Publications

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This work reports measurements of absolute rate coefficients and Rice-Ramsperger-Kassel-Marcus (RRKM) master equation simulations of the C 2H 3 + C 2H 4 reaction. Direct kinetic studies were performed over a temperature range of 300-700 K and pressures of 20 and 133 mbar. Vinyl radicals (H 2C=CH) were generated by laser photolysis of vinyl iodide (C 2H 3I) at 266 nm, and time-resolved absorption spectroscopy was used to probe vinyl radicals through absorption at 423.2 nm. Measurements at 20 mbar are in good agreement with previous determinations at higher temperature. A weighted three-parameter Arrhenius fit to the experimental rate constant at 133 mbar, with the temperature exponent fixed, gives k = (7 ±1) × 10 -14 cm 3 molecule -1 s -1 (T/298 K) 2 exp[-(1430 ± 70) K/T]. RRKM master equation simulations, based on G3 calculations of stationary points on the C 4H 7 potential energy surface, were carried out to predict rate coefficients and product branching fractions. The predicted branching to 1-methylallyl product is relatively small under the conditions of the present experiments but increases as the pressure is lowered. Analysis of end products of 248 nm photolysis of vinyl iodide/ethylene mixtures at total pressures between 27 and 933 mbar provides no direct evidence for participation of 1-methylallyl. © 2007 American Chemical Society.

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In this paper, relative integrated cross sections are measured for spin-orbit-conserving, rotationally inelastic scattering of NO (²Π_1/2), hexapole-selected in the upper Λ-doublet level of the ground rotational state (j = 0.5), in collisions with D₂ at a nominal energy of 551 cm^-1. The final state of the NO molecule is detected by laser-induced fluorescence (LIF). The state-selected NO molecule is oriented with either the N end or the O end toward the incoming D₂ molecule by application of a static electric field E in the scattering region. This field is directed parallel or antiparallel to the relative velocity vector v. Comparison of signals taken for the different applied field directions gives the experimental steric asymmetry SA, defined by SA = (σ_v↑↓E - σ_v↑↑E)/(σ_v↑↓E + σ_v↑↑E), which is equal to within a factor of -1 to the molecular steric effect, S_i→f ≡ (σ_D2→NO - σ_D2→ON)/(σ_D2→NO + σ_D2→ON). The dependence of the integral inelastic cross section on the incoming Λ-doublet component is also measured as a function of the final rotational (j_final) and Λ-doublet (ε_final) state. The measured steric asymmetries are similar to those previously observed for NO-He scattering. Spin-orbit manifold-conserving collisions exhibit a larger propensity for parity conservation than their NO-He counterparts. The results are interpreted in the context of the recently developed quasi-quantum treatment (QQT) of rotationally inelastic scattering. The QQT predictions can be inverted to obtain a fitted hard-shell potential that reproduces the experimental steric asymmetry; this fitted potential gives an empirical estimate of the anisotropy of the repulsive interaction between NO and D₂. Finally, QQT computation of the differential cross section using this simple model potential shows reasonable agreement with the measured differential cross sections.

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Flame-sampling photoionization mass spectrometry is used for measurements of the absolute molar composition of fuel-rich (φ = 1.8) low-pressure laminar flames of allene and propyne. The experiment combines molecular-beam mass spectrometry with photoionization by tunable vacuum-ultraviolet synchrotron radiation. This approach provides selective detection of individual isomers and unambiguous identifications of other flame species of near-equal mass by near threshold photoionization efficiency measurements. Mole fraction profiles for more than 30 flame species with ion masses ranging from 2 to 78 are presented. The isomeric composition is resolved for most intermediates, for example, mole fraction profiles are presented for both benzene and the fulvene isomer. The results are compared with predictions based on current kinetic models. The mole fractions of the major species are predicted quite accurately, however, some discrepancies are observed for minor species. © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The role of ethanol as a fuel additive was investigated in a fuel-rich, non-sooting (C/O = 0.77) flat premixed propene-oxygen-argon flame at 50 mbar (5 kPa). Mole fractions of stable and radical species were derived using two different in situ molecular beam mass spectrometry (MBMS) set-ups, one located in Bielefeld using electron impact ionization (EI), and the other at the Advanced Light Source (ALS) at Berkeley using vacuum UV photoionization (VUV-PI) with synchrotron radiation. A rich propene flame, previously studied in detail experimentally and with flame model calculations, was chosen as the base flame. Addition of ethanol is believed to reduce the concentrations of benzene and small aromatic compounds, while augmenting the formation of other regulated air toxics such as aldehydes. To study the chemical pathways responsible for these effects, quantitative concentrations of about 35 species were determined from both experiments. This is also the first time that a detailed comparison of quantitative species concentrations from these independent MBMS set-ups is available. Effects of ethanol addition on the species pool are discussed with special attention on benzene precursor chemistry and aldehyde formation. © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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This meeting will continue to cover fundamentals and applications of photoionization and photodetachment, including valence and core-level phenomena and applications to reaction dynamics, ultrashort laser pulses and the study of exotic molecules and anions.

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