  We use observations of the CI, CII, HI, 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.
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 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 column
densities (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 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 which 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^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.
