Temperature Dependence Of K1 Research Articles

Rate constants of the reactions of O(3P) atoms with Br2 and NO2 over the temperature range 220‐950 K

AbstractThe kinetics of the reactions of Br2 and NO2 with ground state oxygen atoms have been studied over a wide temperature range, T = 220‐950 K, using a low‐pressure flow tube reactor coupled with a quadrupole mass spectrometer: O + NO2 → NO + O2 (1) and O + Br2 → Br + BrO (2). The rate constant of reaction (1) was determined under pseudo–first‐order conditions, either monitoring the kinetics of O‐atom or NO2 consumption in excess of NO2 or of the oxygen atoms, respectively: k1 = (6.1 ± 0.4) × 10−12 exp((155 ± 18)/T) cm3 molecule−1 s−1 (where the uncertainties represent precision at the 2σ level, the estimated total uncertainty on k1 being 15% at all temperatures). The temperature dependence of k1, found to be in excellent agreement with multiple previous low‐temperature data, was extended to 950 K. The rate constant of reaction (2) determined under pseudo–first‐order conditions, monitoring the kinetics of Br2 consumption in excess of O‐atoms, showed upward curvature at low and high temperatures of the study and was fitted with the following three‐parameter expression: k2 = 9.85 × 10−16 T1.41 exp(543/T) cm3 molecule−1 s−1 at T = (220‐950) K, which is recommended from the present study with an independent of temperature conservative uncertainty of 15% on k2.

Magnetic properties of the nanocomposite CoFe2O4/FeCo-FeO at a high H/T regime

The low temperature magnetic properties of nanopowders of CoFe2O4 and of CoFe2O4/FeCo-FeO with average particle size of about 6nm were investigated for temperatures down to 5K and for applied magnetic fields up to 350kOe. Highly enhanced coercive fields were obtained for the CoFe2O4 nanoparticles. The values of the saturation magnetization MS and of the anisotropy constant K1 were also found to be enhanced for the nanocomposite powder sample. The overall results are discussed in terms of a particle model which takes into account a thin amorphous layer present in the CoFe2O4 particles and due to a FeCo-FeO layer grown during an annealing procedure. Moreover, the temperature dependence of K1 was found to be in good agreement with a model proposed by Tachiki.

Decomposition of nitromethane in shock waves: The primary stage and the kinetics of decomposition at pressures of about 40 atm

The kinetics of nitromethane (NM) decomposition and the observed rate constant of the process were measured behind reflected shock wave using absorption spectroscopy at λ = 230 nm, temperatures of 1060 to 1350 K, pressure of ∼40 atm, and initial reactant concentration within 30–100 ppm. It was observed that, at the initial stage, nitromethane decomposes exponentially, without autoacceleration. The results of numerical simulations with the help of three most known kinetic schemes of nitromethane decomposition proved to be in close to agreement with our experimental data over the entire temperature range covered. It was demonstrated that the measured rate constant is identical to the rate constant of the dissociation CH3NO2 → CH3 + NO2. The temperature dependence of k1 was approximated by the Arrhenius formula k1 = 2.57 × 1014 exp(−52.85/RT) s−1 (activation energy in kcal/mol), which suggests that the nitromethane dissociation proceeds in the falloff pressure region.

HYDRATION KINETICS OF ROASTED LUPIN (LUPINUS ALBUS) SEEDS

Hydration kinetics of roasted lupin (Lupinus albus) seeds was studied by soaking in water at 25, 35, 45 or 55C for 5–8 h. Water absorption rate was high at the early stage of hydration (60–180 min) followed by a decreased rate and finally approached equilibrium. Peleg's equation and first order kinetic model described the hydration characteristics adequately (R2 = 0.95−0.99 and 0.97−0.99). The Peleg rate constant k1 decreased from 1.498 to 0.375 min %-1 while Peleg capacity constant, k2 increased from 0.005 to 0.0068/% (P < 0.05) with increase in temperature. The hydration rate constant k in the first order kinetic model increased from 0.0046 to 0.017/min while the equilibrium moisture content Medecreased from 137.44 to 121.06%. Arrhenius type equation described the temperature-dependence of k1 and k (R2 = 0.98 and 0.97) giving activation energy Eapand Eaeof 39.23 and 37.78 kJ/mol, respectively. The first order kinetic model described the hydration characteristics better than Peleg's equation resulting in lower root mean square error. PRACTICAL APPLICATIONS Soaking beans and peas is an integral part of processing operations like cooking, germination and fermentation. In view of long cooking time required, soaking is predominantly meant to facilitate faster cooking. Soaking or hydration is an essential operation to remove the alkaloids that are responsible for bitter taste of lupin seeds. Optimizing the hydration conditions in order to control and predict the process is vital since hydration governs the subsequent operations and the quality of the final product. Such optimization, process control and prediction primarily call for an in-depth understanding, description and quantification of hydration kinetics of lupin seeds under different soaking conditions.

