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We report here, for the first time, the association of low frequency CRRLwith \HI\ self-absorbing clouds in the inner Galaxy and that the CRRLs from theinnermost $\sim 10^{\circ}$ of the Galaxy arise in the Riegel-Crutcher (R-C)cloud. The R-C cloud is amongst the most well known of \HI\ self-absorbing(HISA) regions located at a distance of about 125 pc in the Galactic centredirection. Taking the R-C cloud as an example, we demonstrate that the physicalproperties of the HISA can be constrained by combining multi-frequency CRRL and\HI\ observations. The derived physical properties of the HISA cloud are usedto determine the cooling and heating rates. The dominant cooling process isemission of the \CII\ 158 \mum line whereas dominant heating process in thecloud interior is photoelectric emission. Constraints on the FUV flux (G0$\sim$ 4 to 7) falling on the R-C cloud are obtained by assuming thermalbalance between the dominant heating and cooling processes. The H$_2$ formationrate per unit volume in the cloud interior is $\sim$ 10$^{-10}$ -- 10$^{-12}$s$^{-1}$ \cmthree, which far exceeds the H$_2$ dissociation rate per unitvolume. We conclude that the self-absorbing cold \HI\ gas in the R-C cloud maybe in the process of converting to the molecular form. The cold \HI\ gasobserved as HISA features are ubiquitous in the inner Galaxy and form animportant part of the ISM. Our analysis shows that combining CRRL and \HI\ datacan give important insight into the nature of these cold gas. We also estimatethe integration times required to image the CRRL forming region with theupcoming SKA pathfinders. Imaging with the MWA telescope is feasible withreasonable observing times.

Carbon Recombination Lines toward the Riegel-Crutcher Cloud and other Cold HI Regions in the inner Galaxy