Output of the Horsehead Nebula photon dominated region time dependent model for observed selected hydrocarbons (CCH, c-C₃H, c-C₃H₂, C₄H). The abundances are shown as a function of visual extinction in the cloud illuminated by the σOri star (see Appendix A for details) and time. For each species the upper panel shows the abundance at time 10³ yr (close to the maximum injection by dust grains, for clarity): initial model without hydrogenated amorphous carbon photolysis (lower curve), with model I including reactions up to CH₄ (dotted line), model II reactions up to C₂Hy (dashed line), model III reactions up to C₃Hy (upper curve). For each species, in the lower panel, the corresponding abundance ratio with the addition of photolytic reactions over the abundance without the addition of the photolytic reactions is shown for all times.