Chemical rates on small grains and PAHs:: C+ recombination and H2 formation

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 similar to 1 x 10(-17) cm(3) s(-1)) along lines of sight with low column density (A(V) less than or similar to 0.25) and low molecular fraction (f(H2) less than or similar to 10(-4)). At higher column densities (0.25 <= A(V) <= 2.13), we find a rate of R similar to 3.5 x 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 x 10(-7) Phi(PAH) T-2(-0.5) cm(3) s(-1) with T-2 = T/100 K, and the reaction C+ + PAH(-) -> C + PAH(0) is best fit with a rate 8.8 x 10(-9) Phi(PAH) cm(3) s(-1). In high-column-density gas, we find Phi(PAH) similar to 0.4. In low-column-density gas, Phi(PAH) is less well constrained, with Phi(PAH) similar to 0.2-0.4.