Pulsed Laval Nozzle Apparatus Research Articles

Experimental and theoretical study of the low-temperature kinetics of the reaction of CN with CH2O and implications for interstellar environments.

Rate coefficients for the reaction of CN with CH2O were measured for the first time below room temperature in the range 32-103 K using a pulsed Laval nozzle apparatus together with the Pulsed Laser Photolysis-Laser-Induced Fluorescence technique. The rate coefficients exhibited a strong negative temperature dependence, reaching (4.62 ± 0.84) × 10-11 cm3 molecule-1 s-1 at 32 K, and no pressure dependence was observed at 70 K. The potential energy surface (PES) of the CN + CH2O reaction was calculated at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory, with the lowest energy channel to reaction characterized by the formation of a weakly-bound van der Waals complex, bound by 13.3 kJ mol-1, prior to two transition states with energies of -0.62 and 3.97 kJ mol-1, leading to the products HCN + HCO or HNC + HCO, respectively. For the formation of formyl cyanide, HCOCN, a large activation barrier of 32.9 kJ mol-1 was calculated. Reaction rate theory calculations were performed with the MESMER (Master Equation Solver for Multi Energy well Reactions) package on this PES to calculate rate coefficients. While this ab initio description provided good agreement with the low-temperature rate coefficients, it was not capable of describing the high-temperature experimental rate coefficients from the literature. However, increasing the energies and imaginary frequencies of both transition states allowed MESMER simulations of the rate coefficients to be in good agreement with data spanning 32-769 K. The mechanism for the reaction is the formation of a weakly-bound complex followed by quantum mechanical tunnelling through the small barrier to form HCN + HCO products. MESMER calculations showed that channel generating HNC is not important. MESMER simulated the rate coefficients from 4-1000 K which were used to recommend best-fit modified Arrhenius expressions for use in astrochemical modelling. The UMIST Rate12 (UDfa) model yielded no significant changes in the abundances of HCN, HNC, and HCO for a variety of environments upon inclusion of rate coefficients reported here. The main implication from this study is that the title reaction is not a primary formation route to the interstellar molecule formyl cyanide, HCOCN, as currently implemented in the KIDA astrochemical model.

Measurements of Low Temperature Rate Coefficients for the Reaction of CH with CH2O and Application to Dark Cloud and AGB Stellar Wind Models

Abstract Rate coefficients have been measured for the reaction of CH radicals with formaldehyde, CH2O, over the temperature range of 31–133 K using a pulsed Laval nozzle apparatus combined with pulsed laser photolysis and laser-induced fluorescence spectroscopy. The rate coefficients are very large and display a distinct decrease with decreasing temperature below 70 K, although classical collision rate theory fails to reproduce this temperature dependence. The measured rate coefficients have been parameterized and used as input for astrochemical models for both dark cloud and Asymptotic Giant Branch stellar outflow scenarios. The models predict a distinct change (up to a factor of two) in the abundance of ketene, H2CCO, which is the major expected molecular product of the CH + CH2O reaction.

Low temperature gas phase reaction rate coefficient measurements: Toward modeling of stellar winds and the interstellar medium

