We report the results from a new , highly sensitive ( \Delta T _ { mb } \sim 3 mK ) survey for thermal OH emission at 1665 and 1667 MHz over a dense , 9 x 9-pixel grid covering a 1 \degr \times 1 \degr patch of sky in the direction of l = 105 \fdg 00 ,b = +2 \fdg 50 towards the Perseus spiral arm of our Galaxy .
We compare our Green Bank Telescope ( GBT ) 1667 MHz OH results with archival ^ { 12 } CO ( 1-0 ) observations from the Five College Radio Astronomy Observatory ( FCRAO ) Outer Galaxy Survey within the velocity range of the Perseus Arm at these galactic coordinates .
Out of the 81 statistically-independent pointings in our survey area , 86 % show detectable OH emission at 1667 MHz , and 19 % of them show detectable CO emission .
We explore the possible physical conditions of the observed features using a set of diffuse molecular cloud models .
In the context of these models , both OH and CO disappear at current sensitivity limits below an A _ { v } of 0.2 , but the CO emission does not appear until the volume density exceeds 100-200 { cm ^ { -3 } } .
These results demonstrate that a combination of low column density A _ { v } and low volume density n _ { H } can explain the lack of CO emission along sight lines exhibiting OH emission .
The 18-cm OH main lines , with their low critical density of n ^ { * } \sim 1 { cm ^ { -3 } } , are collisionally excited over a large fraction of the quiescent galactic environment and , for observations of sufficient sensitivity , provide an optically-thin radio tracer for diffuse H _ { 2 } .