Strength of the CC stretching modes at 6.2 and 7.7 μm relative to the CH out-of-plane bending mode at 11.3 μm. Observed variations (shaded) and mean values (dashed) by Galliano et al. (2008). Full lines at various colors represent band ratios of PAHs with different hydrogenation, α (H/C), and excited by mono-energetic radiation with different hardness (E_γ, as labeled) and constant strength (flux, F). Band ratios derived from PAHs heated by the ISRF are indicated by filled circles and same color index of α. Beside other parameters the degree of ionisation is constant.