AbstractStellar winds of Asymptotic Giant Branch (AGB) stars are responsible for the production of ∼85% of the gas molecules in the interstellar medium (ISM), and yet very few of the gas phase rate coefficients under the relevant conditions (10 – 3000 K) needed to model the rate of production and loss of these molecules in stellar winds have been experimentally measured. If measured at all, the value of the rate coefficient has often only been obtained at room temperature, with extrapolation to lower and higher temperatures using the Arrhenius equation. However, non-Arrhenius behavior has been observed often in the few measured rate coefficients at low temperatures. In previous reactions studied, theoretical simulations of the formation of long-lived pre-reaction complexes and quantum mechanical tunneling through the barrier to reaction have been utilized to fit these non-Arrhenius behaviours of rate coefficients.Reaction rate coefficients that were predicted to produce the largest change in the production/loss of Complex Organic Molecules (COMs) in stellar winds at low temperatures were selected from a sensitivity analysis. Here we present measurements of rate coefficients using a pulsed Laval nozzle apparatus with the Pump Laser Photolysis - Laser Induced Fluorescence (PLP-LIF) technique. Gas flow temperatures between 30 – 134 K have been produced by the University of Leeds apparatus through the controlled expansion of N2 or Ar gas through Laval nozzles of a range of Mach numbers between 2.49 and 4.25.Reactions of interest include those of OH, CN, and CH with volatile organic species, in particular formaldehyde, a molecule which has been detected in the ISM. Kinetics measurements of these reactions at low temperatures will be presented using the decay of the radical reagent. Since formaldehyde and the formal radical (HCO) are potential building blocks of COMs in the interstellar medium, low temperature reaction rate coefficients for their production and loss can help to predict the formation pathways of COMs observed in the interstellar medium.

The importance of OH radical–neutral low temperature tunnelling reactions in interstellar clouds using a new model

Recent laboratory experiments using a pulsed Laval nozzle apparatus have shown that reactions between a neutral molecule and the radical OH can occur efficiently at low temperatures despite activation energy barriers if there is a hydrogen-bonded complex in the entrance channel which allows the system to tunnel efficiently under the barrier. Since OH is a major radical in the interstellar medium, this class of reactions may well be important in the chemistry that occurs in the gas phase of interstellar clouds. Using a new gas-grain chemical network with both gas-phase reactions and reactions on the surfaces of dust particles, we studied the role of OH–neutral reactions in dense interstellar clouds at 10, 50, and 100 K. We determined that at least one of these reactions can be significant, especially at the lowest temperatures studied, where the rate constants are large. It was found in particular that the reaction between CH3OH and OH provides an effective and unambiguous gas-phase route to the production of the gaseous methoxy radical (CH3O), which has been recently detected in cold, dense interstsellar clouds. The role of other reactions in this class is explored.

Measurements of Rate Coefficients for Reactions of OH with Ethanol and Propan-2-ol at Very Low Temperatures.

The low temperature kinetics of the reactions of OH with ethanol and propan-2-ol have been studied using a pulsed Laval nozzle apparatus coupled with pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) spectroscopy. The rate coefficients for both reactions have been found to increase significantly as the temperature is lowered, by approximately a factor of 18 between 293 and 54 K for ethanol, and by ∼10 between 298 and 88 K for OH + propan-2-ol. The pressure dependence of the rate coefficients provides evidence for two reaction channels: a zero pressure bimolecular abstraction channel leading to products and collisional stabilization of a weakly bound OH-alcohol complex. The presence of the abstraction channel at low temperatures is rationalized by a quantum mechanical tunneling mechanism, most likely through the barrier to hydrogen abstraction from the OH moiety on the alcohol.

Low Temperature Kinetics of the CH3OH + OH Reaction

The rate constant of the reaction between methanol and the hydroxyl radical has been studied in the temperature range 56-202 K by pulsed laser photolysis-laser induced fluorescence in two separate experiments using either a low temperature flow tube coupled to a time-of-flight mass spectrometer or a pulsed Laval nozzle apparatus. The two independent techniques yield rate constants that are in mutual agreement and consistent with the results reported previously below 82 K [Shannon et al. Nat. Chem. 2013, 5, 745-749] and above 210 K [Dillon et al. Phys. Chem. Chem. Phys. 2005, 7, 349-355], showing a very sharp increase with decreasing temperature with an onset around 180 K. This onset is also signaled by strong chemiluminescence tentatively assigned to formaldehyde, which is consistent with the formation of the methoxy radical at low temperature by quantum tunnelling, and its subsequent reaction with H and OH. Our results add confidence to the previous low temperature rate constant measurements and consolidate the experimental reference data set for further theoretical work required to describe quantitatively the tunnelling mechanism operating in this reaction. An additional measurement of the rate constant at 56 K yielded a value of (4.9 ± 0.8) × 10(-11) cm(3) molecule(-1) s(-1) (2σ), showing that the rate constant is increasing less rapidly at temperatures below 70 K.

