Laser Photolysis Shock Tube Research Articles

ChemInform Abstract: Thermal Rate Constant Measurements by the Flash or Laser Photolysis Shock Tube Method: Results for the Oxidations of Hydrogen and Deuterium

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High-Temperature Reactions of O + COS and S + SO2. Abstraction versus Substitution Channels

The main concern of this study is to investigate the reaction mechanism of O + COS → products (1). Experiments are conducted by use of an excimer laser photolysis shock tube technique, where mixtures of COS and SO2 diluted in Ar are photolyzed behind reflected shock waves. Time-resolved measurements of O and S atoms are conducted by use of atomic resonance absorption spectroscopy, and the overall rate constant of (1) is determined by the O atom decay at temperatures 1250−1600 K, i.e., k1 = 10-10.18±0.26 exp[−(22.5 ± 7.1) kJ·mol-1/RT] cm3 molecule-1 s-1, which is in good agreement with the former recommendation. It is confirmed in this experiment that the S atom is a direct product of reaction 1. By analysis of time profiles of the S atom, the branching fraction of the S production channel O + COS → S + CO2 (1b), α, is determined against the main channel O + COS → CO + SO (1a), where the S atom consumption reactions, S + SO2 → 2SO (2) and S + COS → S2 + CO (3), are inevitably taken into account. The present experimental result is expressed as α = (0.40 ± 0.10) − (202 ± 137)/T (T = 1120−1540 K). Also, in this kinetic analysis, the rate constant of (2) is simultaneously determined to be k2 = 10-11.01±0.33 exp[−(37.8 ± 8.2) kJ·mol-1/RT]. The reaction mechanism of (1) is examined by comparing the experimental results with those of ab initio potential energy surface/transition state theory calculations.

Direct measurements of the rate coefficients for the reactions of some hydrocarbons with chlorine atoms at high temperatures

The reactions of methane, ethylene, ethane, and propane with chlorine atoms have been studied at hightemperatures by using a laser photolysis-shock tube apparatus coupled with atomic resonance absorption spectroscopy. Chlorine atoms were produced by the laser photolysis of silicon tetrachloride. The overall rate coefficients were experimentally determined from the decay of chlorine atoms as: k(CH4+Cl)=10−9.26±0.26 exp[−(35.7±6.6) kJ mol−1/RT] cm3 molecule−1 s−1 (1100–1550 K) k(C2H4+Cl)=10−9.67±0.16exp[−(35.8±3.9) kJ mol−1/RT] cm3 molecule−1 s−1 (1100–1550 K) k(C2H6+Cl)=10−9.96±0.05 cm3 molecule−1 s−1 (1100–1400 K) k(C3H8+Cl)=10−9.77±0.06 cm3 molecule−1 s−1 (1100–1400 K) where the error limits are given at the two standard deviation level. With systematic errors also considered, the reliabilities, Δ[logk(RH+Cl)], were evaluated to be ±0.2 for reactions CH4+Cl and C2H4+Cl and to be ±0.15 for reactions C2H6+Cl and C3H8+Cl. These results are in good agreement with the extrapolations of the kinetic data measured up to 800 K by Pilgrim et al. To examine the product channels of these reactions, an ab initio molecular orbital calculation was performed at the QCISD(T)/6-311G (d,p)//MP2(full)/6–31G(d) level. The calculation shows that the energetically favorable channels of CH4_Cl, C2H6+Cl, and C3H8+Cl are H abstraction by Cl atoms. In the C2H4+Cl system, the measured rate coefficient has no total density dependence so that the Cl addition to ethylene is negligible in this experimental temperature range, although the addition channel has no energy barrier. Based on transition state theory, the rate coefficients were also calculated and discussed. The reactions of hydrocarbons with Cl atoms are suggested to follow the Evans-Polanyi law.

H+CH 2CO→CH 3+CO at high temperature: A high pressure chemical activation reaction with positive barrier

The Laser Photolysis-Shock Tube (LP-ST) technique coupled with H-atom atomic resonance absorption spectrometry (ARAS) has been used to study the reaction, H+CH2CO→CH3+CO, over the temperature range, 863–1400 K. The results can be represented by the Arrhenius expression, k=(4.85±0.70)×10−11 exp(−2328±155 K/T) cm3 molecule−1 s−1. The present data have been combined with the earlier low temperature flash photolysis-resonance fluorescence measurements to yield a joint three-parameter expression, k=5.44×10−14 T0.851 exp(−1429 K/T) cm3 molecule−1 s−1. This is a chemical activation process that proceeds through vibrationally excited acetyl radicals. However, due to the presence of a low-lying forward dissociation channel to CH3+CO, the present results refer to the high pressure limiting rate constants. Hence, transition state theory with Eckart tunneling is used to explain the data.

