Near‐Infrared Spectroscopy of Nitrogenated Polycyclic Aromatic Hydrocarbon Cations from 0.7 to 2.5 μm

Abstract

The near-infrared (NIR) spectra and absolute band strengths of 10 nitrogenated polycyclic aromatic hydrocarbon (PANH ) radical cations isolated in an argon matrix are presented and compared with the spectra of their parent polycyclic aromatic hydrocarbon (PAH ) radical cations. The 0.7Y2.5 m (14,500Y 4000 cm À1 ) spectrum for the open-shell cation forms of two nitrogenated anthracenes (C 13 H 9 N and C 12 H 8 N 2 ), four isomeric nitrogenated benzanthracenes (C 17 H 11 N ), and four isomeric nitrogenated dibenzanthracenes (C 21 H 13 N ) are reported. These ionized PANHs have allowed electronic transitions that give rise to strong absorption bands in the NIR. Low-lying excited states for these PANH ions are computed using time-dependent density functional theory (TDDFT). The resulting vertical excitation spectrum characterizes the transitions, and leads to a simple model that predicts the qualitative trends in absorption energy. The direction of the shift depends on the position of the nitrogen atom within the PANH and the relative magnitudes of the donor and acceptor molecular orbitals involved in the transitions. As with nonnitrogenated PAHs, ionized interstellar PANHs can be expected to contribute to the mid-IR emission features from UV-rich as well as UV-poor regions, and add weak, broad band structure to the NIR region of the interstellar extinction curve.

The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point

“…Even with the assumed band strength and width modifications, used to give a better match to the astronomical data, it is clear that the discrepancy between the optEC (s) (a) data for a-C(:H) and the astronomical PAHs, as pointed out in Paper III, is still apparent. In the NIR regions (∼1−5 μm) we note that laboratory data do indeed show enhanced extinction with respect to interstellar PAH models (Mattioda et al 2005(Mattioda et al , 2008. Adding more longer wavelength bands, as per the Draine & Li (2007) model, could perhaps bring the optEC (s) (a) model data into closer correspondence with other models.…”

“…The discrepancy could also be due to grain charge effects, which we do not take into account here. For example, aromatic cation species show NIR absorption in the ∼0.8 to >2 μm region (Mattioda et al 2008), which is not apparent in astronomical PAH models. We also note that the tail, extending to high R V and low α NIR in Fig.…”

Infrared vibrational and electronic transitions in the dibenzopolyacene family

“…The matrix-isolation mid-IR and NIR spectroscopic techniques employed in these studies have been described in detail elsewhere [20][21][22][23][24] and are briefly summarized here. Matrix isolated dibenzopolyacene samples were prepared by vapor co-deposition of the species of interest with an over abundance of argon (Ar) onto a 14 K CsI window suspended in a high-vacuum chamber (p < 10 À7 Torr).…”

PRESSURE EFFECTS IN POLYCYCLIC AROMATIC NITROGENATED HETEROCYCLES (PANHs): DIAGNOSTIC QUALITIES AND COSMOBAROMETRY POTENTIAL

“…The infrared spectra of PANHs of various sizes and structurally similar parental polycyclic aromatic hydrocarbons (PAHs) have been extensively studied both computationally (Langhoff et al 1998;Mattioda et al 2005Mattioda et al , 2008Bauschlicher et al 2009Bauschlicher et al , 2008 and experimentally (Bernstein et al 2005;Tielens 2008;Boersma et al 2010), although the latter route is hindered by the difficulty of obtaining PANH standards. The relative position and number of substituted nitrogen atoms have been shown to have a predictable if subtle effect on the infrared spectra proposed by some to account for the 6.2 μm ISM emission band of the ISM .…”