[CII] observations of H$_2$ molecular layers in transition clouds
  We present the first results on the diffuse transition clouds observed in
[CII] line emission at 158 microns (1.9 THz) towards Galactic longitudes near
340deg (5 LOSs) and 20deg (11 LOSs) as part of the GOT C+ survey. Out of the
total 146 [CII] velocity components detected by profile fitting we identify 53
as diffuse molecular clouds with associated $^{12}$CO emission but without
$^{13}$CO emission and characterized by A$_V$ < 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 $^{12}$CO
emission. On the average about 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 $^{12}$CO
intensities in each cloud along with simple chemical models to obtain
constraints on the FUV fields and cosmic ray ionization rates.