Bimolecular Rate Constant and Product Branching Ratio Measurements for the Reaction of C2H with Ethene and Propene at 79 K

The reactions of the ethynyl radical (C(2)H) with ethene (C(2)H(4)) and propene (C(3)H(6)) are studied under low temperature conditions (79 K) in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by 193 nm photolysis of acetylene (C(2)H(2)) and the reactions are studied in nitrogen as a carrier gas. Reaction products are sampled and subsequently photoionized by the tunable vacuum ultraviolet radiation of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. The product ions are detected mass selectively and time-resolved by a quadrupole mass spectrometer. Bimolecular rate coefficients are determined under pseudo-first-order conditions, yielding values in good agreement with previous measurements. Photoionization spectra are measured by scanning the ALS photon energy while detecting the ionized reaction products. Analysis of the photoionization spectra yields-for the first time-low temperature isomer resolved product branching ratios. The reaction between C(2)H and ethene is found to proceed by H-loss and yields 100% vinylacetylene. The reaction between C(2)H and propene results in (85 ± 10)% C(4)H(4) (m/z = 52) via CH(3)-loss and (15 ± 10)% C(5)H(6) (m/z = 66) by H-loss. The C(4)H(4) channel is found to consist of 100% vinylacetylene. For the C(5)H(6) channel, analysis of the photoionization spectrum reveals that (62 ± 16)% is in the form of 4-penten-1-yne, (27 ± 8)% is in the form of cis- and trans-3-penten-1-yne and (11 ± 10)% is in the form of 2-methyl-1-buten-3-yne.

Airfoil sampling of a pulsed Laval beam with tunable vacuum ultraviolet synchrotron ionization quadrupole mass spectrometry: Application to low-temperature kinetics and product detection

A new pulsed Laval nozzle apparatus with vacuum ultraviolet (VUV) synchrotron photoionization quadrupole mass spectrometry is constructed to study low-temperature radical-neutral chemical reactions of importance for modeling the atmosphere of Titan and the outer planets. A design for the sampling geometry of a pulsed Laval nozzle expansion has been developed that operates successfully for the determination of rate coefficients by time-resolved mass spectrometry. The new concept employs airfoil sampling of the collimated expansion with excellent sampling throughput. Time-resolved profiles of the high Mach number gas flow obtained by photoionization signals show that perturbation of the collimated expansion by the airfoil is negligible. The reaction of C(2)H with C(2)H(2) is studied at 70 K as a proof-of-principle result for both low-temperature rate coefficient measurements and product identification based on the photoionization spectrum of the reaction product versus VUV photon energy. This approach can be used to provide new insights into reaction mechanisms occurring at kinetic rates close to the collision-determined limit.

Reaction of the C2H Radical with 1-Butyne (C4H6): Low-Temperature Kinetics and Isomer-Specific Product Detection

