2009ApJ...704..274L

We present an alternate model for the formation of molecular hydrogen in the interstellar medium (ISM) where polycyclic aromatic hydrocarbon (PAH) cations may play a key role as catalysts. The mechanism involves the chemical trapping of H atoms on the periphery of the PAH carbon skeleton and the subsequent release of H₂ through dissociative recombination of the hydrogenated ion with an electron. The estimated rate coefficient of H₂ production using this process compares well with a widely accepted value of 3x10^–17 cm3/s for H₂ formation on grains in the diffuse ISM. It is inferred that PAHs can contribute significantly to the formation of molecular hydrogen, especially in environments where the classical process of H adsorption on grains is impeded by high UV fluxes as in diffuse clouds and photon-dominated regions.