INVESTIGATION OF THE ULTRAVIOLET, VISIBLE, AND NEAR-INFRARED ABSORPTION SPECTRA OF HYDROGENATED POLYCYCLIC AROMATIC HYDROCARBONS AND THEIR CATIONS T. M. Halasinski, 1 F. Salama, 2 and L. J. Allamandola NASA Ames Research Center, Mail Stop 245-6, Moffett Field, CA 94035-1000 Received 2004 November 24; accepted 2005 March 26 ABSTRACT The formation and the presence of PAHs containing excess H atoms, or hydrogenated PAHs (H n -PAHs) in inter- stellar clouds has recently been discussed. It has been suggested that H n -PAHs contribute to the IR emission bands and that they might be among the molecular precursors of the carbon particles that cause the interstellar UV extinction curve. The spectroscopy of H n -PAHs is investigated, and the implications for interstellar spectra are discussed. The UV, visible, and near-IR absorption spectra of a series of H n -PAHs and their photoproducts formed by vacuum UV irradiation were measured for the first time in inert-gas (neon) matrices at 5 K. It is shown that the spectra of both the neutral H n -PAH and cationic H n -PAH + species exhibit vibronic band systems with similar spectral positions and relative spectral intensities to their nonhydrogenated PAH chromophore counterparts. The results are discussed in the context of the nature of the origin of the diffuse interstellar bands. Subject headingg s: astrochemistry — ISM: lines and bands — ISM: molecules — methods: laboratory — molecular data — ultraviolet: ISM 1. INTRODUCTION One of the most ubiquitous spectral signatures of interstellar molecules is a set of IR emission bands, the unidentified infrared bands ( UIRs), which are observed at 3.3, 6.2, 7.7, 8.6, and 11.2 m (Gillett et al. 1973). While it has proved difficult to unambiguously identify the individual molecular species responsible for the UIRs, there is now strong evidence that these emission features arise from carbonaceous aromatic materials and, in particular, from a distri- bution of neutral and ionized polycyclic aromatic hydrocarbons ( PAHs; Allamandola et al. 1999). PAHs are considered good can- didates because they are stable against UV photodissociation, a requirement to maintain the concentration necessary in the inter- stellar medium (ISM), where materials are exposed to harsh far- UV radiation fields. This is particularly true for the diffuse medium, where neutral and ionic PAHs are thought to contribute to the diffuse interstellar bands (DIBs). DIBs are absorption bands from the near-UV to the near-IR seen in the spectra of stars that are obscured by diffuse interstellar molecular clouds (Snow 2001). Although a specific molecular carrier—or a class of molecular carriers—is yet to be unambiguously identified, laboratory stud- ies have shown that several of the observed vibronic transitions of PAH ions are found in the same spectral region where the known DIBs are located and can therefore be considered as good candidates for the origin of some of these astronomical features (Salama et al. 1999; Ruiterkamp et al. 2002). While the IR emission from interstellar sources strongly sug- gests the presence of PAHs, it is difficult to extend this hypoth- esis to include information on which individual PAHs may be found there. The information that is obtained from IR bands only includes the family of structures; little information is provided about individual molecular structures. The absorption arising from electronic transitions, on the other hand, can offer much more specific information regarding the nature of the molecular carrier than the IR emission bands. Thus, identification of the DIBs in the near-UV to the near-IR range offers the potential for a much more complete picture of which specific molecules can be found in diffuse molecular clouds. A more complete understanding of the chemistry that occurs within these regions can be obtained only after specific molecular information is known. Although individual PAHs have yet to be unambiguously iden- tified within the interstellar medium, current experimental as well as theoretical research is focused on the spectral signatures of the many possible different structural variants of PAHs that may exist in these regions. This includes examining spectral signatures as a function of molecular size (Ruiterkamp et al. 2002; Bauschlicher 2002; Weisman et al. 2003), charge (cations and anions; Halasinski et al. 2000), multiply charged ions (Bakes et al. 2001), the ad- dition of oxygen-containing side groups ( Bauschlicher 1998a), the addition of nitrogen-containing side groups as well as ni- trogen substitution within the PAH ring (Bauschlicher 1998b; Mattioda et al. 2003), PAHs containing non–six-membered rings (nonbenzenoid structures; Bauschlicher et al. 1999; Hudgins et al. 2000; Pauzat & Ellinger 2002), open/closed-shell molecular systems (Weisman et al. 2001; Hudgins et al. 2001), and degree of hydrogenation—due to addition of methyl and /or aliphatic substituents to the PAH (Jochims et al. 1999; Pauzat & Ellinger 2001; Petrie et al. 2003) as well as the addition of protons to the PAH (Snow et al. 1998; Bauschlicher 1998c; Le Page et al. 1999; Herbst & Le Page 1999; Bauschlicher & Bakes 2001). In this report, we discuss PAHs containing excess H atoms (H n -PAHs; see Figs. 1 and 2). More specifically, we focus on a subset of H n -PAHs (and their cations) in which an even number of hydrogen atoms have been added to an even number of car- bon atoms ( Fig. 1) and an odd number of hydrogen atoms have been added to an odd number of carbon atoms ( Fig. 2). Thus, the molecules considered here are closed shell in their neutral form and open shell in their singly ionized cationic form. This is the first time that the electronic spectra of these H n -PAHs are re- ported in the literature. We do not focus on species containing an even number of carbon atoms and an odd number of hydro- gen atoms (see Fig. 1) or on species containing an odd number of carbon atoms and an even number of hydrogen atoms (see Fig. 2), as these species are difficult to prepare in situ using ma- trix isolation. 1 Current address: Department of Chemistry, Saint Joseph’s University, Philadelphia, PA 19131. 2 Corresponding author: farid.salama@nasa.gov. 555 The Astrophysical Journal, 628:555–566, 2005 July 20 # 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.