The rate coefficient for the reaction of the ethynyl radical (C(2)H) with 1-butyne (H-C[triple bond]C-CH(2)-CH(3)) is measured in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by laser photolysis of acetylene (C(2)H(2)) at 193 nm and detected via chemiluminescence (C(2)H + O(2) --> CH (A(2)Delta) + CO(2)). The rate coefficients are measured over the temperature range of 74-295 K. The C(2)H + 1-butyne reaction exhibits no barrier and occurs with rate constants close to the collision limit. The temperature-dependent rate coefficients can be fit within experimental uncertainties by the expression k = (2.4 +/- 0.5) x 10(-10)(T/295 K)(-(0.04+/-0.03)) cm(3) molecule(-1) s(-1). Reaction products are detected at room temperature (295 K) and 533 Pa using a multiplexed photoionization mass spectrometer (MPIMS) coupled to the tunable vacuum ultraviolet synchrotron radiation from the Advanced Light Source at the Lawrence Berkeley National Laboratory. Two product channels are identified for this reaction: m/z = 64 (C(5)H(4)) and m/z = 78 (C(6)H(6)) corresponding to the CH(3)-loss and H-loss channels, respectively. Photoionization efficiency (PIE) curves are used to analyze the isomeric composition of both product channels. The C(5)H(4) products are found to be exclusively linear isomers composed of ethynylallene and methyldiacetylene in a 4:1 ratio. In contrast, the C(6)H(6) product channel includes two cyclic isomers, fulvene 18(+/-5)% and 3,4-dimethylenecyclobut-1-ene (DMCB) 32(+/-8)%, as well as three linear isomers, 2-ethynyl-1,3-butadiene 8(+/-5)%, 3,4-hexadiene-1-yne 28(+/-8)%, and 1,3-hexadiyne 14(+/-5)%. Within experimental uncertainties, we do not see appreciable amounts of benzene and an upper limit of 10% is estimated. Diacetylene (C(4)H(2)) formation via the C(2)H(5)-loss channel is also thermodynamically possible but cannot be observed due to experimental limitations. The implications of these results for modeling of planetary atmospheres, especially of Saturn's largest moon Titan and the relationships to combustion reactions, are discussed.

Observation of a large negative temperature dependence for rate coefficients of reactions of OH with oxygenated volatile organic compounds studied at 86–112 K

The rate coefficients (k) for reactions of OH with acetone, methyl ethyl ketone (MEK) and dimethyl ether (DME) have been measured in the temperature range 86-112 K using a pulsed Laval nozzle apparatus. Large increases in k at lower temperatures were observed, with k(86K)/k(295K) = 334 for acetone, and k(93K)/k(295K) = 72 and 3, for MEK and DME respectively. A mechanism involving the formation of a hydrogen bonded complex prior to an overall barrier on the potential energy surface is proposed to explain this behaviour.

Pulsed Laval nozzle study of the kinetics of OH with unsaturated hydrocarbons at very low temperatures

The kinetics of reactions of the OH radical with ethene, ethyne (acetylene), propyne (methyl acetylene) and t-butyl-hydroperoxide were studied at temperatures of 69 and 86 K using laser flash-photolysis combined with laser-induced fluorescence spectroscopy. A new pulsed Laval nozzle apparatus is used to provide the low-temperature thermalised environment at a single density of approximately 4x10(16) molecule cm(-3) in N2. The density and temperature within the flow are determined using measurements of impact pressure and rotational populations from laser-induced fluorescence spectroscopy of NO and OH. For ethene, rate coefficients were determined to be k2=(3.22+/-0.46)x10(-11) and (2.12+/-0.12)x10(-11) cm3 molecule(-1) s(-1) at T=69 and 86 K, respectively, in good agreement with a master-equation calculation utilising an ab initio surface recently calculated for this reaction by Cleary et al. (P. A. Cleary, M. T. Baeza Romero, M. A. Blitz, D. E. Heard, M. J. Pilling, P. W. Seakins and L. Wang, Phys. Chem. Chem. Phys., 2006, 8, 5633-5642) For ethyne, no previous data exist below 210 K and a single measurement at 69 K was only able to provide an approximate upper limit for the rate coefficient of k3<1x10(-12) cm3 molecule(-1) s(-1), consistent with the presence of a small activation barrier of approximately 5 kJ mol(-1) between the reagents and the OH-C2H2 adduct. For propyne, there are no previous measurements below 253 K, and rate coefficients of k4=(5.08+/-0.65), (5.02+/-1.11) and (3.11+/-0.09)x10(-12) cm3 molecule(-1) s(-1) were obtained at T=69, 86 and 299 K, indicating a much weaker temperature dependence than for ethene. The rate coefficient k1=(7.8+/-2.5)x10(-11) cm3 molecule(-1) s(-1) was obtained for the reaction of OH with t-butyl-hydroperoxide at T=86 K. Studies of the reaction of OH with benzene and toluene yielded complex kinetic profiles of OH which did not allow the extraction of rate coefficients. Uncertainties are quoted at the 95% confidence limit and include systematic errors.