HYDRATION KINETICS OF LUPIN (LUPINUS ALBUS) SEEDS

ABSTRACT The hydration kinetics of lupin (Lupinus albus) seeds was studied by soaking in water at temperatures of 20, 30, 40 and 50C (±1C) in constant‐temperature water bath for 9 to 12 h. The weight gain due to the hydration process was determined in terms of moisture content (% dry basis). Water absorption rate was high at the early stage of hydration (20–60 min depending on temperature) followed by a decreased rate and finally approaching equilibrium condition. Peleg's equation adequately described the hydration characteristics of lupin seeds under the experimental condition (R2 = 0.96 to 0.99). The Peleg rate constant k1 decreased from 2.537 to 0.241 min/%, while Peleg capacity constant k2 increased from 0.0036 to 0.0070/% significantly (P &lt; 0.05) with an increase in temperature from 20 to 50C, demonstrating that the water absorption rate increased and water absorption capacity decreased with increase in temperature. The temperature dependence of k1 and k2 was described by Arrhenius type equation with R2 values of 0.98 and 0.97, respectively, and activation energy of 60.44 kJ/mol. The agreement between experimental and estimated values of the hydration data (R2 = 0.97 to 0.99) confirmed that Peleg's equation could be used to describe the hydration characteristics of lupin seeds under the experimental conditions considered.

Rate Coefficients and Equilibrium Constant for the CH2CHO + O2 Reaction System

The kinetics of the CH2CHO + O2 reaction was experimentally studied in two quasi-static reactors and a discharge flow-reactor at temperatures ranging from 298 to 660 K and pressures between 1 mbar and 46 bar with helium as the bath gas. The CH2CHO radicals were produced by the laser-flash photolysis of ethyl vinyl ether at 193 nm and by the reaction F + CH3CHO, respectively. Laser-induced fluorescence excited at 337 or 347.4 nm was used to monitor the CH2CHO concentration. The reaction proceeded via reversible complex formation with subsequent isomerization and fast decomposition: CH2CHO + O2 <= => O2CH2CHO --> HO2CH2CO --> products. The rate coefficients for the first and second steps were determined (k1, k-1, k2) and analyzed by a master equation with specific rate coefficients from the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Molecular and transition-state parameters were obtained from quantum chemical calculations. A third-law analysis led to the following thermodynamic parameters for the first step: Delta(R)S degrees 300K(1) = -144 J K(-1) mol(-1) (1 bar) and Delta(R)H degrees 300K(1) = (-101 +/- 4) kJ mol(-1). From the falloff analysis, the following temperature dependencies for the low- and high-pressure limiting rate coefficients were obtained: k1(0) = 5.14 x 10(-14) exp(210 K/T) cm(-3) s(-1); k1(infinity) = 1.7 x 10(-12) exp(-520 K/T) cm(-3) s(-1); and k2(infinity) = 1.3 x 10(12) exp[-(82 +/- 4) kJ mol(-1)/RT] s(-1). Readily applicable analytical representations for the pressure and temperature dependence of k1 were derived to be used in kinetic modeling.

Oxygen activation by a macrocyclic chromium complex. Mechanism of hydroperoxo-chromium(iii) to oxo-chromium(v) transformation

The transformation of the hydroperoxo complex L1(H2O)CrOOH2+ (L1 = 1, 4 8, 11-tetraazacyclotetradecane) to an oxo-chromium(v) species is a first-order process throughout the pH range examined, 1.7 < pH < 9.2. The pH dependence of the rate constant (k1) yielded an apparent pKa of 5.6 for L1(H2O)CrOOH2+. In the acidic range, (pH <4), the value of k1 is 0.191 s(-1). At the other extreme, pH >7.5, k(1)= 0.025 s(-1). No [H+]-dependence is observed within the two limiting regimes, clearly ruling out a simple attack by H+ at the hydroperoxo group. The temperature dependence of k1 in 0.020 M HClO4 yielded the activation parameters DeltaH++ = 53.7 kJ mol(-1) and DeltaS++ = -80.5 J mol(-1) K(-1).

