Near-infrared spectroscopy of nitrogenated polycyclic aromatic hydrocarbon cations from 0.7 to 2.5 μm

The near-infrared (NIR) spectra and absolute band strengths of 10 nitrogenated polycyclic aromatic hydrocarbon (PANH) radical cations isolated in an argon matrix are presented and compared with the spectra of their parent polycyclic aromatic hydrocarbon (PAH) radical cations. The 0.7-2.5 mu m (14,500-4000 cm(-1)) spectrum for the open-shell cation forms of two nitrogenated anthracenes (C13H9N and C12H8N2), four isomeric nitrogenated benzanthracenes (C17H11N), and four isomeric nitrogenated dibenzanthracenes (C21H13N) are reported. These ionized PANHs have allowed electronic transitions that give rise to strong absorption bands in the NIR. Low-lying excited states for these PANH ions are computed using time-dependent density functional theory (TDDFT). The resulting vertical excitation spectrum characterizes the transitions, and leads to a simple model that predicts the qualitative trends in absorption energy. The direction of the shift depends on the position of the nitrogen atom within the PANH and the relative magnitudes of the donor and acceptor molecular orbitals involved in the transitions. As with non-nitrogenated PAHs, ionized interstellar PANHs can be expected to contribute to the mid-IR emission features from UV-rich as well as UV-poor regions, and add weak, broad band structure to the NIR region of the interstellar extinction curve.