Articles you may be interested inRadical-neutral chemical reactions studied at low temperature with VUV synchrotron photoionization mass spectrometry AIP Conf. Proc. 1501, 1365 (2012); 10.1063/1.4769699 Synchrotron photoionization mass spectrometry study of intermediates in fuel-rich 1,2-dimethoxyethane flame Direct identification of propargyl radical in combustion flames by vacuum ultraviolet photoionization mass spectrometry J. Chem. Phys. 124, 074302 (2006); 10.1063/1.2168448 Photoionization mass spectrometer for studies of flame chemistry with a synchrotron light source Rev. Sci. Instrum. 76, 094102 (2005); 10.1063/1.2010307Photoionization efficiency spectrum and ionization energy of HSO studied by discharge flow-photoionization mass spectrometryWe report the first use of synchrotron radiation, continuously tunable from 8 to 15 eV, for flame-sampling photoionization mass spectrometry ͑PIMS͒. Synchrotron radiation offers important advantages over the use of pulsed vacuum ultraviolet lasers for PIMS; these include superior signal-to-noise, soft ionization, and access to photon energies outside the limited tuning ranges of current VUV laser sources. Near-threshold photoionization efficiency measurements were used to determine the absolute concentrations of the allene and propyne isomers of C 3 H 4 in low-pressure laminar ethylene-oxygen and benzene-oxygen flames. Similar measurements of the isomeric composition of C 2 H 4 O species in a fuel-rich ethylene-oxygen flame revealed the presence of substantial concentrations of ethenol ͑vinyl alcohol͒ and acetaldehyde. Ethenol has not been previously detected in hydrocarbon flames. Absolute photoionization cross sections were measured for ethylene, allene, propyne, and acetaldehyde, using propene as a calibration standard. PIE curves are presented for several additional reaction intermediates prominent in hydrocarbon flames.

Models for chemical mechanisms of hydrocarbon oxidation rely on spectrometric identification of molecular structures in flames. Carbonyl (keto) compounds are well-established combustion intermediates. However, their less-stable enol tautomers, bearing OH groups adjacent to carbon-carbon double bonds, are not included in standard models. We observed substantial quantities of two-, three-, and four-carbon enols by photoionization mass spectrometry of flames burning representative compounds from modern fuel blends. Concentration profiles demonstrate that enol flame chemistry cannot be accounted for purely by keto-enol tautomerization. Currently accepted hydrocarbon oxidation mechanisms will likely require revision to explain the formation and reactivity of these unexpected compounds.

A flame-sampling molecular-beam photoionization mass spectrometer, recently designed and constructed for use with a synchrotron-radiation light source, provides significant improvements over previous molecular-beam mass spectrometers that have employed either electron-impact ionization or vacuum ultraviolet laser photoionization. These include superior signal-to-noise ratio, soft ionization, and photon energies easily and precisely tunable [E/E(FWHM)250–400] over the 7.8–17-eV range required for quantitative measurements of the concentrations and isomeric compositions of flame species. Mass resolution of the time-of-flight mass spectrometer is m/m=400 and sensitivity reaches ppm levels. The design of the instrument and its advantages for studies of flame chemistry are discussed

Absolute rate coefficients for the reactions of chlorine atom with methane and ethane between 292 and 800 K and with propane between 292 and 700 K have been determined using the laser photolysis/continuous wave infrared long-path absorption method, LP/cwIRLPA. A novel reactor design and optical arrangement allow long absorption paths with precise control of the temperature in the probed volume. The rate coefficient for methane exhibits significant curvature between 292 and 800 K and can be described over this temperature range by a modified Arrhenius expression, k CH 4 (T) ) [3.7( -2.5 +8.2 ) × 10 -13 cm 3 molecule -1 s -1 ](T/298) 2.6((0.7) exp [-385((320)/T] (all error bars are (2σ precision only). In the temperature range 292-600 K the rate with ethane agrees well with earlier investigations, fitting a simple Arrhenius expression k C 2 H 6 (T) ) 8.6-((0.5) × 10 -11 exp [-135((26)/T] cm 3 molecule -1 s -1 . However, as the temperature increases beyond 600 K the Arrhenius plot exhibits significant upward curvature. Over the range 292-800 K a three-parameter Arrhenius fit, k C 2 H 6 (T) ) 3.4((1.4) × 10 -11 (T/298) 0.7((0.3) exp[150((110)/T] cm 3 molecule -1 s -1 , models the experimental data adequately. The rate coefficient for propane is found to be independent of temperature and equal to 1.38((0.03) × 10 -10 cm 3 molecule -1 s -1 .