Kinetics of the gas‐phase reaction of CF3OC(O)H with OH radicals at 242–328 K

AbstractThe rate constants, k1, of the reaction of CF3OC(O)H with OH radicals were measured by using a Fourier transform infrared spectroscopic technique in an 11.5‐dm3 reaction chamber at 242–328 K. OH radicals were produced by UV photolysis of an O3–H2O–He mixture at an initial pressure of 200 Torr. Ozone was continuously introduced into the reaction chamber during UV irradiation. With CF3OCH3 as a reference compound, k1 at 298 K was (1.65 ± 0.13) × 10−14 cm3 molecule−1 s−1. The temperature dependence of k1 was determined as (2.33 ± 0.42) × 10−12 exp[−(1480 ± 60)/T] cm3 molecule−1 s−1; possible systematic uncertainty could add an additional 20% to the k1 values. The atmospheric lifetime of CF3OC(O)H with respect to reaction with OH radicals was calculated to be 3.6 years. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 337–344 2004

Laboratory study on heterogeneous decomposition of methyl chloroform on various standard aluminosilica clay minerals as a potential tropospheric sink

Abstract. Methyl chloroform (1,1,1-trichloroethane, CH3CCl3) was found to decompose heterogeneously on seven types of standard clay minerals (23 materials) in dry air at 313 K in the laboratory. All reactions proceeded through the elimination of HCl; CH3CCl3 was converted quantitatively to CH2=CCl2. The activities of the clay minerals were compared via their pseudo-first-order reaction rate constants (k1). A positive correlation was observed between the k1 value and the specific surface area (S) of clay minerals, where the S value was determined by means of the general Brunauer-Emmett-Teller (BET) equation. The k1 value was anti-correlated with the value of n, which was a parameter of the general BET equation and related to the average pore size of the clay minerals, and correlated with the water content that can be removed easily from the clay minerals. The reaction required no special pretreatment of clay minerals, such as heating at high temperatures; hence, the reaction can be expected to occur in the environment. Photoillumination by wavelengths present in the troposphere did not accelerate the decomposition of CH3CCl3, but it induced heterogeneous photodecomposition of CH2=CCl2. The temperature dependence of k1, the adsorption equilibrium coefficient of CH3CCl3 and CH2=CCl2, and the surface reaction rate constant of CH3CCl3 were determined for an illite sample. The k1 value increased with increasing temperature. The amount of CH3CCl3 adsorbed on the illite during the reaction was proportional to the partial pressure of CH3CCl3. The reaction was sensitive to relative humidity and the k1 value decreased with increasing relative humidity. However, the reaction was found to proceed at a relative humidity of 22% at 313 K, although the k1 value was about one-twentieth of the value in non-humidified air. The conditions required for the reaction may be present in major desert regions of the world. A simple estimation indicates that the possible heterogeneous decomposition of CH3CCl3 on the ground surface in arid regions is worth taking into consideration when inferring the tropospheric lifetime of CH3CCl3 and global OH concentration from the global budget concentration of CH3CCl3.

Magnetic anisotropy in Ni–Mn–Ga martensites

We study the temperature dependence of the magnetic anisotropy of three different martensites known to exist in the Ni–Mn–Ga alloys. The anisotropy constants were determined from magnetization curves measured at different temperatures. The anisotropy of five-layered modulated tetragonal martensite is uniaxial with easy magnetization direction along short crystallographic axis. At room temperature K1(rt)=1.65×105 J m−3 and K2 is negligible. Seven-layered modulated orthorhombic martensite exhibits easy magnetization direction along the shortest crystallographic axis. K1(rt)=1.7×105 J m−3 and K2(rt)=0.9×105 J m−3 referring to hard and mid-hard magnetization axes. Nonmodulated tetragonal martensite possesses a uniaxial anisotropy with easy plane and hard magnetization direction along the long crystallographic axis with K1(rt)=−2.3×105 J m−3 and K2(rt)=0.55×105 J m−3. The temperature dependence of K1(T) of five-layered martensite follows magnetization power law with exponent n=3 suggesting a single ion origin of the magnetic anisotropy in Ni–Mn–Ga martensite.

