Molecular Hydrogen Formation Catalyzed by Polycyclic Aromatic Hydrocarbons in the Interstellar Medium

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 H 2 through dissociative recombination of the hydrogenated ion with an electron. The estimated rate coefficient of H 2 production using this process compares well with a widely accepted value of 3 × 10 −17 cm 3 s −1 for H 2 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.