Theoretical evaluation of PAH dication properties

Abstract

Aims. We present a systematic, theoretical study of 40 polycyclic aromatic hydrocarbon dications (PAHs++) containing up to 66 carbon atoms. Methods. We performed our calculations using well-established quantum-chemical techniques in the framework of (i) the density functional theory (DFT) to obtain the electronic ground-state properties and of (ii) 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 ΔI through total energy differences. Conclusions. The ΔI 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 ionisation, 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 neutral and singly ionised species between ∼7 and ∼12 eV. Combining these pieces of information, we found that PAHs++ should actually be stabler against photodissociation than PAHs and PAHs+, if dissociation thresholds are nearly unchanged by ionisation. © ESO 2007.