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Complementary
to the crossed beam experiments, we developed in collaboration with Musahid Ahmed
(Lawrence
Berkeley National Lab) an experimental protocol to investigate the
formation mechanisms of PAHs within a high temperature chemical reactor by
simulating the combustion relevant conditions. Utilizing the indene
molecule as a test case, we successfully conducted a directed synthesis of
indene in situ in a continuous
supersonic molecular beam through reaction of pyrolytically generated
phenyl radicals (C6H5) with hydrocarbons (methylacetylene,
allene) inside a heated silicon carbide tube (chemical reactor). Indene
together with its acyclic isomers were then photoionized by vacuum
ultraviolet (VUV) light from the Advanced
Light Source at the Chemical
Dynamics Beamline at various photon energies from 7.5 to 12.0 eV to
record photoionization efficiency (PIE) curves. Based on known PIE curves
of the PAH and their acyclic isomers, the recorded PIE curves were then
simulated theoretically to extract the nature of the products formed and
their branching ratios. Prior to this study, no experiment has been
conducted in which an individual PAH was formed via a gas phase reaction
under well-defined conditions in a high temperature chemical reactor. The
data analysis of the phenyl/propylene system is in progress.
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The aromatic
indene molecule (C9H8) together with its acyclic
isomers (phenylallene, 1-phenyl-1-propyne, and 3-phenyl-1-propyne) were formed
via a directed synthesis in situ utilizing a high-temperature chemical
reactor under combustion-like conditions (300 Torr, 1200 to 1500 K) through
the reactions of the phenyl radical (C6H5) with
propyne (CH3CCH) and allene (H2CCCH2). The
isomer distributions were probed utilizing tunable vacuum ultraviolet (VUV)
radiation from the Advanced Light Source by recording the photoionization
efficiency (PIE) curves at mass-to-charge of m/z = 116 (C9H8+)
of the products in a supersonic expansion for both the phenyl-allene and
phenyl-propyne systems; branching ratios were derived by fitting the
recorded PIE curves with a linear combination of the PIE curves of the
individual C9H8 isomers. Our data suggest that under
our experimental conditions, the formation of the aromatic indene molecule
via the reaction of the phenyl radical with allene is facile and enhanced
compared to the phenyl - propyne system by a factor of about 7. Reaction
mechanisms and branching ratios are explained in terms of new electronic
structure calculations in collaboration with Alexander Mebel
(Florida International University). Our newly developed high-temperature
chemical reactor presents a versatile approach to study the formation of
combustion-relevant polycyclic aromatic hydrocarbons (PAHs) under
well-defined and controlled conditions
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