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We use observations of the CI, CII, HI, and H_2 column densities along linesof sight in the Galactic plane to determine the formation rate of H_2 on grainsand to determine chemical reaction rates with Polycyclic Aromatic Hydrocarbons.Photodissociation region models are used to find the best fit parameters to theobserved columns. We find the H_2 formation rate on grains has a low rate (R ~1 x 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 columndensities (0.25 < A_V <2.13), we find a rate of R ~ 3.5x10^(-17) cm^(3) s^(-1).The lower rate at low column densities could be the result of grain processingby interstellar shocks which may deplete the grain surface area or process thesites of H +H formation, thereby inhibiting H_2 production. Alternatively, theformation rate may be normal, and the low molecular fraction may be the resultof lines of sight which graze larger clouds. Such lines of sight would have areduced H_2 self-shielding compared to the line-of-sight column. We find thereaction 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^0 --> C + PAH^+ is best fit with a rate 8.8x 10^(-9)\Phi_PAH cm^(3) s^(-1).In high column density gas we find \Phi_PAH ~ 0.4. In low column density gas,\Phi_PAH is less well constrained with \Phi_PAH ~ 0.2 - 0.4.

Chemical Rates on Small Grains and PAHs: C^+ Recombination and H_2 Formation