LARGE MOLECULES, RADICALS, IONS, AND SMALL SOOT PARTICLES IN FUEL-RICH HYDROCARBON FLAMES - PART I - POSITIVE-IONS OF POLYCYCLIC AROMATIC-HYDROCARBONS (PAH) IN LOW-PRESSURE PREMIXED FLAMES OF ACETYLENE AND OXYGEN

Large positive ions with masses up to 10(4) u have been studied in low -pressure, fuel-rich. premixed, flat acetylene/oxygen flames using mol ecular beam sampling with a reflectron time-of-flight mass spectromete r, The ions detected were those of polyynes, polyene-carbenes, polycyc lic aromatic hydrocarbons (PAH(+)), oxygen-containing PAHs, fullerenes , aromers, and small soot particles. Absolute number densities of indi vidual ions were determined through measuring the total ionic number d ensity. This paper focuses on PAH(+) and the transition to small charg ed soot particles. It is shown that the occurrence of large charged PA Hs and the variation of their concentrations are representative of the ir neutral counterparts. In fact, PAH(+) ions with any number of C ato ms up to more than 300 are formed. High-resolution mass spectrometry r eveals the variation in the hydrogen content of PAH(+) with the same n umber of C atoms. PAH(+) ions with an even number of C atoms occur, bo th ionized and protonated. Also reported are C/H diagrams of all molec ular formulae with even- and odd-numbered PAH(+) up to 70 C atoms and also C/H ratios of the larger PAH(+) ions, with up to 320 C atoms. Fro m the relationships bem een molecular formulae and possible structures and also the profiles of individual ions, a growth mechanism of PAH(), which is very probably also valid for uncharged PAHs, is presented and discussed. The hydrogen content and therefore also the carbon stru cture of the PAH(+) ions play an important role in their growth. The d evelopment of the C/H ratio of very large PAH(+) ions is compared to t hat of small soot particles. The continuous mass spectra of aromers as precursors of soot and fullerenes were also detected in these acetyle ne flames. (C) 1998 by The Combustion Institute