Molecular and atomic densities and gas parameters across a gas slab of extinction A_V = 0.5 (top) and A_V = 1.0 (bottom), with fixed pressure P = 10⁷ K cm⁻³ and density ρ = 10⁵ cm⁻³, irradiated by a WD model spectrum of T = 100 000 K (Levenhagen et al. 2017). Left: density of selected species across the visual extinction of the slab. The abundance ratio of HNC/HCN is plotted as a solid purple line, with scale indicated on the right-hand y-axis. Right: temperature and density across the model slab.