Reflectance spectroscopy (350–2500 nm) of solid-state polycyclic aromatic hydrocarbons (PAHs) M.R.M. Izawa ⇑ , D.M. Applin, L. Norman, E.A. Cloutis Hyperspectral Optical Sensing for Extraterrestrial Reconnaissance Laboratory, Dept. Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba R3B 2E9, Canada article info Article history: Received 3 October 2013 Revised 15 April 2014 Accepted 17 April 2014 Available online 30 April 2014 Keywords: Spectroscopy Organic chemistry Asteroids, composition Satellites, composition Comets, composition abstract Polycyclic aromatic hydrocarbons (PAHs) are organic compounds based on fused aromatic rings, and are formed in a variety of astrophysical, solar nebula and planetary processes. Polycyclic aromatic hydrocar- bons are known or suspected to occur in a wide variety of planetary settings including icy satellites, Titan’s hazes, carbonaceous meteorites, comet nuclei, ring particles; and terrestrial organic-rich litholo- gies such as coals, asphaltites, and bituminous sands. Relatively few measurements of the visible and near-infrared spectra of PAHs exist, yet this wavelength region (350–2500 nm) is widely used for remote sensing. This study presents detailed analyses of the 350–2500 nm reflectance spectra of 47 fine-grained powders of different high-purity solid-state PAHs. Spectral properties of PAHs change with variations in the number and connectivity of linked aromatic rings and the presence and type of side-groups and het- erocycles. PAH spectra are characterized by three strong features near 880 nm, 1145 nm, and 1687 nm due to overtones of mCH fundamental stretching vibrations. Some PAHs are amenable to remote detection due to the presence of diagnostic spectral features, including: NAH stretching over- tones at 1490–1515 nm in NH- and NH 2 -bearing PAHs, aliphatic or saturated bond CAH overtone vibra- tions at 1180–1280 nm and 1700–1860 nm; a broad asymmetric feature between 1450 nm and 1900 nm due to OAH stretching overtones in aromatic alcohols, CAH and C@O combinations near 2000–2010 nm and 2060–2270 nm in acetyl and carboxyl-bearing PAHs. Other substituents such as sulphonyl, thioether ether and carboxyl heterocycles, or cyano, nitrate, and aromatic side groups, do not produce well-resolved diagnostic spectral features but do cause shifts in the positions of the aromatic CAH vibrational overtone features. Fluorescence is commonly suppressed by the presence of heterocy- cles, side-groups and in many non-alternant PAHs. The spectral characteristics of PAHs offer the potential, under suitable circumstances, for remote characterization of the classes of PAH present and in some cases, identification of particular heterocyclic or side-group substituents. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are a cosmochemical- ly-important class of organic molecules consisting of multiple linked aromatic rings, possibly including side groups (functional groups replacing a hydrogen atom attached to an aromatic ring) and heterocycles (functional groups replacing a carbon atom within an aromatic ring). Observations of stellar envelopes, nebu- lae and the interstellar medium, and thermodynamic consider- ations, demonstrate the ubiquity of PAHs in astrophysical settings (e.g., Allamandola et al., 1999; Zolotov and Shock, 2001; Peeters et al., 2012). Most current PAH spectral assignments are based on spectral fitting and there have been relatively few specific band assignments. Similarly, the extreme spectral ‘redness’ (i.e., much lower reflectance at shorter wavelengths) of many Solar Sys- tem bodies has commonly been ascribed to organics including PAHs (e.g., Emery et al., 2005; Hardersen et al., 2006; Filacchione et al., 2007; Buratti et al., 2008; Ore et al., 2011, 2012; Cloutis et al., 2012; Pajola et al., 2013). Direct or circumstantial evidence for the presence of PAHs has been reported for numerous Solar System environments, including comet nuclei (Moreels et al., 1994; Lisse et al., 2006), icy satellites and ring particles of the outer planets (Cuzzi and Estrada, 1998; Spilker et al., 2003; Cruikshank et al., 2005; Cruikshank, 2006), the surface and atmosphere of Saturn’s moon Titan (Sagan et al., 1993; Imanaka et al., 2004; e.g., Bernard et al., 2006; McCord et al., 2006; Waite et al., 2007; López-Puertas et al., 2013), Pluto and other Kuiper Belt Objects (KBOs) (Cruikshank and Dalle Ore, 2003), dark asteroids (Sabbah et al., 2010), and planetary regoliths http://dx.doi.org/10.1016/j.icarus.2014.04.033 0019-1035/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author. E-mail address: matthew.izawa@gmail.com (M.R.M. Izawa). Icarus 237 (2014) 159–181 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus