Torr Of N2 Research Articles

Growth of Epitaxial Gan Films Using Zno Buffer Layer by Pulsed Laser Deposition

AbstractGaN films and ZnO buffer layers have been deposited on c-cut sapphire substrates by pulsed laser deposition (PLD) employing a KrF laser (λ = 248 nm). The influence of the deposition parameters, such as substrate temperature and gas pressure during growth, have been studied. GaN films grown above 700 °C are single crystalline and the full width of half-maximum (FWHM) of the GaN (0002) peak decreases to 0.37° as the growth temperature increases to 800 °C. The optimum growth pressure for GaN is determined to be 0.01 torr N2. The optical transmission below the bandgap of the GaN film grown at this pressure is over 85%.

Kinetics of the self-reaction of CF2radical and its reaction with H2, O2, CH4and C2H4over the temperature range 295–873 K

Laser flash photolysis, coupled with time-resolved UV absorption detection, has been used to measure rate coefficients for the self-reaction of CF2: CF2+ CF2→ C2F4(1) and its reaction with the molecular species H2, O2, CH4 and C2H4 over the temperature range 295–873 K. The CF2 radicals were produced by the 193 nm laser photolysis of C2F4. At room temperature, values of k1=(4.01 ± 0.51)× 10–14 cm3 molecule–1 s–1 were obtained for the rate coefficient of reaction (1) and σ249=(1.61 ± 0.21)× 10–17 cm2 molecule–1 for the absorption cross-section of CF2 at 249 nm, where the spectral resolution was 0.5 nm (FWHM). The value of k1 is in good agreement with previous room-temperature measurements, but the value of σ249 is almost a factor of two lower, reflecting the lower spectral resolution used in this study. The absorption cross-section, σ249, was found to decrease with increasing temperature, whereas the rate coefficient, k1, exhibits a positive temperature dependence. The temperature dependence of k1 at 760 Torr N2, when combined with earlier shock-tube determinations at 1200 and 1400 K, could be fitted to a modified Arrhenius expression: k1=(1.39 ± 0.20)× 10–19T2 exp[(347 ± 56)/T] cm3 molecule–1 s–1for the temperature range 295–1400 K. The uncertainties refer to random error in the parameters at the 95% confidence level. Systematic errors are estimated to be ± 10%. However, the variation of k1 with pressure at 873 K indicated that the reaction had not quite reached its high-pressure limit at 760 Torr, so this expression can only be regarded as an empirical expression for the temperature dependence at atmospheric pressure. At lower temperatures, k1 had clearly reached its high-pressure limiting value at 760 Torr. No evidence could be found at any temperature for products other than C2F4, implying that recombination is the only channel occurring.No reaction was observed between CF2 and any of the molecular reagents studied at temperatures up to 873 K. Upper limits were derived for the rate coefficient of CF2 with each reagent at several temperatures. AT 873 K, the values are (in units of cm3 molecule–1 s–1) < 5 × 10–19 for H2, < 3 × 10–17 for O2, < 4 × 10–19 for CH4 and < 2 × 10–16 for C2H4.Experiments performed using the 193 nm photolysis of C3F6 as a source of CF2 suggested another reactive and absorbing species is generated in rather higher yield than CF2, at variance with earlier studies. The results could be rationalised by assuming the species to be C2F3 which reacts with CF2 with a rate coefficient estimated to be ca. 2 × 10–13 cm3 molecule–1 s–1 at 295 K.The implications of the very low reactivity of CF2 with molecular species, compared with its reactivity with radical species, O, H and OH, are discussed in the context of the role played by the CF2 radical in the chemistry of flame inhibition by fluorinated hydrocarbons considered as potential halon replacements.

