OH Survey along Sightlines of Galactic Observations of Terahertz C+

We have obtained OH spectra of four transitions in the ${}^2{\Pi }_{3/2}$ ground state, at 1612, 1665, 1667, and 1720 MHz, toward 51 sightlines that were observed in the Herschel project Galactic Observations of Terahertz C+. The observations cover the longitude range of (32°, 64°) and (189°, 207°) in the northern Galactic plane. All of the diffuse OH emissions conform to the so-called "Sum Rule" of the four brightness temperatures, indicating optically thin emission conditions for OH from diffuse clouds in the Galactic plane. The column densities of the H i "halos" N(H i) surrounding molecular clouds increase monotonically with OH column density, N(OH), until saturating when $N\left(HI\right)=1.0\times {10}^{21}$ cm⁻² and $N\left(OH\right)⩾4.5\times {10}^{15}$ cm⁻², indicating the presence of molecular gas that cannot be traced by H i. Such a linear correlation, albeit weak, is suggestive of $HI$ halos' contribution to the UV shielding required for molecular formation. About 18% of OH clouds have no associated CO emission (CO-dark) at a sensitivity of 0.07 K, but are associated with C⁺ emission. A weak correlation exists between C⁺ intensity and OH column density for CO-dark molecular clouds. These results imply that OH seems to be a better tracer of molecular gas than CO in diffuse molecular regions.