Bath Gas Research Articles

Computational study on mechanisms and pathways of the atmospheric NH2 + IO reaction

ABSTRACTThe potential-energy surfaces of the amino radical (NH2) with IO reaction have been studied at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level. Two kinds of pathways are revealed, namely H-abstraction and addition/elimination. Rice–Ramsperger–Kassel–Marcus theory and transition state theory are employed to calculate the overall and individual rate constants over a wide range of temperatures and pressures. It is predicted that, at atmospheric pressure with N2 as bath gas, the formation of P1 (HI + HNO) is the dominant pathways at 200–700 K, while the direct H-abstraction leading to P3 (3NH + HOI) takes over the reaction at a temperature above 700 K. At the high-pressure limit, IM1 [IONH2] formed by collisional stabilisation is dominant at 200–700 K; the direct H-abstraction resulting in P3 (3NH + HOI) plays an important role at higher temperatures. However, the total rate constants are independence on the pressure; however, the individual rate constants are sensitive to pressure. The atmospheric lifetime of NH2 in IO is around one week. TD-DFT computations imply that IM1 [IONH2], IM1A [IONH2′], IM2 [IN(H2)O], IM3 [OINH2], IM4 [HOINH], tra-IM5 [tra-HON(H)I] and cis-IM5 [cis-HON(H)I] will photolyze under the sunlight.

Line mixing and broadening in the v(1→3) first overtone bandhead of carbon monoxide at high temperatures and high pressures

Temperature-dependent line mixing and line broadening parameters were empirically-determined for 17 rovibrational transitions in the v(1→3) bandhead of carbon monoxide near 2.3 µm. Collisional effects on the high rotational energy lines (E″ = 5500–8600 cm−1) in the R-branch were studied over a range of temperatures from 1200–3750 K in a shock tube and heated gas cell. Measured spectra comprising the target lines in Ar and CO bath gases were fit with Voigt profiles at near-atmospheric pressures to determine line-broadening coefficients, with temperature dependence accounted by a power law. With line broadening established, line-mixing effects were examined at elevated pressures up to 60 atm at similar temperatures, reflecting conditions in high-pressure combustion environments. A modified exponential gap model for line mixing was developed to capture the pressure and temperature dependence of collisional transfer rates for the bandhead region using the relaxation matrix formalism.

Pressure-Dependent Kinetics of the Reaction between CH3O2 and OH: TRIOX Formation.

Reaction of methyl peroxy radicals with hydroxyl radicals (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy at 296 K over the 1-100 bar pressure range (bath gas He): CH3O2 + OH → CH3O + HO2 (1a), CH3O2 + OH → CH2OO + H2O (1b), and CH3O2 + OH → CH3OOOH (TRIOX) (1d). Channel 1a is the dominant channel under ambient conditions, while the contribution of channel 1b is less than 5% at 1 bar and 296 K. Channel 1c is strongly pressure-dependent and becomes dominant at high pressures. The measured branching ratio of channel 1c is α1c = 0.87 ± 0.20 at 100 bar and 296 K. The chain termination channel 1c forming important product TRIOX is experimentally evaluated over an extended pressure range (1-100 bar) for the first time. The stabilization channel 1c might play a role at ambient pressures and low temperatures as well as high pressures at ambient and elevated temperatures.

Kinetics of the OH + NO2 reaction: rate coefficients (217–333 K, 16–1200 mbar) and fall-off parameters for N2 and O2 bath gases

