Theoretical electron affinities of PAHs and electronic absorption spectra of their mono-anions

¹ INAF - Osservatorio Astronomico di Cagliari - Astrochemistry Group, Strada n. 54, Loc. Poggio dei Pini, 09012 Capoterra (CA), Italy e-mail: [gmalloci;gmulas;iporcedd]@ca.astro.it
² Dipartimento di Fisica, Università degli Studi di Cagliari, Complesso Universitario di Monserrato, S. P. Monserrato-Sestu Km 0,700, 09042 Monserrato (CA), Italy e-mail: [giancarlo.cappellini;vincenzo.fiorentini]@dsf.unica.it
³ INFM - Sardinian Laboratory for Computational Materials Science (SLACS)

Received: 25 October 2004
Accepted: 8 November 2004

Abstract

We present theoretical electron affinities, calculated as total energy differences, for a large sample of polycyclic aromatic hydrocarbons (PAHs), ranging in size from azulene (C₁₀H₈) to dicoronylene (C₄₈H₂₀). For 20 out of 22 molecules under study we obtained electron affinity values in the range 0.4-2.0 eV, showing them to be able to accept an additional electron in their LUMO orbital. For the mono-anions we computed the absolute photo-absorption cross-sections up to the vacuum ultraviolet (VUV) using an implementation in real time and real space of the Time-Dependent Density Functional Theory (TD–DFT), an approach which has already been proven to yield accurate results for neutral and cationic PAHs. Comparison with available experimental data hints that this is the case for mono-anions as well. We find that PAH anions, like their parent molecules and the corresponding cations, display strong electronic transitions in the UV. The present results provide a quantitative foundation to estimate the fraction of specific PAHs which can be singly negatively charged in various interstellar environments, to simulate their photophysics in detail and to evaluate their contribution to the interstellar extinction curve.