EDP Sciences
Free access
Volume 432, Number 2, March III 2005
Page(s) 585 - 594
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20042246

A&A 432, 585-594 (2005)
DOI: 10.1051/0004-6361:20042246

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

G. Malloci1, 2, G. Mulas1, G. Cappellini2, 3, V. Fiorentini2, 3 and I. Porceddu1

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)

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 $\pi^\star$ 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 $\pi^*\gets\pi$ 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.

Key words: astrochemistry -- molecular data -- molecular processes -- ISM: molecules -- ultraviolet: ISM -- methods: numerical

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