Infrared spectroscopy of polycyclic aromatic hydrocarbon cations. 1. Matrix-isolated naphthalene and perdeuterated naphthalene.

Ionized polycyclic aromatic hydrocarbons (PAHs) are thought to constitute an important component of the interstellar medium. Despite this fact, the infrared spectroscopic properties of ionized PAHs are almost unknown. The results we present here derive from our ongoing spectroscopic study of matrix isolated PAH ions and include the spectra of the naphthalene cation, C10H8+, and its fully deuterated analog, C10D8+, between 4000 and 200 cm-1. Ions are generated in situ Lyman-alpha photoionization of the neutral precursor. Bands of the C10H8+ ion are observed at 1525.7, 1518.8, 1400.9, 1218.0, 1216.9, 1214.9, 1023.2, and 758.7 cm-1. Positions and relative intensities of these bands agree well with those in the available literature. The 758.7 cm-1 band has not previously been reported. C10D8+ ion bands appear at 1466.2, 1463.8, 1379.4, 1373.8, 1077.3, 1075.4, and 1063.1 cm-1. Compared to the analogous modes in the neutral molecule, the intensities of the cation's CC modes are enhanced by an order of magnitude, while CH modes are depressed by this same factor. Integrated absorption intensities are calculated for the strongest bands of C10H8 and for the observed bands of C10H8+. Absolute intensities derived for the naphthalene cation differ from earlier experimental results by a factor of approximately 50, and from theoretical predictions by a factor of approximately 300. Reasons for these discrepancies and from the astronomical implications of PAH cation spectra are discussed.