Theoretical evaluation of PAH dication properties

Aims. We present a systematic theoretical study on 40 polycyclic aromatic hydrocarbons dications (PAHs ++) containing up to 66 carbon atoms. Methods. We performed our calculations using well established quantum–chemical techniques in the framework of the density functional theory (DFT) to obtain the electronic ground–state properties, and of the time–dependent DFT (TD–DFT) to evaluate the excited–state properties. Results. For each PAH ++ considered we computed the absolute visible–UV photo–absorption cross–section up to about 30 eV. We also evaluated their vibrational properties and compared them to those of the corresponding neutral and singly–ionised species. We estimated the adiabatic and vertical second ionisation energy ∆I through total energy differences. Conclusions. The ∆I values obtained fall in the energy range 8–13 eV, confirming that PAHs could reach the doubly–ionised state in HI regions. The total integrated IR absorption cross–sections show a marked increase upon ionization, being on the average about two and five times larger for PAHs ++ than for PAHs + and PAHs, respectively. The visible–UV photo–absorption cross–sections for the 0, +1 and +2 charge–states show comparable features but PAHs ++ are found to absorb slightly less than their parent neutrals and singly ionized species between ∼ 7 and ∼ 12 eV. Combining these pieces of information we found that PAHs ++ should actually be more stable against photodissociation than PAHs and PAHs + , if dissociation tresholds are nearly unchanged by ionization.