A shock tube study of reactions of CN with HCN, OH, and H2 using CN and OH laser absorption

Quantitative, narrow-line laser absorption measurements of CN time-histories at 388.444 nm were acquired in high-temperature pyrolysis and laser photolysis shock tube experiments. The data were analyzed using a detailed kinetics mechanism to determine the rate coefficients of the reactions for temperatures between 940 and 1860 K. Two independent experimental approaches were utilized: laser photolysis (at 193 nm) of dilute C2N2/HCN/argon and C2N2/H2/argon mixtures in reflected shock wave experiments, and shock heating of HNO3/HCN/argon mixtures in incident and reflected shock wave experiments. Laser absorption measurements of OH at 306.687 nm were also taken in the HNO3/HCN/argon experiments The results are in good agreement with rate coefficient determinations from previous studies at different temperatures. The expression derived by Yang et al (1992) from their k2 measurements in combination with those of Szekely et al (1983), is recommended for the broad temperature range 300–3000 K. The uncertainty factors f and F give the limiting values of the rate coefficient kmin = f kbest fit, kmax = Fkbest fit. The recommended expression for the rate coefficient of reaction (3) also valid for temperatures 300–3000 K, is taken from the transition state theory analysis of the CN + H2 reaction by Wagner and Bair (1986). The rate coefficient for reaction (1) was measured to be 4.0 × 1013cm3mol−1 s−1(f = 0.61, F = 1.40) for the temperature range 1250–1860 K. © 1996 John Wiley & Sons, Inc.

Thermal rate constants for the Cl+H2 and Cl+D2 reactions between 296 and 3000 K

Rate constants for the Cl+H2 and D2 reactions have been measured at room temperature by the laser photolysis-resonance absorption (LP-RA) technique. Measurements were also performed at higher temperatures using two shock tube techniques: laser photolysis-shock tube (LP-ST) technique with Cl-atom atomic resonance absorption spectrometric (ARAS) detection, over the temperature range 699–1224 K; and higher temperature rates were obtained using both Cl-atom and H-atom ARAS techniques with the thermal decomposition of COCl2 as the Cl-atom source. The combined experimental results are expressed in three parameter form as kH2( ± 15%) = 4.78 × 10−16 T1.58 exp(−1610 K/T) and kD2( ± 20%) = 9.71 × 10−17 T1.75 exp(−2092 K/T) cm3 molecule−1 s−1 for the 296–3000 K range. The present results are compared to earlier direct studies which encompass the temperature ranges 199–1283 (H2) and 255–500 K (D2). These data including the present are then used to evaluate the rate behavior for each reaction over the entire experimental temperature range. In these evaluations the present data above 1300 K was given two times more weight than the earlier determinations. The evaluated rate constants are: kH2( ±14%)=2.52×10−11 exp(−2214 K/T) (199≤T<354 K), kH2(±17%)=1.57×10−16 T1.72 exp(−1544 K/T) (354≤T≤2939 K), and kD2(±5%)=2.77×10−16 T1.62 exp(−2162 K/T) (255≤T≤3020 K), in molecular units. The ratio then gives the experimental kinetic isotope effect, KIE ≡ (kH2/kD2). Using 11 previous models for the potential energy surface (PES), conventional transition state theoretical (CTST) calculations, with Wigner or Eckart tunneling correction, are compared to experiment. At this level of theory, the Eckart method agrees better with experiment; however, none of the previous PES’s reproduce the experimental results. The saddle point properties were then systematically varied resulting in an excellent model that explains all of the direct data. The theoretical results can be expressed to within ±2% as kH2th = 4.59 × 10−16 T1.588 exp(−1682 K/ T) (200≤T≤2950 K) and kD2th=9.20×10−16 T1.459 exp(−2274 K/T) cm3 molecule−1 s−1 (255≤T ≤3050 K). The KIE predictions are also compared to experiment. The ‘‘derived’’ PES is compared to a new ab initio calculation, and the differences are discussed. Suggestions are noted for reconciling the discrepancies in terms of better dynamics models.

Kinetics Studies of the O(3P) + CH2Cl2 and CHCl3 Reactions over the 468-1355 and 499-1090 K Ranges Using Two Techniques

The thermal bimolecular rate coefficients, O + CH[sub 2]Cl[sub 2] and O + CHCl[sub 3], have been investigated in this work over the respective temperature ranges 468-1355 and 499-1090 K using two entirely different techniques. The first technique, HTP (high-temperature photochemistry) at Rensselaer, provided results for the lower temperatures of these ranges, while the higher temperature measurements were carried out at Argonne using the LP-ST (laser photolysis-shock tube) method. Substantial variations in source and reactant molecular concentrations, as well as in other experimental parameters specific to each experiment, were employed in each of the studies. Temperature overlap in the databases was possible with CH[sub 2]Cl[sub 2] but not with CHCl[sub 3]. The combined results for (1) O + CH[sub 2]Cl[sub 2] [yields] OH + CHCl[sub 2] and (2) O + CHCl[sub 3] [yields] OH + CCl[sub 3] are k[sub 1] = 1.1 x 10[sup [minus]17] (T/K)[sup 1.99] exp(-2855 K/T) and [kappa][sub 2] = 1.9 x 10[sup [minus]11] (T/K)[sup 0.22] exp(-4755 K/T) cm[sub 3] molecule[sup [minus]1] S[sup [minus]1], with estimated 1 [sigma] accuracy limits of [+-]25%. The present results are compared to the related O + CH[sub 4] and O + CH[sub 3]Cl reactions. The four reactions have nearly equal ratemore » coefficients. This is discussed on the basis of BEBO calculations. 38 refs., 8 figs., 6 tabs.« less