Kinetics and Products of the Reaction SO3 + NH3 + N2

The kinetics of the gas phase reaction SO3 + NH3 + M were measured in N2 at 295 K over the pressure range 10−400 Torr with laminar flow reactors coupled to a chemical ionization mass spectrometer. The pressure dependence of the second-order rate coefficient is fit well by the generalized Troe formalism giving k0 = (3.9 ± 0.8) × 10-30 cm6 molecule-2 s-1 and k∞ = (4.7 ± 1.3) × 10-11 cm3 molecule-1 s-1. The association product of the SO3 + NH3 reaction and secondary acid cluster products were detected by chemical ionization mass spectrometry. Models of the temporal behavior of SO3, sulfamic acid, and sulfamic acid dimer show that sulfamic acid dimerizes at near the hard sphere gas kinetic limit (k > 5 × 10-11 cm3 molecule-1 s-1, 20 Torr N2, 295 K) and the dimer is relatively stable with respect to decomposition to monomers (ΔG°298 ≤ −9 kcal mol-1).

Kinetics and Products of the Gas-Phase Reaction of SO3 with Water

The kinetics of the gas-phase reactions of SO3 with H2O and D2O were studied over the temperature range 250−360 K in N2 with a laminar flow reactor coupled to a chemical ionization mass spectrometer. The SO3 loss is second order in the water concentration, is independent of pressure (20−80 Torr N2, 300 K), and has a strong negative temperature dependence and a significant H/D isotope effect (kH2O ≈ 2kD2O). The yield of sulfuric acid is 1.0 ± 0.5 per SO3 consumed. These observations are consistent with the rapid association of SO3 and H2O to form the adduct H2OSO3 which reacts with water to produce sulfuric acid. The first-order rate coefficients for loss of SO3 by reaction with H2O and D2O are given by kI(s-1) = (2.26 ± 0.85) × 10-43T exp((6544 ± 106)/T)[H2O]2 and (9.45 ± 2.68) × 10-44T exp((6573 ± 82)/T)[D2O]2, where T ≡ K and [H2O, D2O] ≡ molecule cm-3. The errors are the uncertainty at the 95% confidence level for precision only. Analysis of the temperature dependence of the SO3 loss yields an upper limit for the H2O−SO3 bond enthalpy of 13 kcal mol-1.

Quantum yield for carbon monoxide production in the 248 nm photodissociation of carbonyl sulfide (OCS)

Tunable diode laser absorption spectroscopy has been coupled with excimer laser flash photolysis to measure the quantum yield for CO production from 248 nm photo‐dissociation of carbonyl sulfide (OCS) relative to the well‐known quantum yield for CO production from 248 nm photolysis of phosgene (Cl2CO). The temporal resolution of the experiments was sufficient to distinguish CO formed directly by photodissociation from that formed by subsequent S(³PJ) reaction with OCS. Under the experimental conditions employed, CO formation via the fast S(¹D2) + OCS reaction was minimal. Measurements at 297K and total pressures from 4 to 100 Torr N2 + N2O show the CO yield to be greater than 0.95 and most likely unity. This result suggests that the contribution of OCS as a precursor to the lower stratospheric sulfate aerosol layer is somewhat larger than previously thought.

Single-phase polycrystalline Ti1−xWxN alloys (0⩽x⩽0.7) grown by UHV reactive magnetron sputtering: microstructure and physical properties

Single-phase 300 nm thick Bl-NaCl structure polycrystalline Ti1−xWxN alloys, with compositions extending from TiN to Ti0.3W0.7N, have been grown at 500°C on amorphous SiO2 by ultra-high vacuum reactive magnetron sputtering from Ti and W targets in 10 m Torr N2 discharges. The anion-to-cation ratio ranged from slightly overstoichiometric (1 <N(Ti + W) ⩽ 1.1) in TiN-rich alloys to understoichiometric in WN-rich alloys with N(Ti + W) = 0.9 at x = 0.7. The relaxed alloy lattice constant a0 initially increased with increasing W and then decreased below the stoichiometric TiN value, aTiN = 0.42416 nm, for understoichiometric (i.e. N deficient) alloys with x > 0.5. Plan-view and cross-sectional transmission electron microscopy showed that the films exhibited a columnar microstructure, with typical column diameters of 20–25 nm, and a preferred orientation which changed from strong (111) in TiN-rich alloys to strong (002) in WN-rich alloys. The normalized room-temperature resistivity increased linearly at a rate dϱ/ϱTiNdx = 2.5 with increasing WN concentration x due to a combination of alloy, boundary, and free-surface scattering. All of the above results as a function of film composition are directly related to differences in the intensity of low-energy particle irradiation (primarily backscattered N) during film growth.

