Formation mechanism for polycyclic aromatic hydrocarbons in methane flames

A laminar diffusion flame of methane was investigated using time-of-flight mass spectroscopy with two-photon UV laser ionization. Benzenoid polycyclic aromatic hydrocarbons (PAHs) up to 788 amu (C64H20) were detected in the c ombustion gases. Only the most compact PAHs are formed in the flame. The ob served groups of PAH peaks with 24 amu spacings belong to PAHs with constan t hydrogen content and are separated by 26 amu gaps. The sequences of PAH p eaks with 24 amu spacing are explained by a repetitive bay closure mechanis m. The first PAH of a constant H-sequence is proposed to form by a dimeriza tion process. The PAHs observed can be arranged in a repetitive pattern in Dias's formula periodic system. (C) 2000 American Institute of Physics. [S0 021-9606(00)00902-8].