a turbulent flow technique coupled with laser‐induced fluorescence detection of the OH radical. The rate constants for the OH + methyl vinyl ketone and OH + methacrolein reactions were measured to be (1.78 ± 0.08) × 10−11 and (3.22 ± 0.10) × 10−11 cm3 molecule−1 s−1, respectively, and were found to be in excellent agreement with previous studies. In the presence of O2 and NO, the OH radical propagation and the loss of OH through radical termination resulting from the production of methyl vinyl ketone‐ and methacrolein‐based alkyl nitrates were measured at 100 Torr total pressure and compared to the simulations of the kinetics of these reaction systems. The results of these experiments are consistent with an overall rate constant of (2.0 ± 1.3) × 10−11 cm3 molecule−1 s−1 for both the methyl vinyl ketone‐based peroxy radical + NO and methacrolein‐based peroxy radical + NO reactions, each with branching ratios of 0.90 ± 0.10 for the bimolecular channel (oxidation of NO to NO2) and 0.10 ± 0.10 for the termolecular channel (production of methyl vinyl ketone‐ and methacrolein‐based alkyl nitrates). © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 12–25, 2003

Evolution and growth of ZnO thin films on GaN(0 0 0 1) epilayers via metalorganic vapor phase epitaxy

Zinc oxide thin films have been grown via metalorganic vapor phase epitaxy at 450°C and 250 Torr total pressure on O-terminated areas and micro-regions of native oxide formed on GaN(0 0 0 1) epilayers during exposure to the oxygen reactant. Analyses of the Ga3d core level spectra acquired from films grown for progressively longer times and comparisons of these data with both thermodynamic models of growth processes and associated atomic force micrographs revealed that the ZnO nucleated and grew via the Stranski–Krastanov mode. Considerations of (1) differences in surface energetics of the polar (0 0 0 1) surfaces of ZnO and GaN and that of the Ga-based oxide as well as (2) the relatively low stress generated by the moderate lattice mismatches in the a-axis lattice parameters of these compounds supported the observed growth mode. The shifts in the position of the Ga3d core level with increasing deposition time and decreasing peak intensity were caused by surface charging and increasing sampling volumes of the Ga-based oxide as well as the growing ZnO layer. The island/ocean morphology of the Ga-based oxide controlled the morphology of the initial ZnO layer, which, in turn controlled the locations and the morphologies of the subsequently nucleated ZnO islands. Scanning electron microscopy of thicker ZnO films revealed a highly textured microstructure.