We model Carbon Recombination Line ( CRL ) emission from the Photo Dissociation Region ( PDR ) surrounding the Ultra-Compact ( UC ) H ii region W48A .
Our modelling shows that the inner regions ( A _ { V } \sim 1 ) of the C ii layer in the PDR contribute significantly to the CRL emission .
The dependence of line ratios of CRL emission with the density of the PDR and the far ultra-violet ( FUV ) radiation incident on the region is explored over a large range of these parameters that are typical for the environments of UCH ii regions .
We find that by observing a suitable set of CRLs it is possible to constrain the density of the PDR .
If the neutral density in the PDR is high ( \gtrsim 10 ^ { 7 } cm ^ { -3 } ~ { } ) CRL emission is bright at high frequencies ( \gtrsim 20 GHz ) , and absorption lines from such regions can be detected at low frequencies ( \lesssim 10 GHz ) .
Modelling CRL observations towards W48A shows that the UCH ii region is embedded in a molecular cloud of density of about 4 ~ { } \times 10 ^ { 7 } cm ^ { -3 } ~ { } .