Low-Temperature Rate Coefficients for the Reaction of Ethynyl Radical (C2H) with Benzene

The reaction of the C2H radical with benzene is studied at low temperature using a pulsed Laval nozzle apparatus. The C2H radical is prepared by 193-nm photolysis of acetylene, and the C2H concentration is monitored using CH(A2Delta) chemiluminescence from the C2H + O2 reaction. Measurements at very low photolysis energy are performed using CF3C2H as the C2H precursor to study the influence of benzene photodissociation on the rate coefficient. Rate coefficients are obtained over a temperature range between 105 and 298 K. The average rate coefficient is found to be five times greater than the estimated value presently used in the photochemical modeling of Titan's atmosphere. The reaction exhibits a slight negative temperature dependence which can be fitted to the expression k(cm3 molecule(-1) s(-1)) = 3.28(+/-1.0) x 10(-10) (T/298)(-0.18(+/-0.18)). The results show that this reaction has no barrier and may play an important role in the formation of large molecules and aerosols at low temperature. Our results are consistent with the formation of a short lifetime intermediate that decomposes to give the final products.

Rate Coefficients and Kinetic Isotope Effect for the C2H Reactions with NH3 and ND3 in the 104−294 K Temperature Range

Reactions of C2H with NH3 and ND3 are studied at low temperature using a pulsed Laval nozzle apparatus. The C2H radical is prepared by 193 nm photolysis of acetylene, and the C2H concentration is monitored using CH(A2Δ) chemiluminescence from the C2H + O2 reaction. The rate constants for the C2H + NH3 and C2H + ND3 reactions are measured at three temperatures, 104 ± 5 K, 165 ± 15 K, and 296 ± 2 K. Measured rate constants are fit to power law expressions, k(T) = A(T/298)n, for ease of comparison with the results for the related CN + NH3 reaction and to emphasize the importance of the attractive part of intermolecular interaction potential in the reaction mechanism. The rate constants are (2.9 ± 0.7) × 10-11 × (T/298 K)(−0.90 ± 0.15) cm3 molecule-1 s-1 and (1.1 ± 0.2) × 10-11 × (T/298 K)(−0.82 ± 0.026) cm3 molecule-1 s-1 for NH3 and ND3, respectively. A large kinetic isotope effect is observed, k(C2H + NH3)/k(C2H + ND3) = 2.0 ± 0.2, which within experimental uncertainty does not depend on the temperature in the 104−296 K range. Previous theoretical work shows that a hydrogen abstraction channel, C2H + NH3 → C2H2 + NH2, is a possible mechanism for the C2H + NH3 reaction since the minimum energy path for this channel does not have an activation barrier. This theoretical prediction is consistent with the strong negative temperature dependence of the rate coefficients for the C2H + NH3 reaction observed in this work, which clearly shows that the C2H + NH3 reaction does not have a barrier.

Kinetics of C2H Reactions with Hydrocarbons and Nitriles in the 104−296 K Temperature Range

