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 combustion gases. Only the most compact PAHs are formed in the flame. The observed groups of PAH peaks with 24 amu spacings belong to PAHs with constant hydrogen content and are separated by 26 amu gaps. The sequences of PAH peaks with 24 amu spacing are explained by a repetitive bay closure mechanism. The first PAH of a constant H-sequence is proposed to form by a dimerization process. The PAHs observed can be arranged in a repetitive pattern in Dias's formula periodic system. (C) 2000 American Institute of Physics. [S0021-9606(00)00902-8].