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 $\mathrm{A}{\mathrm{r}}^{+2}$ , and $\mathrm{A}{\mathrm{r}}^{+4}$ , we use CHIANTI ( Dere et al (1997); DelZanna et al. (2022)) using the default values for the A values and collision strengths. For $\mathrm{F}{\mathrm{e}}^{+}$ 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 $n_e={10}^2\mathrm{c}{\mathrm{m}}^{-3}$ to ${10}^6\mathrm{c}{\mathrm{m}}^{-3}$.