2004A&A...426..105M

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 (C₁₀H₈) to dicoronylene (C₄₈H₂₀). 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 π*⟵π 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 π*⟵π 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 σ*⟵σ 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.