Electronic absorption spectra of PAHs up to vacuum UV

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 $\pi^*\gets\pi$ 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