Large molecules, ions, radicals, and small soot particles in fuel-rich hydrocarbon flames Part V: Positive ions of polycyclic aromatic hydrocarbons (PAH) in low-pressure premixed flames of benzene and oxygen

Low-pressure benzene-oxygen flames (C/O = 0.30-0.80, p = 2.66 kPa) have been analyzed for large positive PAH ions using molecular beam sampling, combined with reflectron time-of-flight mass spectrometry. In the mass range studied (10-90 C atoms per ion) PAH(+) ions occur with every number of C atoms. Growth of PAH+ is very similar to that of the neutral pendents in following a broad band in the C-H diagram, in which H-rich PAHs play an important role. PAHs with the same number of C atoms always occur with different numbers of H atoms indicating different structures of the carbon skeleton. They are initially Very II-rich, but with increasing temperature their formulae correspond more to maximum pericondensed structures (standard structures) and to H-poor PAHs containing condensed C pentagons. The classification of PAHs according to the oddity of their numbers of C and Il atoms reveals very regular intensities of C distributions for PAHs with a given number (even or odd) of H atoms. These regular C distributions have previously also been observed with neutral PAHs in the same flames. A novelty is the detection of regular C-distributions of very H-rich PAH(+) ions with fewer C atoms than that of the standard PAH structures with the same number of H atoms. They are interpreted as phenyl-substituted pericondensed PAHs, the C-distributions of which can be correlated with those of the unsubstituted pericondensed species. In cooler flame zones very near to the burner, extremely H-rich, oxygen-containing hydrocarbons ions are found in the mass range of about 150 to 400 u; they disappear rapidly with increasing temperature. Their formation is attributed to reactions between oxygen and the fuel, other than the usual combustion reactions, but also initiated by back diffusion of H atoms which attack benzene. (C) 2001 by The Combustion Institute.