Reactions of C 2 H with isobutane (k 1 ), 1-butene (k 2 ), isobutylene (k 3 ), 1,3-butadiene (k4), methyl cyanide (k 5 ), ethyl cyanide (k 6 ), and propyl cyanide (k 7 ) are studied at low temperature using a pulsed Laval nozzleapparatus. The C 2 H radical is prepared by 193-nm photolysis of acetylene, and the C 2 H concentration is monitored using CH(A 2 Δ) chemiluminescence from the C 2 H + O 2 reaction. The rate constants at low and high temperatures are k 1 = (1.3 ′ 0.3) x 10 - 1 0 and (1.0 ′ 0.2) x 10 - 1 0 for isobutane, k 2 = (2.1 ′ 0.4) x 10 - 1 0 and (2.2 ′ 0.4) × 10 - 1 0 for 1-butene, k 3 = (1.4 ′ 0.3) x 10 - 1 0 and (1.2 ′ 0.2) x 10 - 1 0 for isobutylene, and k4 = (2.9 ′ 0.6) x 10 - 1 0 and (3.3 ′ 0.6) x 10 - 1 0 for 1,3-butadiene at T = 104 and 296 K, respectively (in units of cm 3 molecule - 1 s - 1 ) Comparison with existing data shows a trend of a decrease in activation energy with increasing size of the hydrocarbon chain. For these reactions of hydrocarbons containing four carbon atoms, the activation energy is zero within experimental uncertainty, and the rate constants do not depend on temperature in the 104-296 K temperature range. The rate constants for C 2 H reactions with methyl cyanide, ethyl cyanide, and propyl cyanide are measured at three temperatures, 104, 165, and 298 K. Measured rate constants are fit to an Arrhenius expression and are k 5 = (1.8 ′ 0.35) x 10 - 1 1 exp(-766 ′ 38/T), k 6 = (1.5 ′ 0.3) × 10 - 1 1 exp(-145 ′ 10/T), and k 7 = (2.1 ′ 0.4) × 10 - 1 1 exp(-51 ′ 4/T) (in units of cm 3 molecule - 1 s - 1 , T is in Kelvin). At T = 296 K, k 3 , k 5 , k 6 , and k 7 are measured as a function of total pressure and show no pressure dependence in the 0.6-8 Torr (0.08-1.07 kPa) pressure range. Results from this work are compared with the results of previous investigations of C 2 H reactions at low temperature and are discussed in relation to the atmospheres of Saturn and Titan.

Laboratory kinetics of C 2H radical reactions with ethane, propane, and n-butane at T = 96–296 K: implications for Titan

The kinetics of the reactions of C 2H radical with ethane ( k 1), propane ( k 2), and n-butane ( k 3) are studied over the temperature range of T = 96–296 K with a pulsed Laval nozzle apparatus that utilizes a pulsed laser photolysis–chemiluminescence technique. The C 2H decay profiles in the presence of both the alkane reactant and O 2 are monitored by the CH(A 2Δ) chemiluminescence tracer method. The results, together with available literature data, yield the following Arrhenius expressions: k 1( T) = (0.51 ± 0.06) × 10 −10 exp[(−76 ± 30)K/ T] cm 3 molecule −1 s −1 ( T = 96–800 K), k 2( T) = (0.98 ± 0.32) × 10 −10exp[(−71 ± 60)K/ T] cm 3 molecule −1 s −1 ( T = 96–361 K), and k 3( T) = (1.23 ± 0.26) × 10 −10 cm 3 molecule −1 s −1 ( T = 96–297 K). At T = 296 K, k 1 is measured as a function of total pressure and has little or no pressure dependence. The results from this work support a direct hydrogen abstraction mechanism for the title reactions. Implications to the atmospheric chemistry of Titan are discussed.

Kinetics of C 2H radical reactions with ethene, propene and 1-butene measured in a pulsed Laval nozzle apparatus at T=103 and 296 K

Low-temperature kinetics of the reactions of C 2H radical with ethene ( k 1), propene ( k 2), and 1-butene ( k 3) are studied using a pulsed Laval nozzle supersonic expansion. The C 2H decay profiles in the presence of both the alkene reactant and O 2 are monitored by the CH(A 2 Δ) chemiluminescence tracer method. The rate constants k 1, k 2, and k 3 are measured at T=103 K to be (1.4±0.4)×10 −10, (2.4±0.6)×10 −10, and (2.6±0.6)×10 −10 cm 3 molecule −1 s −1 , respectively. Measurements at room temperature (296 K) and previous data suggest a slight negative temperature dependence for all three reactions.

