2010A&A...521L..18V

We present the first results on the diffuse transition clouds observed in [CII] line emission at 158µm (1.9THz) 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 ¹²CO emission but without ¹³CO emission and characterized by A_V<5mag. 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 H₂layer around the CO core. The wide range of clouds in our sample with thin to thick H₂layers suggests that these are at various evolutionary states characterized by the formation of H₂and CO layers from HI and C⁺, respectively. In about 30% of the clouds the H₂column densities (``dark gas'') traced by the [CII] is 50% or more than that traced by ¹²CO emission. On the average ∼25% of the total H₂in these clouds is in an H₂layer which is not traced by CO. We use the HI, [CII], and ¹²CO intensities in each cloud along with simple chemical models to obtain constraints on the FUV fields and cosmic ray ionization rates.