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-benzanth racene (C18H12+) are presented. As with previous PAH cations studied t o date, the CC stretching and CH in-plane bending mode absorptions are about an order of magnitude stronger than the aromatic CH out-of-plan e bending absorptions and nearly 2 orders of magnitude more intense th an the corresponding bands in the neutral molecule. The CH bands arisi ng from the out-of-plane bends in the cation are slightly weaker than the corresponding bands in the neutral species. The strongest cation b ands 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 fr equency than 1300 cm(-1) may be associated with an asymmetric CC stret ching vibration involving rings adjacent to the kink in the chain of a romatic rings.