Abstract. The radical terminating, termolecular reaction between OH and NO2 exerts great influence on the NOy∕NOx ratio and O3 formation in the atmosphere. Evaluation panels (IUPAC and NASA) recommend rate coefficients for this reaction that disagree by as much as a factor of 1.6 at low temperature and pressure. In this work, the title reaction was studied by pulsed laser photolysis and laser-induced fluorescence over the pressure range 16–1200 mbar and temperature range 217–333 K in N2 bath gas, with experiments at 295 K (67–333 mbar) for O2. In situ measurement of NO2 using two optical absorption set-ups enabled generation of highly precise, accurate rate coefficients in the fall-off pressure range, appropriate for atmospheric conditions. We found, in agreement with previous work, that O2 bath gas has a lower collision efficiency than N2 with a relative collision efficiency to N2 of 0.74. Using the Troe-type formulation for termolecular reactions we present a new set of parameters with k0(N2) = 2.6×10-30 cm6 molecule−2 s−1, k0(O2) = 2.0×10-30 cm6 molecule−2 s−1, m=3.6, k∞=6.3×10-11 cm3 molecule−1 s−1, and Fc=0.39 and compare our results to previous studies in N2 and O2 bath gases.

Hot-spot induced mild ignition: Numerical simulation and scaling analysis

Mild ignition is a ‘non-ideal’ autoignition process observed at some conditions in rapid compression machine, shock tube and flow reactor experiments where flames and/or reaction fronts initiate due to non-uniformities within the test mixture. These flames/fronts can consume the entire mixture, or compression-heat the unburned gas, causing bulk ignition at times earlier than in homogeneous environments. This study numerically investigates mild ignition features using computational fluid dynamics techniques under a scenario where a small hot-spot is employed to initiate the process in a spherically-symmetric geometry. Only subsonic regimes are considered. The intent is to develop greater fundamental understandings of competing phenomena, and formulate a robust framework to quantify the perturbative behavior. Syngas blends (CO/H2 = 80/20) are employed as the fuel, mixed with air at lean fuel loadings of φ = 0.2 and 0.5, with the initial temperatures and pressures covering 900–1120 K, and 1.5–15 bar, respectively. Hot-spots ranging from 50 to 2,000 µm in diameter are utilized with temperatures elevated from the surroundings by 1 to 90%, i.e., Thot/T0 = 1.01–1.90. Scaling arguments are derived to analyze competing processes of: (a) quenching vs. ignition, (b) extinction vs. propagation, and (c) flame consumption/forced autoignition vs. homogeneous ignition. This work provides new insight into non-uniform autoignition phenomena, and discusses opportunities and challenges associated with mitigating it. In particular, scaling arguments indicate that experimental parameters could have potential to either suppress hot-spot initiated flame formation (e.g., via bath gases with high thermal diffusivity), or avert flame compression of the end gas (e.g., via lean/diluted mixtures, or bath gases with high heat capacity).

Energy transfer in intermolecular collisions of polycyclic aromatic hydrocarbons with bath gases He and Ar.

Classical trajectory simulations of intermolecular collisions were performed for a series of polycyclic aromatic hydrocarbons interacting with the bath gases helium and argon for bath gas temperature from 300 to 2500 K. The phase-space average energy transferred per deactivating collision, ⟨∆Edown⟩, was obtained. The Buckingham pairwise intermolecular potentials were validated against high-level quantum chemistry calculations and used in the simulations. The reactive force-field was used to describe intramolecular potentials. The dependence of ⟨∆Edown⟩ on initial vibrational energy is discussed. A canonical sampling method was compared with a microcanonical sampling method for selecting initial vibrational energy at high bath gas temperatures. Uncertainties introduced by the initial angular momentum distribution were identified. The dependence of the collisional energy transfer parameters on the type of bath gas and the molecular structure of polycyclic aromatic hydrocarbons was examined.

Comparing Empty and Filled Fullerene Cages with High-Resolution Trapped Ion Mobility Spectrometry.

We have used trapped ion mobility spectrometry (TIMS) to obtain highly accurate experimental collision cross sections (CCS) for the fullerene C80- and the endohedral metallofullerenes La2@C80-, Sc3N@C80-, and Er3N@C80- in molecular nitrogen. The CCS values of the endohedral fullerenes are 0.2% larger than that of the empty cage. Using a combination of density functional theory and trajectory calculations, we were able to reproduce these experimental findings theoretically. Two effects are discussed that contribute to the CCS differences: (i) a small increase in fullerene cage size upon endohedral doping and (ii) charge transfer from the encapsulated moieties to the cage thus increasing the attractive charge-induced dipole interaction between the (endohedral) fullerene ion and the nitrogen bath gas molecules.