Kinetics of reactions of C 2H radical with acetylene, O 2, methylacetylene, and allene in a pulsed Laval nozzle apparatus at [formula omitted

The rate constants of the reactions of C 2H radical with HC CH (k 1) , O 2 (k 2) , CH 3 C CH (k 3) , and H 2 C C CH 2 (k 4) are measured at 103 K using a pulsed Laval nozzle supersonic expansion and the CH(A 2 Δ) chemiluminescence monitor method: k 1=(1.3±0.3)×10 −10, k 2=(3.8±0.9)×10 −11, k 3=(2.7±0.6)×10 −10, and k 4=(2.5±0.6)×10 −10 cm 3 molecule −1 s −1 . These values are compared to previous results and measurements in a continuous Laval nozzle apparatus (accompanying paper by Carty et al.). The non-Arrhenius behavior and possible implications of the results for atmospheric chemistry of Titan are discussed.

Low-Temperature Kinetics of Reactions of the OH Radical with Propene and 1-Butene Studied by a Pulsed Laval Nozzle Apparatus Combined with Laser-Induced Fluorescence

The kinetics of the reactions of the OH radical with propene and 1-butene are studied at T = 103 K. The low-temperature environment is provided by a pulsed Laval nozzle supersonic expansion of nitrogen with admixed radical precursor and reactant gases. The gas number density and temperature distributions in the flow are characterized by both dynamic pressure measurements and laser-induced fluorescence (LIF) spectroscopy of OH radicals excited in the (1,0) band of the A2Σ+−X2Πi transition. For the kinetic measurements, the OH radical decay profiles in the presence of reactants are monitored by LIF. The rate constants of the reactions of OH with propylene and 1-butene are measured at T = 103 K to be (0.81 ± 0.18) × 10-10 and (1.24 ± 0.27) × 10-10 cm3 molecule-1 s-1, respectively. The observed negative temperature dependences of the rate constants for both reactions studied by the pulsed Laval nozzle system show good agreement with both low-temperature and high-temperature kinetic data available in the liter...

Rate coefficients for the reaction of C 2H with O 2 at 90 K and 120 K using a pulsed Laval nozzle apparatus

Absolute rate coefficients for the reaction of C 2H with O 2 are measured at 90 K and 120 K by using a pulsed Laval nozzle apparatus with laser ionization, time-of-flight mass spectrometry (TOF-MS). The C 2H radicals are generated by laser photolysis of C 2H 2 at 193 nm, and the products of the reaction with O 2 and C 2H 2, HCCO and C 4H 2, respectively, are directly detected by laser ionization at 118 nm. Rate coefficients of (9.0±2.0)×10 −11 and (5.0±1.5)×10 −11 cm 3 molecule −1 s −1 are obtained at 90 K and 120 K for the C 2H+O 2 reaction, respectively, by detecting the rise of the product HCCO.

Direct measurements of rate coefficients for the reaction of ethynyl radical (C2H) with C2H2 at 90 and 120 K using a pulsed Laval nozzle apparatus

Rate coefficients for the reaction C2H + C2H2 → C4H2 + H are measured at 90 and 120 K by using a new pulsed Laval nozzle apparatus equipped with laser ionization, time‐of‐flight mass spectrometric detection. The C2H radicals are generated by 193 nm laser photolysis of C2H2, and the reaction product, C4H2, is directly detected by single‐photon ionization at 118 nm (10.5 eV). Rate coefficients of (2.7±0.5) × 10−10 and (2.5±0.4) × 10−10 cm3 molecule−1 s−1 are obtained at 90 and 120 K, compared to the room temperature value of 1.3×10−10 cm3 molecule−1 s−1. The results show a significant negative temperature dependence of the reaction rate coefficient below room temperature. The implications for models of planetary atmospheres, such as for Titan, are discussed.