Time-domain terahertz spectroscopy of polycyclic aromatic hydrocarbons

Abstract

Polycyclic arom. hydrocarbons (PAHs) are believed to be the carriers of the unidentified IR features (UIRs), and as such may account for up to 20̂ of the interstellar carbon budget. In addn., PAHs may play a significant role in carbon grain formation, and affect many chem. and phys. processes in the interstellar medium. However, it is difficult to distinguish individual PAHs in the near- to mid-IR region of the spectrum, and our failure to unambiguously detect PAHs hinders our ability to evaluate the role they play. One possible soln. is observations in the TeraHertz (THz) (or far-IR, from 0.1 to 10 THz or 3 to 330 cm^(-1)) regime. Recent work in the THz region of the electromagnetic spectrum has shown that the identity of complex mol. solids can be detd. from their THz spectra, and the time-domain THz spectrometer developed in our lab. has the ability to measure the full complex index of refraction in a single measurement, thus aiding in the identification of complex solids. In addn., recent advances in THz technol. have enabled both lab. spectroscopy and astronomical observations in this region. A first step in both lab. and astronomical studies of PAHs is the acquisition of spectra of pure PAH samples. We present the THz time-domain spectra (0.5 - 7 THz) of several PAHs, including naphthalene, anthracene, and pyrene, and discuss the utility of these spectra for future lab. and astronomical studies.

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