Exploring the Conformational Space of Growth-Hormone-Releasing Hormone Analogues Using Dopant Assisted Trapped Ion Mobility Spectrometry-Mass Spectrometry.

Recently, we proposed a high-throughput screening workflow for the elucidation of agonistic or antagonistic growth hormone-releasing hormone (GHRH) potencies based on structural motif descriptors as a function of the starting solution. In the present work, we revisited the influence of solution and gas-phase GHRH molecular microenvironment using trapped ion mobility-mass spectrometry (TIMS-MS). The effect of the starting solvent composition (10 mM ammonium acetate (NH4Ac), 50% methanol (MeOH), 50% acetonitrile (MeCN), and 50% acetone (Ac)) and gas-phase modifiers (N2, N2 + MeOH, N2 + MeCN, and N2 + Ac) on the conformational states of three GHRH analogues, GHRH (1-29), MR-406, and MIA-602, is described as a function of the trapping time (100-500 ms). Changes in the mobility profiles were observed showing the dependence of the conformational states of GHRH analogues according to the molecular microenvironment in solution, suggesting the presence of solution memory effects on the gas-phase observed structures. Modifying the bath gas composition resulted in smaller mobilities that are correlated with the size and mass of the organic modifier, and more importantly led to substantial changes in relative abundances of the IMS profiles. We attributed the observed changes in the mobility profiles by a clustering/declustering mechanism between the GHRH analogue ions and the gas modifiers, redefining the free energy landscape and leading to other local minima structures. Moreover, inspection of the mobility profiles as a function of the trapping time (100-500 ms) allowed for conformational interconversions toward more stable "gas-phase" structures. These experiments enabled us to outline a more detailed description of the structures and intermediates involved in the biological activity of GHRH, MR-406, and MIA-602.

Can soot primary particle size distributions be determined using laser-induced incandescence?

Soot from combustion processes often takes the form of fractal-like aggregates, assembled of primary particles, both of which obey polydisperse size distributions. In this work, the possibility of determining the primary particle size distribution through time-resolved laser-induced incandescence (TiRe-LII) under the influence of thermal shielding of polydispersely distributed aggregates is critically investigated for two typical measurement situations: in-flame measurements at high temperature and a soot-laden aerosol at room temperature. The uncertainty attached to the quantities is evaluated through Bayesian inference. We show how different kinds of prior knowledge concerning the aggregation state of the aerosol affect the uncertainties of the recovered size distribution parameters of the primary particles. To obtain reliable estimates for the primary particle size distribution parameters, specific information about the aggregate size distribution is required. This is especially the case for cold bath gases, where thermal shielding has a large effect. Furthermore, it is crucial to use the full duration of the usable LII signal trace to recover the width of the size distribution with small uncertainties. The uncertainty attached to TiRe-LII inferred primary particle size parameters becomes considerably larger when additional model parameters are considered.

Rate Coefficients for the Gas-Phase Reaction of ( E)- and ( Z)-CF3CF═CFCF3 with the OH Radical and Cl-Atom.

