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 H i ‘ halos ’ N ( H i ) surrounding molecular clouds increase monotonically with OH column density , N ( OH ) , until saturating when N ( H { \textsc { i } } ) = 1.0 \times 10 ^ { 21 } cm ^ { -2 } and N ( OH ) \geq 4.5 \times 10 ^ { 15 } cm ^ { -2 } , indicating the presence of molecular gas that can not be traced by H i .
Such a linear correlation , albeit weak , is suggestive of H i 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 .