UV absorption studies with the FUSE satellite have made important observations of H _ { 2 } molecular gas in Galactic interstellar translucent and diffuse clouds .
Observations of the 158 \mu m [ C ii ] fine structure line with Herschel trace the same H _ { 2 } molecular gas in emission .
We present [ C ii ] observations along 27 lines of sight ( LOSs ) towards target stars of which 25 have FUSE H _ { 2 } UV absorption .
Two stars have only HST STIS C ii \lambda 2325 absorption data .
We detect [ C ii ] 158 \mu m emission features in all but one target LOS .
For three target LOSs which are close to the Galactic plane , { | \it b| } < 1 ^ { \circ } , we also present position-velocity maps of [ C ii ] emission observed by Herschel HIFI in on-the-fly spectral line mapping .
We use the velocity resolved [ C ii ] spectra observed by the HIFI instrument towards the target LOSs observed by FUSE to identify [ C ii ] velocity components associated with the H _ { 2 } clouds .
We analyze the observed velocity integrated [ C ii ] spectral line intensities in terms of the densities and thermal pressures in the H _ { 2 } gas using the H _ { 2 } column densities and temperatures measured by the UV absorption data .
We present the H _ { 2 } gas densities and thermal pressures for 26 target LOSs and from the [ C ii ] intensities derive a mean thermal pressure in the range of \sim 6100 to 7700 K cm ^ { -3 } in diffuse H _ { 2 } clouds .
We discuss the thermal pressures and densities towards 14 targets , comparing them to results obtained using the UV absorption data for two other tracers C i and CO. Our results demonstrate the richness of the far-IR [ C ii ] spectral data which is a valuable complement to the UV H _ { 2 } absorption data for studying diffuse H _ { 2 } molecular clouds .
While the UV absorption is restricted to the directions of the target star , far-IR [ C ii ] line emission offers an opportunity to employ velocity resolved spectral line mapping capability to study in detail the clouds ’ spatial and velocity structures .