[CII] observations of H2 molecular layers in transition clouds

We present the first results on the diffuse transition clouds observed in [CII] line emission at 158 mu m (1.9 THz) towards Galactic longitudes near 340. (5 LOSs) & 20. (11 LOSs) as part of the HIFI tests and GOT C+ survey. Out of the total 146 [CII] velocity components detected by profile fitting we identify 53 as diffuse molecular clouds with associated (CO)-C-12 emission but without 13CO emission and characterized by AV < 5 mag. We estimate the fraction of the [CII] emission in the diffuse HI layer in each cloud and then determine the [CII] emitted from the molecular layers in the cloud. We show that the excess [CII] intensities detected in a few clouds is indicative of a thick H-2 layer around the CO core. The wide range of clouds in our sample with thin to thick H-2 layers suggests that these are at various evolutionary states characterized by the formation of H-2 and CO layers from HI and C+, respectively. In about 30% of the clouds the H-2 column densities (" dark gas") traced by the [CII] is 50% or more than that traced by (CO)-C-12 emission. On the average similar to 25% of the total H-2 in these clouds is in an H-2 layer which is not traced by CO. We use the HI, [CII], and (CO)-C-12 intensities in each cloud along with simple chemical models to obtain constraints on the FUV fields and cosmic ray ionization rates.