IONS AND CHARGED SOOT PARTICLES IN HYDROCARBON FLAMES .2. POSITIVE ALIPHATIC AND AROMATIC IONS IN ETHYNE OXYGEN FLAMES

Positive and negative ions up to masses >103 u in premixed low-pressure ethyne/oxygen flames were analyzed by means of a time-of-flight mass spectrometer. This work deals with positive ions. Absolute concentrations of different groups of ions were calculated from comparison with absolute total ion concentrations under different conditions. After a very fast formation of aliphatic CnH5+ and substituted oxomethylium ions, mono- and polyaromatic hydrocarbon ions (PAH+) were formed in the oxidation zone even at C/O below the threshold of soot formation. In nonsooting flames they were totally consumed by thermal decomposition, generating polyynic ions C3+2nH3+ and C4+2nH3+, n = 0-≈6. At lower temperatures c-C3H3+ was present while at maximum temperature the linear form probably prevailed. The polyynic ions were in equilibrium with ethyne and polyynes. Enthalpies of formation were obtained for ions up to C14H3+ from the equilibrium constants. Mass distributions of odd- and even-carbon-numbered CmH3+ could clearly be distinguished and their resemblance with Poisson distributions is discussed. In sooting flames, PAH+ up to masses of about 325 u were decomposed before the flame reached its maximum temperature while larger species continued to grow forming charged soot. After thermal decomposition of the lower mass PAH+ at temperatures ≳1400 K they were formed again at a much slower rate when the burned gas cooled. However, they did not exceed a mass of about 400 u. The different mechanisms of PAH+ formation in the oxidation zone and in the postflame gas is discussed. © 990 American Chemical Society.