We have combined experimental and theoretical approaches to gain new insight into the mechanisms of PAH growth and soot formation. The experimental approach involves aerosol-mass spectrometry in conjunction with vacuum-ultraviolet photoionization of volatile species vaporizing from particles sampled from an Ar-diluted C2H2/O2 counter-flow diffusion flame at nearly atmospheric pressure (700 Torr). We recorded aerosol mass spectra at different distances from the fuel outlet for fixed ionization energies and in a fixed position while tuning the photoionization energy. The mass spectra contain a large distribution of peaks, highlighting the importance of small building blocks and showing a variety of chemical species that extends beyond the traditional classification of PAHs based on thermodynamic stability. In addition, we performed stochastic simulations of PAH growth in the flame in order to provide better insight into the chemical composition of species associated with peaks in the measured mass spectra. These simulations were conducted using a stochastic nanoparticle simulator (SNAPS). Synthesis of experimental and simulated results showed that peaks in the observed mass spectra generally consisted of a mixture of PAH isomers. At m/z =154 and 202, for example, experiments and simulations suggested that additional isomers than biphenyl and pyrene are important. Furthermore, the results highlight the importance of odd-carbon numbered species and complex growth paths. The experimental results suggest that species of higher masses can build up concentration ahead of species of lower masses. Our experimental results show, for example, that the peak at m/z = 278 appears closer to the burner outlet than the peak at m/z = 202, i.e., suggesting that a single monotonic growth mechanism is not enough.
In this study, two flames of iso-pentanol were stabilized on a 60-mm flat flame burner at a low pressure of 15 Torr and analyzed by a flame-sampling molecular-beam setup coupled to a mass spectrometer (MBMS). Singlephoton ionization by synchrotron-generated vacuum-UV radiation with high energy resolution (E/ΔE ∼0.04 eV) and/or electron ionization was combined with a custom-built reflectron time-of-flight spectrometer providing high mass resolution (m/Δm = 3000). Mole fraction profiles for more than 40 flame species and the temperature profile were determined experimentally. The flame temperatures were measured using OH laser induced fluorescence and used as input parameters for the model calculations. The experimental dataset was used to guide the development of a combustion chemistry model for the high-temperature oxidation chemistry of iso-pentanol. The chemical kinetic model is herein validated for the first time against detailed speciation profiles of combustion intermediates and product species including C5 branched aldehydes, enols, and alkenes. In a separated study, the model was validated against a number of different datasets including low and high temperature ignition delay in rapid compression machines and shock tubes, jet stirred reactor speciation data, premixed laminar flame speed, and opposed-flow diffusion flame strained extinction.
Cyclic ethers, like tetrahydrofuran (THF), are formed during the autoignition of alkanes and subsequently influence their combustion chemistry. To learn more about the oxidation chemistry of these ether intermediates, a fuel-rich THF flame (π = 1.75) has been studied using the versatile technique of flame-sampling Molecular Beam Mass Spectrometry (MBMS) in combination with single-photon ionization. Several cyclic intermediates which are potentially formed by dehydrogenation of the fuel are identified by their ionization energies. Ethylene, propene, ketene and formaldehyde are major stable decomposition products of THF and their mole fraction profiles are presented. Detected oxygenated species include ethenol, acetaldehyde and propanal. Despite the fuel-rich conditions, the concentrations of benzene and other aromatic hydrocarbons are near the detection limit.
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.