High-resolution NMR of low pressure laser-polarized 129Xe gas

Hypermagnetized 129Xe (0.5% magnetization) was generated, by spin exchange with optically pumped Rb, in Xe gas (100 Torr natural isotopic composition, 50 Torr N2), contained in a sealed glass bulb connected to an NMR tube. After optical pumping with a 3 mW diode laser and spin exchange at 32 mT, 129Xe NMR spectra were recorded at 7 T (83 MHz) in a standard high-resolution probe. Linewidths were 0.35 Hz in the gas and 300 Hz in the solid. Shielding in the solid is 301 ppm relative to the gas. From the responses to pulse pairs, the diffusion constant of Xe gas was determined to be 17 mm2 s−1 at 298 K.

Reactions of NO3‐naphthalene adducts with O2 and NO2

AbstractThe kinetics of the gas‐phase reaction of the NO3 radical with naphthalene have been investigated at 150 torr O2 + 590 torr N2 and 600 torr O2 + 140 torr N2 at 298 ± 2 K. Relative rate measurements were carried out in reacting NO3N2O5‐naphthalene‐propene‐O2N2 mixtures by longpath Fourier transform infrared absorption spectroscopy. A rate constant ratio for the reactions of O2 and NO2 with the NO3‐naphthalene adduct of k/k &lt; 4 × 10−7 was obtained from the competition between O2 and NO2 for reaction with the NO3‐naphthalene adduct and thermal decomposition of the adduct back to reactants. Atmospheric pressure ionization MS/MS measurements of the nitronaphthalene products of the NO3 radical‐initiated reaction of naphthalene are consistent with the proposed reaction mechanism, and the atmospheric implications of the data are discussed. © 1994 John Wiley &amp; Sons, Inc.

Interferometric calorimeter for the measurement of water-vapor absorption

An interferometric calorimeter has been developed to measure the optical-absorption coefficient of gases at the level of 10−10 cm−1. The system operates by measuring the phase shift on a cw He–Ne laser probe beam, which is caused by the expansion of the gas after it has been heated in the focal volume of a pulsed excitation laser beam. The minimum detectable fringe shift is approximately 2 × 10−5 waves. For a 1-J excitation laser this shift corresponds to an absorption coefficient of 2 × 10−10 cm−1. The continuum absorption of water vapor at 9466 cm−1 has been measured to be 6 × 10−10 cm−1 at a 16.5-Torr partial pressure in a 1-atm (760 Torr) N2 buffer at 30°C.

Experimental study of correspondence between the electron distribution function in plasma and the second derivative of the Langmuir probe I-V characteristic

Experimental results are presented that were obtained in a glow discharge in coaxial geometry. Crossed E and H fields in the gas mixture of 0.8 Torr CO2+2.2 Torr N2+5 Torr He were used. A calculation was made of the radiation power P of a gas-discharge laser (utilizing the studied plasma as the active element) on the basis of the measured probe’s I-V characteristics. The calculated dependence P(H) agrees with the results of direct calorimetric measurements. This agreement verifies that the second derivative of a Langmuir probe I-V characteristic varies in the same way as the distribution function of plasma electrons.

