We carried out a comprehensive far-UV survey of 12 CO and H 2 column densities along diffuse molecular Galactic sight lines. This sample includes new measurements of CO from HST spectra along 62 sight lines and new measurements of H 2 from FUSE data along 58 sight lines. In addition, high-resolution optical data were obtained at the McDonald and European Southern Observatories, yielding new abundances for CH, CH + , and CN along 42 sight lines to aid in interpreting the CO results. These new sight lines were selected according to detectable amounts of CO in their spectra and provide information on both lower density ( 100 cm À3) and higher density diffuse clouds. A plot of log N (CO) versus log N (H 2 ) shows that two power-law relationships are needed for a good fit of the entire sample, with a break located at log N (CO; cm À2 ) ¼ 14:1 and log N (H 2 ) ¼ 20:4, corresponding to a change in production route for CO in higher density gas. Similar logarithmic plots among all five diatomic molecules reveal additional examples of dual slopes in the cases of CO versus CH (break at log N ¼ 14:1, 13.0), CH + versus H 2 (13.1, 20.3), and CH + versus CO (13.2, 14.1). We employ both analytical and numerical chemical schemes in order to derive details of the molecular environments. In the denser gas, where C 2 and CN molecules also reside, reactions involving C + and OH are the dominant factor leading to CO formation via equilibrium chemistry. In the low-density gas, where equilibrium chemistry studies have failed to reproduce the abundance of CH + , our numerical analysis shows that nonequilibrium chemistry must be employed for correctly predicting the abundances of both CH + and CO.

We examine 20 diffuse and translucent Galactic sight lines and extract the column densities of the 12 CO and 13 CO isotopologues from their ultraviolet AYX absorption bands detected in archival Space Telescope Imaging Spectrograph data with k/Ák ! 46;000. Five more targets with Goddard High-Resolution Spectrograph data are added to the sample that more than doubles the number of sight lines with published Hubble Space Telescope observations of 13 CO. Most sight lines have 12 CO-to-13 CO isotopic ratios that are not significantly different from the local value of 70 for 12 C/ 13 C, which is based on millimeter-wave observations of rotational lines in emission from CO and H 2 CO inside dense molecular clouds, as well as on results from optical measurements of CH + . Five of the 25 sight lines are found to be fractionated toward lower 12 C/ 13 C values, while three sight lines in the sample are fractionated toward higher ratios, signaling the predominance of either isotopic charge exchange or selective photodissociation, respectively. There are no obvious trends of the 12 CO-to-13 CO ratio with physical conditions such as gas temperature or density, yet 12 CO/ 13 CO does vary in a complicated manner with the column density of either CO isotopologue, owing to varying levels of competition between isotopic charge exchange and selective photodissociation in the fractionation of CO. Finally, rotational temperatures of H 2 show that all sight lines with detected amounts of 13 CO pass through gas that is on average colder by 20 K than the gas without 13 CO. This colder gas is also sampled by CN and C 2 molecules, the latter indicating gas kinetic temperatures of only 28 K, enough to facilitate an efficient charge exchange reaction that lowers the value of 12 CO/ 13 CO.

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