We investigated the physical and chemical changes induced in soot aggregates exposed to laser radiation using a scanning mobility particle sizer, a transmission electron microscope, and a scanning transmission x-ray microscope to perform near-edge x-ray absorption fine structure spectroscopy. Laser-induced nanoparticle production was observed at fluences above 0.12 J/cm(2) at 532 nm and 0.22 J/cm(2) at 1064 nm. Our results indicate that new particle formation proceeds via (1) vaporization of small carbon clusters by thermal or photolytic mechanisms, followed by homogeneous nucleation, (2) heterogeneous nucleation of vaporized carbon clusters onto material ablated from primary particles, or (3) both processes.

We investigated the physical and chemical changes induced in soot aggregates exposed to laser radiation using a scanning mobility particle sizer, a transmission electron microscope, and a scanning transmission x-ray microscope to perform near-edge x-ray absorption fine structure spectroscopy. Laser-induced nanoparticle production was observed at fluences above 0.12 J/cm(2) at 532 nm and 0.22 J/cm(2) at 1064 nm. Our results indicate that new particle formation proceeds via (1) vaporization of small carbon clusters by thermal or photolytic mechanisms, followed by homogeneous nucleation, (2) heterogeneous nucleation of vaporized carbon clusters onto material ablated from primary particles, or (3) both processes.

We measured aerosol mass spectra and mobility size distributions of particles formed during the pyrolysis of ethylene, indene, and a mixture of ethylene with a small amount of indene. Measurements were recorded with a scanning mobility particle sizer and an aerosol mass spectrometer employing vacuum ultraviolet photoionization using tunable radiation from the Advanced Light Source at Lawrence Berkeley National Laboratory. The results demonstrate that particle formation occurs at a temperature of ∼1123±50 K for ethylene alone, at ∼923±50 K for indene alone, and at a comparable temperature to that of indene for ethylene seeded with a small amount of indene. Our results demonstrate that indene and indenyl, a resonance-stabilized radical (RSR) formed from indene pyrolysis, promote particle formation at lower temperatures and in the absence of acetylene, supporting the hypothesis that RSRs promote soot-particle inception. Even a small amount of indene added to ethylene promotes particle formation while circumventing the traditional pathways for mass growth by acetylene-addition reactions. Further support is provided by the appearance in the aerosol mass spectra of prominent peaks for masses corresponding to other RSRs from particles formed at lower pyrolysis temperatures. In addition, odd-numbered carbon species, associated with RSRs, appear in higher concentrations at low temperatures and are closely tied to particle inception. In indene pyrolysis, mass signals indicative of covalently bound indenyl dimers and trimers are prominent at pyrolysis temperatures near soot onset. Photoionization efficiency (PIE) curves demonstrate that, while the isomeric composition for some of these peaks may differ between ethylene and indene, the addition of indene to ethylene does not notably alter the isomers formed at these peaks between ethylene and the ethylene-indene mixture. PIE curves also show that m/z 202, which is commonly assumed to be pyrene, is composed principally of fluoranthene under these conditions.

We investigated the pyrolysis of propyne and propene using aerosol mass spectrometry coupled with tunable synchrotron vacuum ultraviolet photoionization. The results show that propyne forms particles at pyrolysis temperatures approximately 100 K below the lowest temperature at which propene produces particles. The aerosol mass spectra show that the carbon-to-hydrogen (C/H) ratios of the pyrolysis products from both reactants increase with temperature between the particle onset temperature and 1275 K. In this temperature range, the mass peaks corresponding to species with 18 or more carbon atoms are more saturated (i.e., have lower C/H ratios) than species predicted by the stabilomer grid. The observation of relatively high saturation (low C/H ratios) at temperatures near particle onset is consistent with the presence of aliphatically linked hydrocarbons during the preliminary stages of soot formation. The masses of the species presumed to be aliphatically linked suggest that these linked species are adducts of species smaller than chrysene (C18H12). We performed a comparative analysis of the initial PAH-growth pathways consistent with aerosol mass spectra and associated mass-specific photoionization-efficiency curves. The results from the comparative analysis indicate that the development of PAHs up to the size of acenaphthylene (C12H8) follow similar formation pathways for propyne and propene, but the isomeric composition of species larger than acenaphthylene diverges for the two reactants.

We have measured photoionization-efficiency curves for pyrene, fluoranthene, chrysene, perylene, and coronene in the photon energy range of 7.5-10.2 eV and derived their photoionization cross-section curves in this energy range. All measurements were performed using tunable vacuum ultraviolet (VUV) radiation generated at the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory. The VUV radiation was used for photoionization, and detection was performed using a time-of-flight mass spectrometer. We measured the photoionization efficiency of 2,5-dimethylfuran simultaneously with those of pyrene, fluoranthene, chrysene, perylene, and coronene to obtain references of the photon flux during each measurement from the known photoionization cross-section curve of 2,5-dimethylfuran.