Energy requirements for the dissociative adsorption of hydrogen on Cu(110)

The activation energy for the dissociative adsorption of H2 on Cu(110) is determined to be 14.3±1.4 kcal/mol (95% confidence limit). This value represents a true Arrhenius activation energy from Boltzmann distributions of impinging H2 gas at ∼623 K. The results were obtained by titrating atomic oxygen from a Cu(110) surface under conditions where the rate of titration was equal to the rate of dissociative hydrogen adsorption and where the oxygen coverage did not affect the rate. The hot copper surface was exposed to ∼10−4 Torr H2 in a microreactor whose walls were at 300 K. The degree to which the H2 translational, rotational, and vibrational populations approach the surface temperature was controlled by adding N2 as an energy transfer medium. The dissociative adsorption probability at 623 K increased from ∼5×10−7 to ∼10−5 when the N2 pressure was increased from 0 to ∼80 Torr. About 70% of this increase occurred below 2 Torr. Known gas‐phase energy transfer rates suggest that the translational temperature of H2 hitting the Cu surface was already at the surface temperature after addition of only 2 Torr N2, but that the v=1 vibrational population was still unchanged. It approached the population of the surface temperature only at 80–300 Torr N2. These results together with beam experiments suggest either translational or vibrational energy can dominate in overcoming the activation barrier to dissociation of an H2 molecule.

Ultraviolet absorption spectra of the CH2Cl and CHCl2 radicals and the kinetics of their self-recombination reactions from 273 to 686 K

The UV absorption spectra of the chloromethyl (CH2Cl) and dichloromethyl (CHCl2) radicals have been determined between 197.5 and 230 nm, together with the absolute rate constants for their association reactions: CH2Cl + CH2Cl → Products (1a) CHCl2+ CHCl2→ Products (1b) as a function of temperature from 273 to 686 K and between 29 and 760 Torr N2 total pressure. The transient decays of the radicals were monitored by time-resolved UV absorption following the flash photolysis of Cl2 mixed with the parent molecules (CH3Cl or CH2Cl2). At a resolution of 2 nm, no vibrational structure was detected in either spectrum which both appear as strong broad electronic bands with maxima around 200 nm (CH2Cl) and 215 nm (CHCl2). At these wavelengths the absolute absorption cross-sections were measured as (1.45 ± 0.16)× 10–17 and (1.37 ± 0.24)× 10–17 cm2 molecule–1, respectively, relative to that of CH3O2 at 240 nm (4.55 × 10–18 cm2 molecule–1). The experimental results, supported by RRKM calculations, demonstrate that the measured rate constants for removal of the radicals correspond to the high-pressure limiting rate constants for recombination, both of which exhibit a negative temperature dependence, represented by the following expressions: CH2Cl: k1a=(2.8 ± 0.3)× 10–11(T/298)–(0.85 ± 0.14) cm3 molecule–1 s–1. CHCl2: k1b=(9.3 ± 1.7)× 10–12(T/298)–(0.74 ± 0.10) cm3 molecule–1 s–1. Errors are 1 σ. The present results are compared to existing data on the self-recombination reactions of the CH3 and CCl3 radicals; the negative temperature dependences of the self-association rate constants for the series CH3, CH2Cl, CHCl2 and CCl3 are shown to be consistent within the framework of a recently developed variational transition-state theory method.

ChemInform Abstract: Rate Constants for the Reactions of Br Atoms with a Series of Alkanes, Alkenes, and Alkynes in the Presence of O2