Rate constants for the N2O reaction system: Thermal decomposition of N2O; N+NO→N2+O; and implications for O+N2→NO+N

Experiments on the thermal decomposition of N2O have been carried out in reflected shock waves using the atomic resonance absorption spectrometric (ARAS) technique for monitoring the product O atoms. The results provide a calibration for the ARAS system as well as values for the decomposition rate constant, k1, between 1540 and 2500 K. The present results can be represented as k1=5.25×10−10 exp(−27 921 K/T) cm3 molecule−1 s−1. Additionally, rate constants for the reaction, (2) N+NO→N2+O, have been measured by the laser photolysis-shock tube (LP-ST) technique between 1251 and 3152 K. NO serves as both the photolytic source of N(4S) atoms and the reactant molecule. N atoms are monitored by the ARAS method. The results do not show significant temperature dependence and can be represented by k2=(3.7±0.8)×10−11 cm3 molecule−1 s−1 over the experimental temperature range. Values for the reaction, (−2) O+N2→NO+N, can be derived from these results. The results are compared with earlier studies and are then theoretically discussed in terms of transition states taken from an ab initio electronic structure calculation and also from a dispersion force model. Neither model accurately predicts the observed behavior, and this suggests that further theoretical work with trajectory calculations be attempted to assess the importance of recrossing.

A kinetics study of the O( 3P)+CH 3Cl reaction over the 556–1485 K range by the HTP and LP-ST techniques

The high-temperature photochemistry (HTP) and laser photolysis-shock tube (LP-ST) techniques have been combined to study the kinetics of the reaction between ground-state oxygen atoms with CH3Cl over the temperature range, 556–1485 K. In the HTP reactor, used for the 556–1291 K range, O atoms were generated by flash photolysis of O2, CO2 or SO2, and the atom concentrations were monitored by resonance fluorescence, while with the LP-ST technique, used for the 916–1485 K range, O atoms were generated by the photolysis of either SO2 or NO with the 193 nm light from a pulsed ArF excimer laser, and atomic resonance absorption spectroscopy (ARAS) was used to monitor [O]t. In both studies, rate coefficients were derived from the [O] profiles under the pseudo-first-order condition, [O] ≪[CH3Cl]. The data obtained by the two techniques are in excellent agreement and are best represented by the expression, k(T)=2.57×10−11 (T/K)0.31 exp(−5633 K/T) cm3 molecule−1 s−1 with a 2σ precision varying from ±6 to ±22% and an estimated 2σ accuracy of ±21% to ±30%, depending on temperature. The rate coefficients for the title reaction are essentially identical to those for the O+CH4 reaction over the observed temperature range, the reasons for which are discussed.

Rate constants for the reactions H+O2→OH+O and D+O2→OD+O over the temperature range 1085–2278 K by the laser photolysis–shock tube technique

Rate constants for the reactions (1) H+O2→OH+O and (2) D+O2→OD+O have been measured over the temperature ranges 1103–2055 K and 1085–2278 K, respectively. The experimental method that has been used is the laser-photolysis–shock-tube technique. This technique utilizes atomic resonance absorption spectrophotometry (ARAS) to monitor H- or D-atom depletion in the presence of a large excess of reactant, O2. The results can be well represented by the Arrhenius expressions k1(T)=(1.15±0.16)×10−10 exp(−6917±193 K/T) cm3 molecule−1 s−1, and k2(T)=(1.09±0.20)×10−10 exp(−6937±247 K/T) cm3 molecule−1 s−1. Over the experimental temperature range, the present results show that the isotope effect is unity within experimental uncertainty. The Arrhenius equations, k−1(T)=(8.75±1.24) ×10−12 exp(1121±193 K/T) cm3 molecule−1 s−1 and k−2 (T)=(9.73±1.79)×10−12 exp(526±247 K/T) cm3 molecule−1 s−1, for the rate constants of the reverse reactions were calculated from the experimentally measured forward rate constants and expressions for the equilibrium constants that have been derived from the JANAF thermochemical database. The theoretical implications of the present results are also discussed.

Laser photolysis shock tube for combustion kinetics studies

An excimer laser photolysis method has been developed for shock tube kinetics studies of elementary combustion reactions. The excimer laser provides a high intensity source of UV photons well suited to the rapid production of reactive radicals, while shock wave heating provides a convenient means of varying the reaction environment over a wide temperature range. An ArF excimer (193.3 nm) was used to photolyze H 2 O behind reflected shock waves to produce H and OH radicals. The temporal variation of the OH concentration was monitored using a cw, narrow-linewidth laser absorption diagnostic, and the rate coefficient of the dominant reaction after photolysis, H+H 2 O→OH+H 2 , was determined. The measured rate coefficient, k =2.4(±.25)×10 14 exp (−10750 (±500)/T,K) cm 3 mole −1 s −1 , T=1600–2500 K, is in good agreement with other recent studies of this reaction in both the forward and reverse directions.