The Mid‐Infrared Absorption Spectra of Neutral Polycyclic Aromatic Hydrocarbons in Conditions Relevant to Dense Interstellar Clouds

Polycyclic aromatic hydrocarbons (PAHs) are common throughout the universe and are expected to be present in dense interstellar clouds. In these environments, some PAHs may be present in the gas phase, but most should be frozen into ice mantles or adsorbed onto dust grains and their spectral features are expected to be seen in absorption. HereweextendourpreviousworkontheinfraredspectralpropertiesofthesmallPAHnaphthalene(C10H8)inseveral media to include the full mid-infrared laboratory spectra of 11other PAHs and related aromatic species frozen in H2O ices. Theseincludethe molecules1,2-dihydronaphthalene,anthracene,9,10-dihydroanthracene,phenanthrene, pyrene, benzo(e)pyrene, perylene, benzo(k)fluoranthene, pentacene, benzo(ghi)perylene, and coronene. These results demon- strate that PAHs and related molecules, as a class, show the same spectral behaviors as naphthalene when incorporated into H2O-rich matrices. When compared to the spectra of these same molecules isolated in inert matrices (e.g., Ar or N2), the absorption bands produced when they are frozen in H2O matrices are broader (factors of 3-10), show small position shifts in either direction (usually < 4c m 1 , always <10 cm 1 ), and show variable changes in relative band strengths(typicallyfactorsof1-3).Thereisnoevidenceofsystematicincreasesordecreasesintheabsolutestrengths of the bands of these molecules when they are incorporated in H2O matrices. In H2O-rich ices, their absorption bands are relatively insensitive to concentration over the range of 10 < H2O/PAH < 200. The absorption bands of these molecules are also insensitive to temperature over the 10 K < T < 125 K range, although the spectra can show dra- maticchangesastheicesarewarmedthroughthetemperaturerangeinwhichamorphousH2Oiceconvertstoitscubic and hexagonal crystalline forms (T > 125 K). Given the small observed band shifts cause by H2O, the current data- base of spectra from Ar matrix-isolated neutral PAHs and related molecules should be useful for the search for these species in dense clouds on the basis of observed absorption band positions. Furthermore, these data permit deter- mination of column densities to better than a factor of 3 for PAHs in dense clouds. Column density determination of detected aromaticsto better than afactor of 3will,however, require good knowledge about the natureof the matrixin which the PAH is embedded and laboratory studies of relevant samples. Subject headinggs: infrared: ISM — ISM: lines and bands — ISM: molecules — line: formation — line: identification — molecular data