Rate constants of Br atom reactions have been determined using a relative kinetic method in a 20 l reaction chamber at total pressures between 25 and 760 torr in N2 + O2 diluent over the temperature range 293–355 K. The measured rate constants for the reactions with alkynes and alkenes showed dependence upon temperature, total pressure, and the concentration of O2 present in the reaction system. Values of (6.8 ± 1.4) × 10−15, (3.6 ± 0.7) × 10−14, (1.5 ± 0.3) × 10−12, (1.6 ± 0.3) × 10−13, (2.7 ± 0.5) × 10−12, (3.4 ± 0.7) × 10−12, and (7.5 ± 1.5) × 10−12 (units: cm3 s−1) have been obtained as rate constants for the reactions of Br with 2,2,4-trimethylpentane, acetylene, propyne, ethene, propene, 1-butene, and trans-2-butene, respectively, in 760 torr of synthetic air at 298 K with respect to acetaldehyde as reference, k = 3.6 × 10−12 cm3 s−1. Formyl bromide and glyoxal were observed as primary products in the reaction of Br with acetylene in air which further react to form CO, HBr, HOBr, and H2O2. Bromoacetaldehyde was observed as an primary product in the reaction of Br with ethene. Other observed products included CO, CO2, HBr, HOBr, BrCHO, bromoethanol, and probably bromoacetic acid.

Rate constants for the reactions of Br atoms with a series of alkanes, alkenes, and alkynes in the presence of O2

AbstractRate constants of Br atom reactions have been determined using a relative kinetic method in a 20 l reaction chamber at total pressures between 25 and 760 torr in N2 + O2 diluent over the temperature range 293–355 K. The measured rate constants for the reactions with alkynes and alkenes showed dependence upon temperature, total pressure, and the concentration of O2 present in the reaction system.Values of (6.8 ± 1.4) × 10−15, (3.6 ± 0.7) × 10−14, (1.5 ± 0.3) × 10−12, (1.6 ± 0.3) × 10−13, (2.7 ± 0.5) × 10−12, (3.4 ± 0.7) × 10−12, and (7.5 ± 1.5) × 10−12 (units: cm3 s−1) have been obtained as rate constants for the reactions of Br with 2,2,4‐trimethylpentane, acetylene, propyne, ethene, propene, 1‐butene, and trans‐2‐butene, respectively, in 760 torr of synthetic air at 298 K with respect to acetaldehyde as reference, k = 3.6 × 10−12 cm3 s−1.Formyl bromide and glyoxal were observed as primary products in the reaction of Br with acetylene in air which further react to form CO, HBr, HOBr, and H2O2. Bromoacetaldehyde was observed as an primary product in the reaction of Br with ethene. Other observed products included CO, CO2, HBr, HOBr, BrCHO, bromoethanol, and probably bromoacetic acid.

Infrared line intensity measurements in the v = 0–1 band of the ClO radical

Line intensities of the ClO radical in the fundamental vibrational band, v = 0–1, have been measured using a high‐resolution Fourier transform spectrometer coupled to a long path length absorption cell. The ClO infrared absorption spectra were recorded at 0.004 cm−1 unapodized resolution in 50 coadded scans at total pressures less than or equal to 0.4 torr, He buffer gas. Spectra were recorded at calibrated [ClO] over the range 4 × 1012 to 1.4 × 1013 molecule cm−3, using two different chemical sources of ClO: Cl + O3 and NO + OClO. The band intensity was measured as S = 9.68 ± 1.45 cm−2 atm−1 (95% confidence level) at 296 K, which is in agreement with a recent ClO band intensity determination using the concentration independent Herman‐Wallis method of band intensity analysis (Burkholder et al, 1987a). This value of the ClO band intensity is a factor of 2 greater than the value recently reported by Kostiuk et al. (1986). A N2 collisional broadening coefficient of γ0 = 0.093 ± 0.018 cm−1 atm−1 (95% confidence level) was determined from spectra recorded with 10 torr N2 added. A source of systematic error in the use of the Cl + O3 reaction as a quantitative source of ClO radicals is discussed.