The rate coefficients, k, for the gas-phase reaction of the OH radical and Cl-atom with ( E)- and ( Z)-CF3CF═CFCF3 were measured using a relative rate technique over a range of temperature (240-375 K) and bath gas pressure (50-630 Torr, He). The obtained rate coefficients were found to be independent of pressure under these conditions. The obtained rate coefficients for the reaction of Cl-atom with ( E)- and ( Z)-CF3CF═CFCF3 at 296 K were k1(296 K) = (7.23 ± 0.3) × 10-12 cm3 molecule-1 s-1 and k2(296 K) = (6.70 ± 0.3) × 10-12 cm3 molecule-1 s-1, respectively, with the temperature dependence described by the Arrhenius expressions: k1( T) = (3.47 ± 0.35) × 10-12 exp[(210 ± 25)/ T] cm3 molecule-1 s-1 and k2( T) = (3.37 ± 0.35) × 10-12 exp[(199 ± 25)/ T] cm3 molecule-1 s-1. The rate coefficients for the OH radical reaction with ( E)- and ( Z)-CF3CF═CFCF3 were found to be k3(296-375 K) = (4.34 ± 0.45) × 10-13 cm3 molecule-1 s-1 and k4(296-375 K) = (3.30 ± 0.35) × 10-13 cm3 molecule-1 s-1, respectively. The quoted rate coefficient uncertainties are 2σ (95% confidence level) and include estimated systematic errors. The rate coefficients for the reaction of OH with a mixture of the two stereoisomers were determined using a pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) technique for comparison with previous kinetic measurements using stereoisomer mixtures. The effective rate coefficient for the 0.7/0.3 ( E)/( Z) stereoisomer sample was found to be nearly independent of temperature over the range 222-375 K with a value of (4.47 ± 0.36) × 10-13 cm3 molecule-1 s-1. The atmospheric lifetimes for ( E)- and ( Z)-CF3CF═CFCF3 due to OH-reactive loss are estimated to be 25 and 35 days, respectively. The lifetime-corrected radiative efficiencies (W m-2 ppb-1) and 100 year time horizon global warming potentials derived in this work are 0.05 and 1.2 for ( E)-CF3CF═CFCF3 and 0.13 and 4.1 for ( Z)-CF3CF═CFCF3. The photochemical ozone creation potentials for ( E)- and ( Z)-CF3CF═CFCF3 are estimated to be 2.5 and 2.1, respectively.

Ligation Kinetics as a Probe for Non-Covalent Electrostatic Bonding and Electron Solvation of Alkali and Alkaline Earth Cations with Ammonia.

Mono-ligation kinetics were measured for ammonia reacting with atomic cations in the first two groups of the periodic table (K+, Rb+, Cs+ and Ca+, Sr+, Ba+). Also, mono-ligation energies were computed using density functional theory (DFT) in an attempt to assess the role of non-covalent electrostatic interactions in these chemical reactions. The measurements were performed at room temperature in helium bath gas at 0.35Torr using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients are reported for ammonia addition, the only reaction channel that was observed with all these cations. A systematic decrease in the rate of addition of NH3 was observed for both group 1 and 2 cations going down the periodic table. The computational studies predict a decrease in the adduct binding energy and an increase in the bond separation going down groups 1 and 2 of the periodic table and provide some insight into the role of the extra selectron in the group 2 radical cations in ligand bonding. A correlation is seen between the efficiency of ligation and the binding energy of the adduct ion and attributed to the lifetime of the intermediate encounter complex against back dissociation which is dependent on its well depth. Higher-order additions of ammonia were also observed. Remarkable differences in the extent and kinetics were seen between the group 1 and 2 cations, and these were attributed to the occurrence of ammonia solvation of the extra s electron in the higher-order adducts of the alkaline earth cations.

Effects of Bath Gas and NOx Addition on n-Pentane Low-Temperature Oxidation in a Jet-Stirred Reactor

The oxidation of n-pentane (C5H12) in different bath gases (He, Ar, and CO2) and in Ar with NO2 or NO addition has been studied in a jet-stirred reactor at 107 kPa, temperatures between 500 and 110...

The impact of bath gas composition on the calibration of photoacoustic spectrometers with ozone at discrete visible wavelengths spanning the Chappuis band

