Near- and Mid-Infrared Laboratory Spectra of PAH Cations in Solid H₂O

Polycyclic aromatic hydrocarbons (PAHs) have been observed in absorption along lines of sight toward embedded protostars. In such cold space environments PAHs should condense into H₂O-rich ice mantles at low temperature and be exposed to ionizing radiation. In this paper we present the first infrared spectra of PAH cations in solid H₂O generated under conditions that resemble dense molecular clouds. After exposing PAHs in solid H₂O at 15 K to low doses of UV radiation, we have observed both the vibrational absorptions of PAH cations in the mid-IR and electronic transitions in the near-IR. The PAHs observed as ions in solid H₂O were naphthalene, anthracene, phenanthrene, benzo[k]fluoranthene, and benzo[ghi]perylene. Peak positions, strengths, and temperature dependence are reported for the detected ion bands, and their astrophysical significance is discussed. These laboratory measurements suggest that absorption bands of PAH cations in H₂O ice may be observable by astronomers in the near- and mid-infrared.