I_FIR emitted from the surface of a PDR as a function of the cloud density n and the FUV flux incident on the cloud G₀. We assume that the line emission is in the optically thin limit so that the ratio of emissivities is given by the ratio of volume emissivity. For \({\rm Ar^{+2}}\) , and \({\rm Ar^{+4}}\) , we use CHIANTI ( Dere et al (1997); DelZanna et al. (2022)) using the default values for the A values and collision strengths. For \({\rm Fe^+}\) we substituted the default values in CHIANTI with Einstein A values from Deb & Hibbert (2011) and collision strengths from Smyth et al. (2019) . The emissivity ratios are found in the temperature range from \(T_e=10^3\) K to \(10^4\) K, and the density range from \(n_e = 10^2~{\rm cm^{-3}}\) to \(10^6~{\rm cm^{-3}}\).