Magnetic anisotropy and magnetostriction in a Lu2Fe17 intermetallic single crystal

The magnetic anisotropy and magnetostriction of a Lu2Fe17 single crystal are investigated. The temperature dependence of the magnetic anisotropy constant K 1 is measured in the range 4.2–300 K. The results obtained are compared with the data calculated using the Callen theoretical formula. It is found that the temperature dependence of K 1 for the Lu2Fe17 single crystal deviates from the temperature curve predicted by the localized single-ion model. The inference is drawn that a certain contribution to the magnetic anisotropy of the Lu2Fe17 compound is made by the magnetic anisotropy of band electrons. The longitudinal, transverse, and volume magnetostrictions of the Lu2Fe17 single crystal are studied, and the magnetostriction constants are calculated. It is demonstrated that the exchange integral of Lu2Fe17, as for the Y2Fe17 compound, substantially depends on the atomic volume. This dependence is responsible for the considerable difference between the Curie temperatures of the Lu2Fe17 and Y2Fe17 compounds. It is revealed that the magnetostriction of the Lu2Fe17 single crystal in the temperature range of the magnetic phase transition is determined by the two-ion exchange and single-ion contributions.

Kinetics of the Reactions of Allyl and Propargyl Radicals with CH3

The allyl-methyl and propargyl-methyl cross-radical reactions were studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants were obtained in direct real-time experiments in the temperature region 301−800 K and bath gas (helium) density (3−36) × 1016 atom cm-3. The observed overall C3H5 + CH3 (1) and C3H3 + CH3 (2) rate constants demonstrate negative temperature dependencies. Master equation modeling of collisional effects indicates that the C3H5 + CH3 reaction is near its high-pressure limit under all experimental conditions used. Minor corrections for the falloff effects (on average, 18% at the highest temperature) applied to the experimental values result in the high-pressure-limit temperature dependence of the rate constant of reaction 1: k1∞ = 1.55 × 10-9 T -0.54exp(117 K/T) cm3 molecule-1 s-1. The temperature dependence of k1∞ combined with the thermochemistry of reaction 1 results in the rate constants of the reverse reaction of thermal decomposition of 1-C4H8 k-1∞(T) =...

N-carboxymethanofuran (carbamate) formation from methanofuran and CO2 in methanogenic archaea. Thermodynamics and kinetics of the spontaneous reaction.

N-carboxymethanofuran (carbamate) formation from unprotonated methanofuran (MFR) and CO2 is the first reaction in the reduction of CO2 to methane in methanogenic archaea. The reaction proceeds spontaneously. We address here the question whether the rate of spontaneous carbamate formation is high enough to account for the observed rate of methanogenesis from CO2. The rates of carbamate formation (v1) and cleavage (v2) were determined under equilibrium conditions via 2D proton exchange NMR spectroscopy (EXSY). At pH 7.0 and 300 K the second order rate constant k1* of carbamate formation from 'MFR'(MFR + MFRH+) and 'CO2' (CO2 + H2CO3 + HCO3-+ CO32-) was found to be 7 M-1.s-1 (v1 = k1* ['MFR'] ['CO2']) while the pseudo first order rate constant k2* of carbamate cleavage was 12 s-1 (v2 = k2* [carbamate]). The equilibrium constant K* = k1*/k2* = [carbamate]/['MFR']['CO2'] was 0.6 M-1 at pH 7.0 corresponding to a free energy change DeltaG degrees ' of + 1.3 kJ.mol-1. The pH and temperature dependence of k1*, of k2* and of K* were determined. From the second order rate constant k1* it was calculated that under physiological conditions the rate of spontaneous carbamate formation is of the same order as the maximal rate of methane formation and as the rate of spontaneous CO2 formation from HCO3- in methanogenic archaea, the latter being important as CO2 is mainly present as HCO3- which has to be converted to CO2 before it can react with MFR. An enzyme catalyzed carbamate formation thus appears not to be required for methanogenesis from CO2. Consistent with this conclusion is our finding that the rate of carbamate formation was not enhanced by cell extracts of Methanosarcina barkeri and Methanobacterium thermoautotrophicum or by purified formylmethanofuran dehydrogenase which catalyzes the reduction of N-carboxymethanofuran to N-formylmethanofuran. From the concentrations of 'CO2' and of 'MFR' determined by 1D-NMR spectroscopy and the pKa of H2CO3 and of MFRH+ the concentrations of CO2 and of MFR were obtained, allowing to calculate k1 (v1 = k1 [MFR] [CO2]). The second order rate constant k1 was found to be approximately 1000 M-1 x s-1 at 300 K and pH values between 7.0 and 8. 0 which is in the order of k1 values determined for other carbamate forming reactions by stopped flow.

