The Impact of Deuteration on the Infrared Spectra of Interstellar Polycyclic Aromatic Hydrocarbons

In this paper, we present the results of a computational study of the effects of a low absolute level of deuteration ( 5%) on the observed mid-IR spectrum of the interstellar polycyclic aromatic hydrocarbon (PAH) population. The analyses are based on a global average spectrum, including all the possible singly deuterated isomers of an ensemble of partially superhydrogenated PAH species (i.e., Hn-PAHs) in both neutral and cationic forms. The distinctive spectroscopic properties considered in this paper include three classes of D-involved vibrational modes: (1) CD in-plane bending features in the 9.5-12 mr egion, (2) CD out-of-plane bending features in the 14.5-17.9 m region, and (3) aromatic and aliphatic CD stretching features in the 4.4-4.7 m region. In addition, D substitution also has the potential to alter the appearance of the CH out-of-plane bending features in the 11-14 m region by disrupting the adjacency of the H atoms on the PAH edge rings. The spectroscopic impacts of a low level of deuteration in the interstellar PAH population are considered, and their implications for the detailed appearance of the astronomical mid-IR PAH emission spectrum is explored. Overall, the results of this study indicate that the CD stretching features offer the greatest prospects for the detection of deuterated interstellar PAHs. Since these features are effectively obscured by telluric CO2 absorptions, the information presented herein should be useful in the planning and design of future space-based IR observing platforms. Subject heading gs: astrochemistry — infrared: ISM — ISM: evolution — ISM: molecules — molecular processes