Abstract
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.
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