The Mid-Infrared Laboratory Spectra of Naphthalene (C₁₀H₈) in Solid H₂O

Polycyclic aromatic hydrocarbons (PAHs) are common throughout the universe. Gas-phase PAHs are easily detected in radiation-rich environments by the characteristic infrared (IR) emission bands they produce when they are excited by higher energy radiation. In contrast, in dense interstellar clouds, where the PAHs could be present as unexcited gas-phase molecules or dust grains or frozen into ice mantles, they are expected to be seen in absorption. We present full mid-IR spectra of the PAH naphthalene (C₁₀H₈) in several media to facilitate searches for PAHs in absorption in dense clouds. The positions, widths, and strengths of the absorption bands of naphthalene are significantly altered from the gas phase when it is placed in a matrix, with the extent of the alteration depending on the nature of the matrix interaction. The infrared features of gas-phase naphthalene show P, Q, and R branches characteristic of rovibrational transitions, while naphthalene in condensed forms does not. When condensed in a relatively inert matrix (e.g., Ar or N₂), the IR features produced by naphthalene are narrow (generally FWHM < 2 cm^-1) and may show multiple components associated with different matrix sites. In more interactive matrices (e.g., pure naphthalene and H₂O), the bands become considerably broader (factors of 3-10), show small band position shifts (0-10 cm^-1 in either direction), and show variable changes in relative band strengths (typically factors of 1-3). There is little systematic increase or decrease in the band strengths (perhaps none). In H₂O-rich ices, naphthalene bands are relatively insensitive to concentration and temperature, with the exception that all the bands show dramatic changes as the ices are warmed through the temperature range in which amorphous H₂O ice converts to its cubic and hexagonal crystalline forms. Given the small observed band shifts (<10 cm^-1; <0.04 μm at 6 μm), the current database of spectra from Ar matrix-isolated PAHs should be useful for the search for PAHs in dense clouds on the basis of observed absorption band positions. Furthermore, these data permit determination of column densities to better than a factor of 3 for PAHs in dense clouds. Column density determination of detected aromatics to better than a factor of 3 will, however, require good knowledge about the nature of the matrix in which the PAH is embedded and laboratory studies of relevant samples.