Atomic Resonance Absorption Spectroscopy with Flash or Laser Photolysis in Shock Wave Experiments

Michael, Joe V.; Lifshitz, Assa

doi:10.1016/b978-012086430-0/50039-7

Cited by 17 publications

(30 citation statements)

References 82 publications

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“…Due to lamp gas hydrogeneous impurities in research grade He (even cooled with liquid N 2 ), Lyman-α H radiation is emitted from the lamp along with a low percent of radiation that is extraneous (nonresonant). In order to measure the fraction of nonresonant radiation present in the lamp, an H 2 discharge flow system, an atom filter, is used to create large [H] (∼1 × 10 14 atoms cm –3 ) between the lamp and shock tube window ,− thereby removing all of the Lyman-α H in the emission lamp. The path length of the atomic filter section is 3 cm.…”

Section: Experimental Sectionmentioning

confidence: 99%

“…However, these experiments require metering very small amounts of D 2 into the resonance lamp such that the lamp intensity is similar to that for the H lamp. This ensures that the D-atom concentration will be very low in the lamp, and that the lamp will be effectively unreversed; i.e., a completely defined Ladenburg-Reiche Gaussian line shape. , In this case, D-atoms in the presence of H-atoms can be directly detected by carrying out the experiment with the H 2 discharge flow system turned on (i.e., removing Lyman-α H ) during the D-atom experiment.…”

Section: Experimental Sectionmentioning

confidence: 99%

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High Temperature Shock Tube and Theoretical Studies on the Thermal Decomposition of Dimethyl Carbonate and Its Bimolecular Reactions with H and D-Atoms

2013

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The shock tube technique was used to study the high temperature thermal decomposition of dimethyl carbonate, CH3OC(O)OCH3 (DMC). The formation of H-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range of 1053-1157 K at pressures ∼0.5 atm. The H-atom profiles were simulated using a detailed chemical kinetic mechanism for DMC thermal decomposition. Simulations indicate that the formation of H-atoms is sensitive to the rate constants for the energetically lowest-lying bond fission channel, CH3OC(O)OCH3 → CH3 + CH3OC(O)O [A], where H-atoms form instantaneously at high temperatures from the sequence of radical β-scissions, CH3OC(O)O → CH3O + CO2 → H + CH2O + CO2. A master equation analysis was performed using CCSD(T)/cc-pv∞z//M06-2X/cc-pvtz energetics and molecular properties for all thermal decomposition processes in DMC. The theoretical predictions were found to be in good agreement with the present experimentally derived rate constants for the bond fission channel (A). The theoretically derived rate constants for this important bond-fission process in DMC can be represented by a modified Arrhenius expression at 0.5 atm over the T-range 1000-2000 K as, kA(T) = 6.85 × 10(98)T (-24.239) exp(-65250 K/T) s(-1). The H-atom temporal profiles at long times show only minor sensitivity to the abstraction reaction, H + CH3OC(O)OCH3 → H2 + CH3OC(O)OCH2 [B]. However, H + DMC is an important fuel destruction reaction at high temperatures. Consequently, measurements of D-atom profiles using D-ARAS allowed unambiguous rate constant measurements for the deuterated analog of reaction B, D + CH3OC(O)OCH3 → HD + CH3OC(O)OCH2 [C]. Reaction C is a surrogate for H + DMC since the theoretically predicted kinetic isotope effect at high temperatures (1000 - 2000K) is close to unity, kC ≈ 1.2 kB. TST calculations employing CCSD(T)/cc-pv∞z//M06-2X/cc-pvtz energetics and molecular properties for reactions B and C are in good agreement with the experimental rate constants. The theoretical rate constants for these bimolecular processes can be represented by modified Arrhenius expressions over the T-range 500-2000 K as, kB(T) = 1.45 × 10(-19)T(2.827) exp(-3398 K/T) cm(3) molecule(-1) s(-1) and kC(T) = 2.94 × 10(-19)T(2.729) exp(-3215 K/T) cm(3) molecule(-1) s(-1).