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.

Rate constants for reactions of OH radicals with a series of asymmetrical ethers andtert-Butyl alcohol

Absolute rate constants for the reactions of OH radicals with butyl ethyl ether (k1), methyl tert-butyl ether (k2), ethyl tert-butyl ether (k3) tert-amyl methyl ether (k4) and tert-butyl alcohol (k5) have been measured over the temperature range 230–372 K using a pulsed laser photolysis-laser induced fluorescence (PLP-LIF) technique. The temperature dependence of k1 − k5 when expressed in Arrhenius form gave: k1 = (6.59 ± 0.66) × 10 −12 exp|(362 ± 60)/T|, k2 = (5.03 ± 0.27) × 10−12 exp|&minus(133 ± 30)/T|, k3 = (4.40 ± 0.24) × 10−12 exp|(210 ± 37)/T|,k4 = (4.7 ± 0.7) × 10−12 exp|(82 ± 85)/T|, and k5 = (2.66 ± 0.48) × 10−12 exp| −(270 ± 130)/T|. However, the Arrhenius plots for k1–k5, were slightly curved and are best fitted by the three parameter fits which are given in the article. The room temperature values of k1, k2, k3, k4, and k5 are (2.08 ± 0.23) × 10−11, (3.13 ± 0.36) × 10−12, (8.80 ± 0.50) × 10−12, (6.28 ± 0.45) × 10−12, and (1.08 ± 0.10) × 10−12, respectively, in cm3 molecule−1 s−1. © 1996 John Wiley & Sons, Inc.

Kinetics of OH radical reactions with a series of ethers

AbstractThe rate constants for the reactions of OH with dimethyl ether (k1), diethyl ether (k2), di‐n‐propyl ether (k3), di‐isopropyl ether (k4), and di‐n‐butyl ether (k5) have been measured over the temperature range 230–372 K using the pulsed laser photolysis‐laser induced fluorescence (PLP‐LIF) technique. The temperature dependence of k1,k4, can be expressed in the Arrhenius plots form: k1 = (6.30 ± 0.10) × 10−12 exp[−(234 ± 34)/T] and k4 = (4.13 ± 0.10) × 10−12 exp[(274 ± 26)/T]. The Arrhenius plots for k2,k3, and k5, were curved and they were fitted to the three parameter expressions: k2 = (1.02 ± 0.08) × 10−17 T2 exp[(797 ± 24)/T], k3 = (1.84 ± 0.23) × 10−17T2 exp[(767 ± 34)/T], and k5 = (6.29 ± 0.74) × 10−18T2 exp[(1164 ± 34)/T]. The values at 298 K are (2.82 ± 0.21) × 10−12, (1.36 ± 0.11) × 10−11,(2.17 ± 0.16) × 10−11, (1.02 ± 0.10) × 10−11, and (2.69 ± 0.22) × 10−11 for k1, k2, k3, k4, and k5, respectively, (in cm3 molecule−1 s−1). These results are compared to the literature data. © 1995 John Wiley &amp; Sons, Inc.

Laser photolysis studies of hydrazine vapor: 193 and 222‐nm H‐atom primary quantum yields at 296 K, and the kinetics of H + N2H4 reaction over the temperature range 222–657 K

AbstractThe primary quantum yield of H‐atom production in the pulsed‐laser photolysis of hydrazine vapor, N2H4 + hν → H + N2H3, was measured to be (1.01 ± 0.12) at 193 nm relative to HBr photolysis, and (1.06 ± 0.16) at 222 nm relative to 248‐nm N2H4 photolysis, in excess He buffer gas at 296 K. The H‐atoms were directly monitored in the photolysis by cw‐resonance fluorescence detection of H(2S) at 121.6 nm. The high H‐atom yield observed in the photolysis is consistent with the continuous ultraviolet absorption spectrum of N2H4 involving unit dissociation of the diamine from repulsive excited singlet state(s). The laser photodissociation of N2H4 was thus used as a ‘clean’ source of H‐atoms in excess N2H4 and He buffer gas to study the gas‐phase reaction, H + N2H4 → products; (k1), in a thermostated photolysis reactor made of quartz or Pyrex. The pseudo‐first‐order temporal profiles of [H] decay immediately after photolysis were determined for a range of different hydrazine concentrations employed in the experiments to calculate the absolute second‐order reaction rate coefficient, k1. The Arrhenius expression was determined to be k1 = (11.7 ± 0.7) × 10−12 exp[−(1260 ± 20)/T] cm3 molec−1 s−1 in the temperature range 222–657 K. The rate coefficient at room temperature was, within experimental errors, independent of the He buffer gas pressure in the range 24.5–603 torr. The above temperature dependence of k1 is in excellent agreement to that we determine in our discharge flow‐tube apparatus in the temperature range 372–252 K and in 9.5 torr of He pressure. The Arrhenius parameters we report are consistent with a metathesis reaction mechanism involving the abstraction of hydrogen from N2H4 by the H‐atom. © 1995 John Wiley &amp; Sons, Inc.

