Simple schematic of a PDR. The incoming UV radiation from the star at left ionizes the material and dissociates molecules. When the UV field penetrates further inside the PDR, its intensity decreases and more complex molecules can be built. The dashed envelopes represent the typical transition layers within the PDR, moving from atomic and ionic species at the left to molecular and neutral species at the right at higher extinction. Cells 1– N are sections of the PDR aligned with the star, which are sufficiently small for the different parameters to be constant within the cell (UV flux and density of each species). The H ₂ density is calculated in each cell from left to right according to the model presented in Section 4.1. The H ₂ formation leads to a decrease of UV intensity through molecular absorption as outlined in the graph at the bottom of the figure.