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Quantitative understanding of the interstellar medium requires knowledge ofits physical conditions. Low frequency carbon radio recombination lines (CRRLs)trace cold interstellar gas, and can be used to determine its physicalconditions (e.g., electron temperature and density). In this work we presentspatially resolved observations of the low frequency ($\leq390$ MHz) CRRLscentered around C$268\alpha$, C$357\alpha$, C$494\alpha$ and C$539\alpha$towards Cassiopeia A on scales of $\leq1.2$ pc. We compare the spatialdistribution of CRRLs with other ISM tracers. This comparison reveals a spatialoffset between the peak of the CRRLs and other tracers, which is verycharacteristic for photodissociation regions and that we take as evidence forCRRLs being preferentially detected from the surfaces of molecular clouds.Using the CRRLs we constrain the gas electron temperature and density. Theseconstraints on the gas conditions suggest variations of less than a factor oftwo in pressure over $\sim1$ pc scales, and an average hydrogen density of$200$-$470$ cm$^{-3}$. From the electron temperature and density maps we alsoconstrain the ionized carbon emission measure, column density and path length.Based on these, the hydrogen column density is larger than $10^{22}$ cm$^{-2}$,with a peak of $\sim4\times10^{22}$ cm$^{-2}$ towards the South of CassiopeiaA. Towards the southern peak the line of sight length is $\sim40$ pc over a$\sim2$ pc wide structure, which implies that the gas is a thin surface layeron a large (molecular) cloud that is only partially intersected by CassiopeiaA. These observations highlight the utility of CRRLs as tracers of low densityextended HI and CO-dark gas halo's around molecular clouds.

Mapping low frequency carbon radio recombination lines towards Cassiopeia A at 340, 148, 54 and 43 MHz