Model⁵ of the C⁺ layer with [C]/[S] = 10. Lower panel: abundance profiles as a function of A_V along the UV-illumination direction: from the IF to the cloud interior (lower axis) and the equivalent angular distance perpendicular to the Bar (upper axis). Upper panel: normalized emissivities of the [C II] 158 μm, S54α, and C54α lines. For the RRLs, we adopted an LTE model (see Appendix A).