Theoretical evaluation of PAH dication properties^*

¹ Centre d'Étude Spatiale des Rayonnements, CNRS et Université Paul Sabatier Toulouse 3, Observatoire Midi-Pyrénées, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 04, France e-mail: [giuliano.malloci;christine.joblin]@cesr.fr
² Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Cagliari, Strada n.54, Loc. Poggio dei Pini, 09012 Capoterra (CA), Italy e-mail: [gmalloci;gmulas]@ca.astro.it

Received: 17 July 2006
Accepted: 25 September 2006

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 through total energy differences.

Conclusions.The 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.