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

Abstract

Large positive ions with masses up to 10 4 u have been studied in low-pressure, fuel-rich, premixed, flat acetylene/oxygen flames using molecular beam sampling with a reflectron time-of-flight mass spectrometer. The ions detected were those of polyynes, polyene-carbenes, polycyclic aromatic hydrocarbons (PAH +), oxygen-containing PAHs, fullerenes, aromers, and small soot particles. Absolute number densities of individual ions were determined through measuring the total ionic number density. This paper focuses on PAH + and the transition to small charged soot particles. It is shown that the occurrence of large charged PAHs and the variation of their concentrations are representative of their neutral counterparts. In fact, PAH + ions with any number of C atoms up to more than 300 are formed. High-resolution mass spectrometry reveals the variation in the hydrogen content of PAH + with the same number of C atoms. PAH + ions with an even number of C atoms occur, both ionized and protonated. Also reported are C/H diagrams of all molecular 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. From the relationships between 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 structure of the PAH + ions play an important role in their growth. The development of the C/H ratio of very large PAH + ions is compared to that of small soot particles. The continuous mass spectra of aromers as precursors of soot and fullerenes were also detected in these acetylene flames.