Electronic absorption spectra of PAHs up to vacuum UV

Free access article

Issue		A&A Volume 426, Number 1, October IV 2004


Page(s)		105 - 117
Section		Interstellar and circumstellar matter
DOI		http://dx.doi.org/10.1051/0004-6361:20040541

A&A 426, 105-117 (2004)
DOI: 10.1051/0004-6361:20040541

Towards a detailed model of interstellar PAH photophysics

G. Malloci^{1, 2}, G. Mulas^{1, 2} and C. Joblin³

¹ INAF - Osservatorio Astronomico di Cagliari - Astrochemistry Group, Strada n.54, Loc. Poggio dei Pini, 09012 Capoterra (CA), Italy
e-mail: [gmalloci;gmulas]@ca.astro.it
² Astrochemical Research in Space Network, http://www.ars-network.org
³ Centre d'Étude Spatiale des Rayonnements, CNRS et Université Paul Sabatier, 9 avenue du colonel Roche, 31028 Toulouse, France
e-mail: christine.joblin@cesr.fr

(Received 29 March 2004 / Accepted 17 June 2004 )

Abstract
We computed the absolute photo-absorption cross-sections up to the vacuum ultaviolet (VUV) of a total of 20 Polycyclic Aromatic Hydrocarbons (PAHs) and their respective cations, ranging in size from naphthalene (C ₁₀H ₈) to dicoronylene (C ₄₈H ₂₀). We used an implementation in real time and real space of the Time-Dependent Density Functional Theory (TD-DFT), an approach which was proven to yield accurate results for conjugated molecules such as benzene. Concerning the low-lying excited states of $\pi^*\gets\pi$ character occurring in the near-IR, visible and near-UV spectral range, the computed spectra are in good agreement with the available experimental data, predicting vertical excitation energies precise to within a few tenths of eV, and the corresponding oscillator strengths to within experimental errors, which are indeed the typical accuracies currently achievable by TD-DFT. We find that PAH cations, like their parent molecules, display strong $\pi^*\gets\pi$ electronic transitions in the UV, a piece of information which is particularly useful since a limited amount of laboratory data is available concerning the absorption properties of PAH ions in this wavelength range. Moreover, a detailed discussion of the UV-VUV properties of both neutral and cation species is presented. Concerning neutrals, the agreement with existing laboratory data is very good, the specific TD-DFT implementation used in this work apparently being able to reproduce the overall far-UV behaviour, including the broad absorption peak dominated by $\sigma^*\gets\sigma$ transitions, which matches well both in position and width. The implications of these results are discussed in conjunction with the contribution PAH-like molecules are expected to give to the interstellar extinction curve.

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

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