Conventional and photothermally modulated ferromagnetic resonance investigations of anisotropy fields in an epitaxial Fe(001) film

Conventional angle-dependent ferromagnetic resonance (FMR) measurements on an epitaxial (001) Fe film grown on a GaAs substrate were performed as a function of the orientation of the external magnetic field Bext for three configurations: One with Bext in the film plane and two with Bext out of plane starting in plane in different crystallographic orientations. From these measurements the magnetization M, the crystalline anisotropy constants K1 and K2, the surface anisotropy Ks and an additional uniaxial anisotropy Ku were deduced self-consistently. The temperature dependence of K1 and M was investigated for the in-plane [110] orientation in the range from 100 to 570 K. In addition, locally resolved photothermally modulated FMR measurements were carried out to study the homogeneity of the magnetization and of the crystalline anisotropy across the film. Disturbances of both magnetic parameters as a function of position were observed which are related to local inhomogeneities in the semiconducting substrate.

Laser flash photolysis studies of radical–radical reaction kinetics: The O(3P J)+BrO reaction

A novel dual laser flash photolysis-long path absorption-resonance fluorescence technique has been employed to study the kinetics of the important stratospheric reaction O(3PJ)+BrO→k1Br(2PJ)+O2 as a function of temperature (231–328 K) and pressure (25–150 Torr) in N2 buffer gas. The experimental approach preserves the principal advantages of the flash photolysis method, i.e., complete absence of surface reactions and a wide range of accessible pressures, but also employs techniques which are characteristic of the discharge flow method, namely chemical titration as a means for deducing the absolute concentration of a radical reactant and use of multiple detection axes. We find that k1 is independent of pressure, and that the temperature dependence of k1 is adequately described by the Arrhenius expression k1(T)=1.91×10−11 exp(230/T) cm3 molecule−1 s−1; the absolute accuracy of measured values for k1 is estimated to vary from ±20% at T∼230 K to ±30% at T∼330 K. Our results demonstrate that the O(3PJ)+BrO rate coefficient is significantly faster than previously ‘‘guesstimated,’’ and suggest that the catalytic cycle with the O(3PJ)+BrO reaction as its rate-limiting step is the dominant stratospheric BrOx odd-oxygen destruction cycle at altitudes above 24 km.

Kinetics of the reactions of OH with alkanes

AbstractThe rate coefficients for the reactions of OH with ethane (k1), propane (k2), n‐butane (k3), iso‐butane (k4), and n‐pentane (k5) have been measured over the temperature range 212–380 K using the pulsed photolysis‐laser induced fluorescence (PP‐LIF) technique. The 298 K values are (2.43±0.20) × 10−13, (1.11 ± 0.08) × 10−12, (2.46 ± 0.15) × 10−12, (2.06 ± 0.14) × 10−12, and (4.10 ± 0.26) × 10−12 cm3 molecule−1 s−1 for k1, k2, k3, k4, and k5, respectively. The temperature dependence of k1 and k2 can be expressed in the Arrhenius form: k1 = (1.03 ± 0.07) × 10−11 exp[−(1110 ± 40)/T] and k2 = (1.01 ± 0.08) × 10−11 exp[−(660 ± 50)/T]. The Arrhenius plots for k3 – k5 were clearly curved and they were fit to three parameter expressions: k3 = (2.04 ± 0.05) × 10−17 T2 exp[(85 ± 10)/T] k4 = (9.32 ± 0.26) × 10−18 T2 exp[(275 ± 20)/T]; and k5 = (3.13 ± 0.25) × 10−17 T2 exp[(115 ± 30)/T]. The units of all rate constants are cm3 molecule−1 s−1 and the quoted uncertainties are at the 95% confidence level and include estimated systematic errors. The present measurements are in excellent agreement with previous studies and the best values for atmospheric calculations are recommended. © 1994 John Wiley &amp; Sons, Inc.