2008ApJ...680..384W

We use observations of the C I, C II, H I, and H₂ column densities along lines of sight in the Galactic plane to determine the formation rate of H₂ on grains and to determine chemical reaction rates with polycyclic aromatic hydrocarbons (PAHs). Photodissociation region models are used to find the best-fit parameters to the observed columns. We find the H₂formation rate on grains has a low rate (R∼1x10^–17 cm3/s) along lines of sight with low column density (A_V≲0.25) and low molecular fraction (f_H2≲10^–4). At higher column densities (0.25≤A_V≤2.13), we find a rate of R∼3.5x10^–17 cm3/s. The lower rate at low column densities could be the result of grain processing by interstellar shocks, which may deplete the grain surface area or process the sites of H+H formation, thereby inhibiting H₂production. Alternatively, the formation rate may be normal, and the low molecular fraction may be the result of lines of sight that graze larger clouds. Such lines of sight would have a reduced H₂self-shielding compared to the line-of-sight column. We find the reaction C⁺+PAH^–⟶C+PAH⁰ is best fit with a rate 2.4x10^–7Φ_PAHT^–0.5₂cm3/s with T₂=T/100 K, and the reaction C⁺+PAH⁰⟶C+PAH⁺ is best fit with a rate 8.8x10^–9Φ_PAHcm3/s. In high-column-density gas, we find Φ_PAH∼0.4. In low-column-density gas, Φ_PAH is less well constrained, with Φ_PAH∼0.2-0.4.