We have characterized soot particles measured in situ in a laminar co-flow ethylene-air diffusion flame using small-angle X-ray scattering (SAXS). The analysis includes temperature measurements made with coherent anti-Stokes Raman spectroscopy (CARS) and complements soot volume-fraction and maturity measurements made with laser-induced incandescence (LII). We compared the results of fits to the SAXS measurements using a unified model and a fractal core-shell model. Power-law parameters yielded by the unified model indicate that aggregates of primary particles are in the mass-fractal regime, whereas the primary particles are in the surface-fractal regime in the middle of the flame. Higher and lower in the flame, the primary-particle power-law parameter approaches 4, suggesting smooth primary particles. These trends are consistent with fits using the fractal core-shell model, which indicate that particles have an established core-shell structure in the middle of the flame and are internally homogeneous at higher and lower heights in the flame. Primary-particle size distributions derived using the fractal core-shell model demonstrate excellent agreement with distributions inferred from transmission electron microscopy (TEM) images in the middle of the flame. Higher in the flame, a second small mode appears in the size distributions, suggesting particle fragmentation during oxidation. Surface oxidation would explain (1) aggregate fragmentation and (2) loss of core-shell structure leading to smoother primary-particle surfaces by removal of carbon overlayers. SAXS measurements are much more sensitive to incipient and young soot particles than LII and demonstrate significant volume fraction from particles low in the flame where the LII signal is negligible.

We present a combined experimental and probabilistic simulation study of soot-precursor. The experiments were conducted using aerosol mass spectrometry coupled with tunable vacuum ultraviolet radiation from the Advanced Light Source at Lawrence Berkeley National Laboratory. Mass spectra and photoionization efficiency (PIE) curves of soot precursor species were measured at different heights in a premixed flat flame and in a counter-flow diffusion flame fueled by ethylene and oxygen. The PIE curves at the pyrene mass from these flames were compared with reference PIE scans recorded for pyrene. The results demonstrate that other C16H10 isomers than pyrene are major components among species condensed onto incipient soot in this study, which is in agreement with the simulations. Species with mass 202 u only have a high prevalence in incipient soot particles drawn from the premixed flame, but hydrocarbon species with sizes in the range 200-400 u are important to incipient-soot formation in both flames. The simulations predict that some species form through combination reactions involving relatively large radicals and bypass traditional molecular-growth pathways through addition of small hydrocarbon species. The experimental results support this prediction; they demonstrate that these species have higher relative abundances in particles formed close to the fuel outlet than smaller, lighter molecular species and indicate that these species are important to early formation of incipient-soot precursors. The results also imply that a leading role in incipient-soot precursor formation is played by species with lower thermal stability than the even-carbon numbered, unsubstituted polycyclic aromatic hydrocarbons known as "stabilomers".

We have developed a strategy for distinguishing between small-angle X-ray scattering (SAXS) from gas-phase species and newly formed nanoparticles in mixed gas- and particle-phase reacting flows. This methodology explicitly accounts for temperature-dependent scattering from gases. We measured SAXS in situ in a sooting linear laminar partially premixed co-flow ethylene/air diffusion flame. The scattering signal demonstrates a downward curvature as a function of the momentum transfer (q) at q values of 0.2-0.57 Å^-1. The q-dependent curvature is consistent with the Debye equation and the independent-atom model for gas-phase scattering. This behavior can also be modeled using the Guinier approximation and could be characterized as a Guinier knee for gas-phase scattering. The Guinier functional form can be fit to the scattering signal in this q range without a priori knowledge of the gas-phase composition, enabling estimation of the gas-phase contribution to the scattering signal while accounting for changes in the gas-phase composition and temperature. We coupled the SAXS measurements with in situ temperature measurements using coherent anti-Stokes Raman spectroscopy. This approach to characterizing the gas-phase SAXS signal provides a physical basis for distinguishing among the contributions to the scattering signal from the instrument function, flame gases, and nanoparticles. The results are particularly important for the analysis of the SAXS signal in the q range associated with particles in the size range of 1-6 nm.

We present a critical evaluation of photoionization efficiency (PIE) measurements coupled with aerosol mass spectrometry for the identification of condensed soot-precursor species extracted from a premixed atmospheric-pressure ethylene/oxygen/nitrogen flame. Definitive identification of isomers by any means is complicated by the large number of potential isomers at masses likely to comprise particles at flame temperatures. This problem is compounded using PIE measurements by the similarity in ionization energies and PIE-curve shapes among many of these isomers. Nevertheless, PIE analysis can provide important chemical information. For example, our PIE curves show that neither pyrene nor fluoranthene alone can describe the signal from C₁₆H₁₀ isomers and that coronene alone cannot describe the PIE signal from C₂₄H₁₂ species. A linear combination of the reference PIE curves for pyrene and fluoranthene yields good agreement with flame-PIE curves measured at 202 u, which is consistent with pyrene and fluoranthene being the two major C₁₆H₁₀ isomers in the flame samples, but does not provide definite proof. The suggested ratio between fluoranthene and pyrene depends on the sampling conditions. We calculated the values of the adiabatic-ionization energy (AIE) of 24 C₁₆H₁₀ isomers. Despite the small number of isomers considered, the calculations show that the differences in AIEs between several of the isomers can be smaller than the average thermal energy at room temperature. The calculations also show that PIE analysis can sometimes be used to separate hydrocarbon species into those that contain mainly aromatic rings and those that contain significant aliphatic content for species sizes investigated in this study. Our calculations suggest an inverse relationship between AIE and the number of aromatic rings. We have demonstrated that further characterization of precursors can be facilitated by measurements that test species volatility.