Infrared spectroscopy of matrix-isolated polycyclic aromatic hydrocarbon cations. 4. The tetracyclic PAH isomers chrysene and 1,2-benzanthracene.

The mid-infrared spectra of the polycyclic aromatic hydrocarbon (PAH) cations of the tetracyclic isomers chrysene (C18H12+) and 1,2-benzanthracene (C18H12+) are presented. As with previous PAH cations studied to date, the CC stretching and CH in-plane bending mode absorptions are about an order of magnitude stronger than the aromatic CH out-of-plane bending absorptions and nearly 2 orders of magnitude more intense than the corresponding bands in the neutral molecule. The CH bands arising from the out-of-plane bends in the cation are slightly weaker than the corresponding bands in the neutral species. The strongest cation bands of these species fall between 1300 and 1330 cm-1, close to the peak of the most intense interstellar emission feature in HII regions and reflection nebulae. A strong PAH cation band at slightly higher frequency than 1300 cm-1 may be associated with an asymmetric CC stretching vibration involving rings adjacent to the kink in the chain of aromatic rings.