Reactive Dimerization: A New PAH Growth Mechanism in Flames

Abstract

The formation of polycyclic aromatic hydrocarbon (PAH) molecules in laminar diffusion flames is investigated by time-of-flight mass spectrometry combined with laser two-photon ionization. Gas samples from the flames are analyzed on-line with the help of a new inlet system that permits the recording of high-resolution height profiles. Such curves from 13 different flame species of pure and Ar-, C0₂- or H₂- diluted methane flames were measured. Larger PAH are generally found higher in the flame than smaller ones. This behaviour is described by a simple model for successive growth. Some PAH however deviate from this trend and are formed faster than their precursors. This is inconsistent with successive growth and suggests the occurrence of growth by reactive coagulation.