Abstract. Photoacoustic spectroscopy is a sensitive in situ technique for measuring the absorption coefficient for gas and aerosol samples. Photoacoustic spectrometer (PAS) instruments require accurate calibration by comparing the measured photoacoustic response with a known level of absorption for a calibrant. Ozone is a common calibrant of PAS instruments, yet recent work by Bluvshtein et al. (2017) has cast uncertainty on the validity of ozone as a calibrant at a wavelength of 405 nm. Moreover, Fischer and Smith (2018) demonstrate that a low O2 mass fraction in the bath gas can bias the measured PAS calibration coefficient to lower values for wavelengths in the range 532–780 nm. In this contribution, we present PAS sensitivity measurements at wavelengths of 405, 514 and 658 nm using ozone-based calibrations with variation in the relative concentrations of O2 and N2 bath gases. We find excellent agreement with the results of Fischer and Smith at the 658 nm wavelength. However, the PAS sensitivity decreases significantly as the bath gas composition tends to pure oxygen for wavelengths of 405 and 514 nm, which cannot be rationalised using arguments presented in previous studies. To address this, we develop a model to describe the variation in PAS sensitivity with both wavelength and bath gas composition that considers Chappuis band photodynamics and recognises that the photoexcitation of O3 leads rapidly to the photodissociation products O(3P) and O2(X, v > 0). We show that the rates of two processes are required to model the PAS sensitivity correctly. The first process involves the formation of vibrationally excited O3(X̃) through the reaction of the nascent O(3P) with bath gas O2. The second process involves the quenching of vibrational energy from the nascent O2(X, v > 0) to translational modes of the bath gas. Both of these processes proceed at different rates in collisions with N2 or O2 bath gas species. Importantly, we show that the PAS sensitivity is optimised for our PAS instruments when the ozone-based calibration is performed in a bath gas with a similar composition to ambient air and conclude that our methods for measuring aerosol absorption using an ozone-calibrated PAS are accurate and without detectable bias. We emphasise that the dependence of PAS sensitivity on bath gas composition is wavelength-dependent, and we recommend strongly that researchers characterise the optimal bath gas composition for their particular instrument.

Parameterization Strategies for Intermolecular Potentials for Predicting Trajectory-Based Collision Parameters.

The accuracy of separable strategies for constructing full-dimensional potential energy surfaces for collisional energy transfer and collision rate calculations is studied systematically for three alcohols (A = methanol, ethanol, and butanol) and three bath gases (M = Ar, N2, and H2O). The fitting efficiency (defined as the number of ab initio data required to achieve parametrizations of a desired accuracy) is quantified for both pairwise (Buckingham or "exp6") and nonpairwise (permutationally invariant polynomials, PIPs, of Morse variables) functional forms, for four sampling strategies, and as a function of the complexity and anisotropy of the interaction potential. We find that convergence with respect to the number of sampled ab initio data is largely independent of the choice of functional form but instead varies nearly linearly with the number of adjustable parameters and depends strongly on the sampling strategy. Specifically, the use of biased Sobol quasirandom sampling is ∼7× more efficient than using unbiased pseudorandom sampling, on average, requiring just ∼3 computed ab initio energies per adjustable fitting parameter. The pairwise exp6 functional form is shown to provide accurate and transferable parametrizations for M = Ar but is unable to accurately describe alcohol interactions with M = N2 and H2O. The nonpairwise PIP functional form, which is systematically improvable, can produce separable parametrizations with arbitrarily small fitting errors. However, these can suffer from overfitting, which is demonstrated using dynamics calculations of collision parameters for a large number of exp6 and PIP parametrizations. The tests described here validate a robust strategy for automatically generating A + M potential energy surfaces with minimal human intervention, including a quantifiable out of sample metric for judging the accuracy of the fitted surface. We further analyze this set of automatically generated potential energy surfaces to identify areas where more sophisticated fitting strategies may be desired, including pruning of the PIP expansions for large systems and improved sampling strategies more closely coupled with the description of the functional form.

Kinetic study of the OH + ethylene reaction using frequency‐modulated laser absorption spectroscopy

AbstractThe pressure dependence of the OH + C2H4 addition reaction has been investigated using frequency‐modulated laser absorption spectroscopy to monitor OH kinetics. Bimolecular rate coefficients for the title reaction are reported in argon bath gas at room temperature and total pressures ranging from 2 to 361 Torr. The pressure‐dependent rate coefficients measured here agree well with the majority of published kinetic studies under similar conditions. Previous high‐level ab initio calculations have identified a prereaction complex on the OH + C2H4 potential energy surface. The influence of this complex on the OH + C2H4 kinetics has been investigated using one‐dimensional master equation analyses of the current and previous experimental measurements.

