Infrared Complex Refractive Indices of Seven Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are environmentally and astrophysically significant toxic organic compounds, several of which are carcinogenic. Infrared spectroscopy provides a nondestructive, standoff method of detecting and monitoring PAHs. Analysis of any infrared measurements, however, is complicated by the fact that measured extinction depends not only on molecular absorption features but also on electromagnetic scattering due to particle size and morphology. Quantitative interpretation therefore requires knowledge of the morphologically independent optical properties of the material, specifically the complex refractive indices n(ν̃) and k(ν̃). This work reports n(ν̃) and k(ν̃) of seven condensed-phase PAHs: naphthalene, phenanthrene, pyrene, benzo[a]pyrene, benzo[e]pyrene, benzo[ghi]perylene, and coronene. Wavelength-dependent complex refractive indices were derived in the mid-infrared (7800–400 cm–1) at room temperature (296 K) using quantitative, gravimetric potassium bromide (KBr) pellet transmission spectroscopy. The precision of the method was evaluated through a variability study of 15 KBr pellets of pyrene. The resulting complex refractive indices are then applied to model pyrene aerosol absorption spectra, demonstrating their utility for future morphology-dependent identification and quantitative interpretation of PAHs in infrared observations.