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Section: Experimental Sectionmentioning

confidence: 99%

Section: Experimental Sectionmentioning

confidence: 99%

High Temperature Shock Tube and Theoretical Studies on the Thermal Decomposition of Dimethyl Carbonate and Its Bimolecular Reactions with H and D-Atoms

2013

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“…In the present work, we report on a complementary shock-tube study of NCN decomposition. Whereas in ref time-resolved laser absorption was used to monitor the reactant NCN, we use atomic resonance absorption spectroscopy (ARAS) , to monitor C atoms formed as products of reaction . Besides the direct detection of a product, which could verify the predicted dominance of the C + N 2 channel, the high sensitivity of ARAS enables us to apply very low initial concentrations of NCN, which suppresses competing bimolecular reactions.…”

Section: Introductionmentioning

confidence: 99%

“…, 28 ] to monitor C atoms formed as products of reaction R1. Besides the direct detection of a product, which could verify the predicted dominance of the C + N 2 channel, the high sensitivity of ARAS enables us to apply very low initial concentrations of NCN, which suppresses competing bimolecular reactions.…”

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confidence: 99%

Thermal Decomposition of NCN: Shock-Tube Study, Quantum Chemical Calculations, and Master-Equation Modeling

et al. 2015

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“…This prior theoretical analysis and the earlier experimental result for propane, coupled with the potential for an enhanced “roaming” contribution in larger molecules, has supplied the esoteric motivation for the present experiments on isobutane and neopentane. In these experiments, the absolute H-atom rates of production and H-atom yields have been measured using ultrasensitive H-atom atomic resonance absorption spectrometry (ARAS). − From the data, H-atom branching ratios and dissociation rate constants can be obtained. The measured dissociation rate constants are compared to earlier studies.…”

Section: Introductionmentioning

confidence: 99%

Shock Tube Explorations of Roaming Radical Mechanisms: The Decompositions of Isobutane and Neopentane

et al. 2012

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The thermal decompositions of isobutane and neopentane have been studied using both shock tube experiments and ab initio transition state theory based master equation calculations. Dissociation rate constants for these molecules have been measured at high temperatures (1260-1566 K) behind reflected shock waves using high-sensitivity H-ARAS detection. The two major dissociation channels at high temperature are iso-C(4)H(10) → CH(3) + i-C(3)H(7) (1a) and neo-C(5)H(12) → CH(3) + t-C(4)H(9) (2a). Ultrahigh-sensitivity ARAS detection of H-atoms produced from the rapid decomposition of the product radicals, i-C(3)H(7) in (1a) and t-C(4)H(9) in (2a), through i-C(3)H(7) + M → H + C(3)H(6) + M (3a) and t-C(4)H(9) + M → H + i-C(4)H(8) + M (4a) allowed measurements of both the total decomposition rate constants, k(total), and the branching to radical products, which were observed to be equivalent in both systems, k(1a)/k(total) and k(2a)/k(total) = 0.79 ± 0.05. Theoretical analyses indicate that in isobutane, the non-H-atom fraction has two contributions, the dominant fraction being due to the roaming radical mechanism leading to molecular products through iso-C(4)H(10) → CH(4) + C(3)H(6) (1b) with k(1b)/k(total) = 0.16, and a minor fraction that involves the isomerization of i-C(3)H(7) to n-C(3)H(7) that then subsequently forms methyl radicals, i-C(3)H(7) + M → n-C(3)H(7) + M → CH(3) + C(2)H(4) + M (3b). In contrast to isobutane, in neopentane, the contribution to the non-H-atom fraction is exclusively through the roaming radical mechanism that leads to neo-C(5)H(12) → CH(4) + i-C(4)H(8) (2b) with k(2b)/k(total) = 0.21. These quantitative measurements of larger contributions from the roaming mechanism for larger molecules are in agreement with the qualitative theoretical arguments that suggest long-range dispersion interactions (which become increasingly important for larger molecules) may enhance roaming.

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Atomic Resonance Absorption Spectroscopy with Flash or Laser Photolysis in Shock Wave Experiments

Cited by 17 publications

References 82 publications

High Temperature Shock Tube and Theoretical Studies on the Thermal Decomposition of Dimethyl Carbonate and Its Bimolecular Reactions with H and D-Atoms

High Temperature Shock Tube and Theoretical Studies on the Thermal Decomposition of Dimethyl Carbonate and Its Bimolecular Reactions with H and D-Atoms

Thermal Decomposition of NCN: Shock-Tube Study, Quantum Chemical Calculations, and Master-Equation Modeling

Shock Tube Explorations of Roaming Radical Mechanisms: The Decompositions of Isobutane and Neopentane

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