  High quality archival spectra of interstellar absorption from C I toward 9
stars, taken with the Goddard High Resolution Spectrograph on the Hubble Space
Telescope, were analyzed. Our sample was supplemented by two sight lines, 23
Ori and B1 Sco, for which the C I measurements of Federman, Welty, & Cardelli
were used. Directions with known CH+ absorption, but only upper limits on
absorption from C2 and CN, were considered for our study. This restriction
allows us to focus on regions where CH+ chemistry dominates the production of
carbon-bearing molecules. Profile synthesis of several multiplets yielded
column densities and Doppler parameters for the C I fine structure levels.
Equilibrium excitation analyses, using the measured column densities as well as
the temperature from H2 excitation, led to values for gas density. These
densities, in conjunction with measurements of CH, CH+, C2, and CN column
densities, provided estimates for the amount of CH associated with CH+
production, which in turn set up constraints on the present theories for CH+
formation in this environment. We found for our sample of interstellar clouds
that on average, 30--40 % of the CH originates from CH+ chemistry, and in some
cases it can be as high as 90 %. A simple chemical model for gas containing
non-equilibrium production of CH+ was developed for the purpose of predicting
column densities for CH, CO, HCO+, CH2+, and CH3+ generated from large
abundances of CH+. Again, our results suggest that non-thermal chemistry is
necessary to account for the observed abundance of CH and probably that of CO
in these clouds.