Influence of gas modifiers on the TIMS analysis of familiar explosives

In the present work, we studied the influence of the bath composition (e.g., organic modifiers) on the mobility resolving power, resolution, and lifetime of familiar explosives during trapped ion mobility spectrometry (TIMS). Experimental results showed the dependence of the mobility with the organic modifiers (mass and size) for the case of TIMS-MS. Different from trends observed in drift tube like IMS devices, no correlation between the mobility resolving power and resolution in TIMS was observed with the bath gas composition (e.g., air, air + methanol, air +2-propanol, and air + acetone). Time decay plots showed that common explosives with adduct complexes signal decrease over time as a function of the trapping time, without any significant improvement with the addition of the organic modifiers. Theoretical calculation of potential clustering and dissociation pathways supported the time decay findings since no major energetic differences between the pathways were observed as a function of the organic modifiers. Our findings suggest that beside the size of the collision partner, there are specific intermolecular dynamics that drive the trapping behavior of familiar explosives.

Pressure effects on the vibrational and rotational relaxation of vibrationally excited OH (ν, J) in an argon bath.

Quasi-classical molecular dynamics simulations were used to study the energy relaxation of an initially non-rotating, vibrationally excited (ν = 4) hydroxyl radical (OH) in an Ar bath at 300 K and at high pressures from 50 atm to 400 atm. A Morse oscillator potential represented the OH, and two sets of interaction potentials were used based on whether the Ar-H potential was a Buckingham (Exp6) or a Lennard-Jones (LJ) potential. The vibrational and rotational energies were monitored for 25 000-90 000 ps for Exp6 trajectories and 5000 ps for LJ trajectories. Comparisons to measured vibrational relaxation rates show that Exp6 rates are superior. Simulated initial vibrational relaxation rates are linearly proportional to pressure, implying no effect of high-pressure breakdown in the isolated binary collision approximation. The vibrational decay curves upward from single-exponential decay. A model based on transition rates that exponentially depend on the anharmonic energy gap between vibrational levels fits the vibrational decay well at all pressures, suggesting that anharmonicity is a major cause of the curvature. Due to the competition of vibration-to-rotation energy transfer and bath gas relaxation, the rotational energy overshoots and then relaxes to its thermal value. Approximate models with adjustable rates for this competition successfully reproduced the rotational results. These models show that a large fraction of the vibrational energy loss is initially converted to rotational energy but that fraction decreases rapidly as the vibrational energy content of OH decreases. While simulated rates change dramatically between Exp6 and LJ potentials, the mechanisms remain the same.

Experimental and Computational Studies of Unimolecular 1,1-HX (X = F, Cl) Elimination Reactions of C2D5CHFCl: Role of Carbene:HF and HCl Adducts in the Exit Channel of RCHFCl and RCHCl2 Reactions.

The gas-phase unimolecular reactions of C2D5CHFCl molecules with 94 kcal mol-1 of vibrational energy have been studied by the chemical-activation experimental technique and by electronic-structure computations. Products from the reaction of C2D5CHFCl molecules, formed by the recombination of C2D5 and CHFCl radicals in a room temperature bath gas, were measured by gas chromatography-mass spectrometry. The 2,1-DCl (81%) and 1,1-HCl (17%) elimination reactions are the principal processes, but 2,1-DF and 1,1-HF elimination reactions also are observed. Comparison of experimental rate constants to calculated statistical rate constants provides threshold energies. The potential surfaces associated with C2D5(F)C: + HCl and C2D5(Cl)C: + HF reactions are of special interest because hydrogen-bonded adducts with HCl and HF with dissociation energies of 6.4 and 9.3 kcal mol-1, respectively, are predicted by calculations. The relationship between the geometries and threshold energies of transition states for 1,1-HCl elimination and carbene:HCl adducts is complex, and previous studies of related molecules, such as CD3CHFCl, CD2ClCHFCl, C2D5CHCl2, and halogenated methanes are included in the computational analysis. Extensive calculations for CH3CHFCl as a model for 1,1-HCl reactions illustrate properties of the exit-channel potential energy surface. Since the 1,1-HCl transition state is submerged relative to dissociation of the adduct, inner and outer transition states should be considered for analysis of rate constants describing 1,1-HCl elimination and addition reactions of carbenes to HCl.

