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$^\circ$, 64$^\circ$) and
(189$^\circ$, 207$^\circ$) 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 condition for OH from diffuse
clouds in the Galactic plane. The column densities of the HI `halos' N(HI)
surrounding molecular clouds increase monotonically with OH column density,
N(OH), until saturating when N(HI)=1.0 x 10$^{21}$ cm$^{-2}$ and N (OH) $\geq
4.5\times 10^{15}$ cm$^{-2}$, indicating the presence of molecular gas that
cannot be traced by HI. 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.
