Example of temperature and abundance profiles obtained with the Meudon PDR code. Top panel: gas temperature and abundances, while bottom panel: line brightness temperature of Cnα lines with principal quantum numbers n = 30, 65, 91 and 157, and of the 158μm-[CII] line. The input conditions for the model are a radiation field of G₀ = 1 × 10⁴, in Mathis units, and a total gas density of n_H = 1 × 10⁴ cm⁻³. The difference between the abundance of free electrons and the abundance of ionized carbon is produced by the ionization of species such as sulfur or hydrogen.