ChemInform Abstract: The UV Absorption Spectra and Kinetics of the Self Reactions of CH2ClO2 and CH2FO2 Radicals in the Gas Phase

The ultraviolet absorption spectra of chloromethylperoxy and fluoromethylperoxy radicals, CH2ClO2 and CH2FO2, and the kinetics of their respective self reactions have been studied in the gas phase using a flash photolysis technique. The absorption spectra for both radicals were quantified over the wavelength range 210 and 290 nm. The measured absorption cross-sections were used to derive the observed self-reaction rate constants (for reactions 1 and 2) over the temperature range 228–380 K, defined as –d[CH2XO2]/dt = 2k[CH2XO2]2, where X represents Cl or F. The rate constants at 298 K were found to be independent of pressure over the range 25–400 torr N2 with values of k1(298 K) = (3.78 ± 0.45) × 10−12 and k2(298 K) = (3.07 ± 0.65) × 10−12 in units of cm3 molecule−1 s−1. The kinetic data over the complete temperature range are represented by the Arrhenius expressions: where the error limits represent 2σ from linear least squares analysis. These results are discussed with respect to previous measurements of the absorption spectra and reactions of alkylperoxy radicals.

Primary electron avalanche under an induced toroidal electric field in H2, He and N2

Reports a study on the development of primary electron avalanche under a toroidal electric field. The applied toroidal field is an induced electric field of E0=86.8 V cm-1 and the gases used are H2, He and N2. The avalanches are generated and make steady progress in the torus at 0.8-2.4 Torr in H2, at 1.6-6.3 Torr in He and at 0.4-1.2 Torr in N2 under the given field. It is suggested that the considerably high pressure range of He is closely related to the large value of alpha /p at low E/p. The development velocities of avalanche in N2 and He are nearly equal to the electron drift velocity at the E/p used. The development velocity in H2 is about twice as large as that of the others. This difference is presumably due to the multiplied effect of the large ionisation coefficient and the small absorption coefficient.

The UV absorption spectra and kinetics of the self reactions of CH2ClO2 and CH2FO2 radicals in the gas phase

AbstractThe ultraviolet absorption spectra of chloromethylperoxy and fluoromethylperoxy radicals, CH2ClO2 and CH2FO2, and the kinetics of their respective self reactions have been studied in the gas phase using a flash photolysis technique. The absorption spectra for both radicals were quantified over the wavelength range 210 and 290 nm. The measured absorption cross‐sections were used to derive the observed self‐reaction rate constants (for reactions 1 and 2) over the temperature range 228–380 K, defined as –d[CH2XO2]/dt = 2k[CH2XO2]2, where X represents Cl or F. magnified image The rate constants at 298 K were found to be independent of pressure over the range 25–400 torr N2 with values of k1(298 K) = (3.78 ± 0.45) × 10−12 and k2(298 K) = (3.07 ± 0.65) × 10−12 in units of cm3 molecule−1 s−1. The kinetic data over the complete temperature range are represented by the Arrhenius expressions: where the error limits represent 2σ from linear least squares analysis. These results are discussed with respect to previous measurements of the absorption spectra and reactions of alkylperoxy radicals.

A kinetics study of the reaction of Cl with NO2

AbstractThe third order rate coefficients for the addition reaction of Cl with NO2, Cl + NO2 + M → ClNO2 (ClONO) + M; k1, were measured to be k1(He) = (7.5 ± 1.1) × 10−31 cm6 molecule−2 s−1 and k1(N2) = (16.6 ± 3.0) × 10−31 cm6 molecule−2 s−1 at 298 K using the flash photolysis‐resonance fluorescence method. The pressure range of the study was 15 to 500 torr He and 19 to 200 torr N2. The temperature dependence of the third order rate coefficients were also measured between 240 and 350 K. The 298 K results are compared with those from previous low pressure studies.

FTIR study of the DMS/NO2/I2/N2 photolysis system: The reaction of IO radicals with DMS

AbstractA steady‐state system involving the photolysis of NO2 in an excess of I2 as a source of IO radicals has been used to study the reaction IO + DMS in 760 Torr N2 at 296 K. IO radicals were found to react rapidly with DMS, one molecule of DMSO being produced for each molecule of DMS consumed. Numerical analysis of the experimental results yielded a rate constant of (3.0 ± 1.5) × 10−11 cm3 s−1 for the reaction IO + DMS → DMSO + I.