INFRARED-SPECTROSCOPY OF MATRIX-ISOLATED POLYCYCLIC AROMATIC HYDROCARBON CATIONS .2. THE MEMBERS OF THE THERMODYNAMICALLY MOST FAVORABLE SERIES THROUGH CORONENE

Gaseous, ionized polycyclic aromatic hydrocarbons (PAHs) are thought t o be responsible for a very common family of interstellar infrared emi ssion bands. Here the near- and mid-infrared spectra of the cations of the five most thermodynamically favored PAHs up to coronene:phenanthr ene, pyrene, benzo[e]pyrene, benzo[ghi]perylene, and coronene, are pre sented to test this hypothesis. For those molecules that have been stu died previously (pyrene, pyrene-d(10), and coronene), band positions a nd relative intensities are in agreement. In all of these cases we rep ort additional features. Absolute integrated absorbance values are giv en for the phenanthrene, perdeuteriophenanthrene, pyrene, benzo[ghi]pe rylene, and coronene cations. With the exception of coronene, the cati on bands corresponding to the CC modes are typically 2-5 times more in tense than those of the CH out-of-plane bending vibrations. For the ca tions, the CC stretching and CH in-plane bending modes give rise to ba nds that are an order of magnitude stronger than those of the neutral species, and the CH out-of-plane bends produce bands that are 5-20 tim es weaker than those of the neutral species. This behavior is similar to that found in most other PAH cations studied to date. The astronomi cal implications of these PAH cation spectra are also discussed.