Theoretically calculated ionization potentials, IP^(Z+1)+, experimentally obtained IP^exp and appearance energies, AE^(Z+1)+, as a functionof the number of carbon atoms. For Z = 0, 1, 2, the values refer to the transition . The black and gray curves are the modeled IP from Weingartner & Draine (2001b) and IP from Bakes & Tielens (1994b), respectively, as an estimate of the IP^(Z+1)+ evolution as a function of PAH size and charge, Z (see Eqs. (1) and (3) and text for details). The dashed horizontal line marks the 13.6 eV photon energy cut-off for H I regions. Notes: (a) taken from Malloci et al. (2007b); (b) taken from Clar et al. (1981); (c) taken from Hager & Wallace (1988); (d) taken from Zhen et al. (2016); (e) this work.