Context : H i and CO large scale surveys of the Milky Way trace the diffuse atomic clouds and the dense shielded regions of molecular hydrogen clouds , respectively .
However , until recently , we have not had spectrally resolved C ^ { + } surveys in sufficient lines of sight to characterize the ionized and photon dominated components of the interstellar medium , in particular , the H _ { 2 } gas without CO , referred to as CO–dark H _ { 2 } , in a large sample of interstellar clouds .

Aims : To use a sparse Galactic plane survey of the 1.9 THz ( 158 \mu m ) [ C ii ] spectral line from the Herschel Open Time Key Programme , Galactic Observations of Terahertz C+ ( GOT C+ ) , to characterize the H _ { 2 } gas without CO in a statistically significant sample of interstellar clouds .

Methods : We identify individual clouds in the inner Galaxy by fitting the [ C ii ] and CO isotopologue spectra along each line of sight .
We then combine these spectra with those of H i and use them along with excitation models and cloud models of C ^ { + } to determine the column densities and fractional mass of CO–dark H _ { 2 } clouds .

Results : We identify1804 narrow velocity [ C ii ] components corresponding to interstellar clouds in different categories and evolutionary states .
About 840 are diffuse molecular clouds with no CO , \sim 510 are transition clouds containing [ C ii ] and ^ { 12 } CO , but no ^ { 13 } CO , and the remainder are dense molecular clouds containing ^ { 13 } CO emission .
The CO–dark H _ { 2 } clouds are concentrated between Galactic radii of \sim 3.5 to 7.5 kpc and the column density of the CO–dark H _ { 2 } layer varies significantly from cloud-to-cloud with a global average of 9 \times 10 ^ { 20 } cm ^ { -2 } .
These clouds contain a significant fraction by mass of CO–dark H _ { 2 } , that varies from \sim 75 \% for diffuse molecular clouds to \sim 20 \% for dense molecular clouds .

Conclusions : We find a significant fraction of the warm molecular ISM gas is invisible in H i and CO , but is detected in [ C ii ] .
The fraction of CO–dark H _ { 2 } is greatest in the diffuse clouds and decreases with increasing total column density , and is lowest in the massive clouds .
The column densities and mass fraction of CO-dark H _ { 2 } is less than predicted by models of diffuse molecular clouds using solar metallicity , which is not surprising as most of our detections are in Galactic regions where the metallicity is larger and shielding more effective .
There is an overall trend towards a higher fraction of CO–dark H _ { 2 } in clouds with increasing Galactic radius , consistent with lower metallicity there .