Comparison of dissociation rates for coronene C₂₄H₁₂ (solid lines) and ovalene C₃₆H₁₈ (dashed lines). Only for low energies where H and C dissociation starts to appear do H₂ and C₂H₂ loss have comparable rates to H loss; for high energies H loss always dominates. Dissociation of a dimer is always favoured because of the much weaker binding energy between two PAHs. For ovalene, the reactions appear at nearly the same temperature but at higher energies because of the larger heat capacity. Hence, the increasing size of PAHs and PAH clusters increases their stability against X-rays as the deposited energy is independent of the structure. The dissociation rate for clusters additionally depends on the size of the cluster. The typical rates for IR cooling and energy transfer to dust grains are indicated by the grey lines.