TD-DFT calculations of electronic spectra of hydrogenated protonated polycyclic aromatic hydrocarbon (PAH) molecules: implications for the origin of the diffuse interstellar bands?

We report the application of time-dependent density functional theory (TD-DFT) to the calculation of electronic spectra of hydrogenated protonated polycyclic aromatic hydrocarbon (PAH) molecules. The hydrogen atoms lie on the periphery of the PAH structure and those considered here may be written Hn-HPAH+, where n is even. It is found, in common with protonated PAH molecules, HPAH+, that some of the electronic transitions fall in the visible spectral region. The implications of the results are discussed in the context of the long-standing enigmatic astronomical problem of the diffuse interstellar absorption bands.