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Mode–mode vibrational coupling in the acetylinic CH stretch at 3330 cm−1 of 1-butyne and 1-pentyne is studied via high-resolution, direct absorption infrared spectroscopy. As in our previous study of propyne, mixing of the CH stretch vibration carrying oscillator strength (the bright state) with the bath of multiquantum combination states (the dark, or background, states) manifests itself in the spectrum via fragmentation of the isolated bright state transitions into clusters of closely spaced spectral lines in a ∼0.01 cm−1 window about the zeroth order acetylinic CH stretch position. In the 1-butyne spectrum, we find an experimental density of mixed states of 114±30 states/cm−1 compared to a direct state count prediction of 90 total states/cm−1, and thus quantitatively determine that all possible states appear in the spectrum. The 1-butyne line spacing distribution suggests the Wigner distribution expected for a quantum mechanically ergodic system. Analysis of the mode mixing as a function of J′ shows that anharmonic terms in the potential, rather than Coriolis effects, contribute most strongly to the coupling. The acetylinic CH stretch spectrum of 1-pentyne (2400 states/cm−1) reveals only broad rovibrational transitions with ∼0.01 cm−1 Lorentzian width, even at our 10−4 cm−1 resolution. J′ independent, anharmonic coupling with a minimum of 1/3 of all states must be invoked to reproduce the observed broadening. In contrast, the 1-pentype methyl CH stretch spectrum shows broadening greater than five times larger than that observed at the acetylinic end. Via Fourier transform methods, the spectra for both 1-butyne and 1-pentyne indicate vibrational energy localization in the CH stretch for ∼500 ps. However, for the methyl CH stretch, energy redistribution takes place in <40 ps, clearly indicating the presence of mode specific, nonRRKM vibrational relaxation pathways.

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The high resolution, slit jet cooled infrared v=1←0 methyl asymmetric stretch spectra of trans-2-butene and 1-butene are reported. Both of these molecules are singly unsaturated butene chains, have 30 vibrational degrees of freedom, and yield nearly equivalent vibrational state densities (ρvib≊200 states/cm−1) at CH stretch levels of excitation. The key difference between these two molecules is the presence of a large amplitude C–C–C skeletal torsional coordinate in 1-butene corresponding to a low barrier, internal isomerization pathway which is completely absent in trans-2-butene. The trans-2-butene asymmetric CH stretch (ν16) spectrum is fully discrete at 0.002 cm−1 resolution, and the coarse structure readily assigned to zero order rovibrational transitions (J′K′aK′c ← J″K″aK″c) in an asymmetric top. Fragmentation of these zero order transitions into spectral ‘‘clumps’’ of fine structure provides direct evidence for coupling of the CH stretch to vibrational bath states, but no evidence for loss of Ka′ and Kc′ as good quantum labels in the spectrum. The average density of coupled states is found directly from the spectrum to be 114 states/cm−1, i.e., on the order of 0.5 ρvib. In contrast to the behavior in trans-2-butene, the 1-butene v=1←0 methyl asymmetric stretch spectrum exhibits an essentially continuous absorption contour even at Trot=6 K and 0.002 cm−1 resolution. On closer inspection, the 1-butene spectral envelope exhibits reproducible, intramolecular vibrational relaxation (IVR) induced fine structure limited by apparatus resolution and characteristic of highly congested IVR coupling. Analysis of this fine structure indicates a density of coupled states on the order of 1 000–10 000 states/cm−1, i.e., 20–30-fold in excess of ρvib, and 1–2 orders of magnitude larger than observed in trans-2-butene. In order to model the degree of fine structure observed in the spectrum, this level of spectral congestion essentially requires complete mixing of all ρvib⋅(2J′+1) rovibrational states consistent with conservation of total energy and angular momentum. The qualitatively dramatic differences between 1-butene and trans-2-butene behavior support a simple model for strong vibration-rotation (V-R) coupling in the bath states due to large amplitude skeletal motion in the C–C–C torsional mode which greatly enhances the available state density for IVR. Hence, the presence of a low barrier, skeletal isomerization coordinate may prove to be a general, moiety specific promoter for IVR processes in CH stretch excited hydrocarbons.