INFRARED FREQUENCIES AND INTENSITIES FOR ASTROPHYSICALLY IMPORTANT POLYCYCLIC AROMATIC HYDROCARBON CATIONS

Polycyclic aromatic hydrocarbons (PAHs) have been implicated as the ca rriers of the '' unidentified infrared'' (UIR) emission bands observed from the interstellar medium. It has long been thought that these mol ecules, if present, probably exist as cations. Laboratory-based spectr a of the PAH cations in the visible and ultraviolet regions have been known for years, but comparable spectra in the infrared region have be en lacking. In this paper we present infrared spectra of the cations o f five moderate-sized PAHs. The PAH cations have been produced by low- energy electron impact and then trapped and stabilized in argon matric es at 12 K. To date, results have been obtained on naphthalene, anthra cene, pyrene, perylene, and coronene. A common feature of the infrared spectra of all these cations is the very different intensity pattern of the ions compared to the neutral parents. Visible and (partial) inf rared spectra of the coronene cation are also presented. It is shown t hat the out-of-plane CH bending mode shifts to a position very close t o the UIR band at 11.3 mum. The astrophysical impact of these observat ions is discussed.