Chemical Rates on Small Grains and PAHs: C⁺Recombination and H₂Formation

We use observations of the C I, C II, H I, and H 2 column densities along lines of sight in the Galactic plane to determine the formation rate of H 2 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 H 2 formation rate on grains has a low rate ( R ∼ 1 × 10 −17 cm 3 s −1 ) along lines of sight with low column density ( A V ≲ 0.25) and low molecular fraction ( f H 2 ≲ 10 −4 ). At higher column densities (0.25 ⩽ A V ⩽ 2.13), we find a rate of R ∼ 3.5 × 10 −17 cm 3 s −1 . 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 H 2 production. 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 H 2 self-shielding compared to the line-of-sight column. We find the reaction C + + PAH − → C + PAH 0 is best fit with a rate 2.4 × 10 −7 Φ PAH T −0.5 2 cm 3 s −1 with T 2 = T/100 K, and the reaction C + + PAH 0 → C + PAH + is best fit with a rate 8.8 × 10 −9 Φ PAH cm 3 s −1 . 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.