Thermal decomposition of CH3I revisited: Consistent calibration of I‐atom concentrations behind shock waves with dual I‐/H‐ARAS

AbstractIodinated hydrocarbons are often used as precursors for hydrocarbon radicals in shock‐tube experiments. The radicals are produced by C─I bond fission reaction, and their formation can be followed through time‐resolved monitoring of the complementary I‐atom concentrations, for example, by I‐atom resonance absorption spectroscopy (I‐ARAS). This very sensitive technique requires, however, an independent calibration. As a very clean source of I atoms, CH3I is particularly well suited as calibration system for I‐ARAS presumed the yield of I atoms and the rate coefficient of I‐atom formation from CH3I are known with sufficient accuracy. But if the formation of I atoms from CH3I by I‐ARAS is to be characterized, an independent calibration system is required.In this study, we propose a cross‐calibration approach for I‐ARAS based on the simultaneous time‐resolved monitoring of I and H atoms by ARAS in C2H5I pyrolysis experiments. For this reaction system, it can be shown that at sufficiently short reaction times very similar amounts of I and H atoms are formed (difference <1%). As calibration of H‐ARAS, with mixtures of N2O and H2, is a well‐established technique, we calibrated I‐atom absorption–time profiles with respect to simultaneously recorded H‐atom concentration–time profiles. Using this approach, we investigated the thermal decomposition of CH3I in the temperature range 950–2050 K behind reflected shock waves at two different nominal pressures (p ∼ 0.4 and 1.6 bar, bath gas: Ar). From the obtained absolute I‐atom concentration–time profiles at temperatures T < 1250 K, we inferred a second‐order rate coefficient k(T) = (1.7 ± 0.7) × 1015 exp(–20020 K/T) cm3 mol–1 s–1 for the reaction CH3I + Ar → CH3 + I + Ar. A small mechanism to describe the pyrolysis of CH3I under shock‐tube conditions is presented and discussed.

Measurement of the reaction rate of H + O2 + M → HO2 + M, for M= Ar, N2, CO2, at high temperature with a sensitive OH absorption diagnostic

The reaction rate of H + O2+M = HO2+M in the low-pressure limit was determined in the temperature range of 1450– 2000 K, with Argon, Nitrogen, and Carbon Dioxide as the third-body collision partners, by measuring the OH time-history after the induction time during lean oxidation of Hydrogen. Test conditions were optimized to suppress the sensitivity of OH to interfering reactions after the induction time, using dilute and extremely lean mixtures at these temperatures. The strong Q1(5) transition in the A-X(0,0) band of OH was used as the probing wavelength for the diagnostic. Calibration measurements were performed prior to the experiments to estimate the effects of pressure shift and collision broadening on the absorption coefficient of OH for each of the bath gas species, which enabled a reduction in the uncertainty in the absorption cross-section of OH. Aided by the calibrated and improved OH diagnostic, the reaction rate constants were determined at high temperatures, with low scatter, and tight uncertainty bounds. The measured rate constants also agree with the extrapolation of previous investigations at lower temperatures. Combined rate constant expressions based on the lower temperature measurements of Shao et al. (2018) and the current high-temperature measurements are proposed, which are valid over the temperature range of 1000–2000 K:k2,Ar=(2.66*1019)*T−1.36cm6mol−2s−1(±9%)k2,N2=(2.25*1021)*T−1.95cm6mol−2s−1(±23%)k2,CO2=(2.23*1018)*T−0.79cm6mol−2s−1(±28%)