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

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are common throughout theuniverse 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 dustgrains and their spectral features are expected to be seen in absorption. Here we extend our previous work on the infrared spectral propertiesof the small PAH naphthalene (C 10 H 8 ) in several media to includethe full mid-infrared laboratory spectra of 11 other PAHs and relatedaromatic species frozen in H 2 O ices. These include the molecules1,2-dihydronaphthalene, anthracene, 9,10-dihydroanthracene, phenanthrene,pyrene, benzo[e]pyrene, perylene, benzo(k)fluoranthene, pentacene,benzo[ghi]perylene, and coronene. These results demonstrate thatPAHs and related molecules, as a class, show the same spectral behaviorsas naphthalene when incorporated into H 2 O-rich matrices. When comparedto the spectra of these same molecules isolated in inert matrices(e.g., Ar or N 2 ), the absorption bands produced when they are frozenin H 2 O matrices are broader (factors of 3-10), show small positionshifts in either direction (usually <4 cm -1 , always <10 cm -1 ),and show variable changes in relative band strengths (typically factorsof 1-3). There is no evidence of systematic increases or decreasesin the absolute strengths of the bands of these molecules when theyare incorporated in H 2 O matrices. In H 2 O-rich ices, their absorptionbands are relatively insensitive to concentration over the rangeof 10 < 200. The absorption bands of these moleculesare also insensitive to temperature over the 10 K < T < 125 K range,although the spectra can show dramatic changes as the ices are warmedthrough the temperature range in which amorphous H 2 O ice convertsto its cubic and hexagonal crystalline forms ( T > 125 K). Giventhe small observed band shifts cause by H 2 O, the current databaseof spectra from Ar matrix-isolated neutral PAHs and related moleculesshould be useful for the search for these species in dense cloudson the basis of observed absorption band positions. Furthermore,these data permit determination of column densities to better thana factor of 3 for PAHs in dense clouds. Column density determinationof detected aromatics to better than a factor of 3 will, however,require good knowledge about the nature of the matrix in which thePAH is embedded and laboratory studies of relevant samples.