Abstract. IR spectroscopy provides a valuable tool for the characterisation and identification of interstellar molecular species.Here, we present 6-9 µm spectra of a sample of reflection nebulae, HII regions, YSOs, evolved stars and galaxies that show strong unidentified infrared bands, obtained with the SWS spectrograph on board ISO. The IR emission features in this wavelength region show pronounced variations. 1) The 6.2 µm feature shifts from 6.22 to 6.3 µm and clearly shows profile variations.2) The 7.7 µm complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7 µm. In some cases the main peak can apparently shift up to 8 µm. Two sources do not exhibit a 7.7 µm complex but instead show a broad emission feature at 8.22 µm.3) The 8.6 µm feature has a symmetric profile in all sources and some sources exhibit this band at slightly longer wavelengths. For the 6.2, 7.7 and 8.6 µm features, the sources have been classified independently based on their profile and peak position. The classes derived for these features are directly linked with each other. Sources with a 6.2 µm feature peaking at ∼6.22 µm exhibit a 7.7 µm complex dominated by the 7.6 µm component. In contrast, sources with a 6.2 µm profile peaking longwards of 6.24 µm show a 7.7 µm complex with a dominant peak longwards of 7.7 µm and a 8.6 µm feature shifted toward the red. Furthermore, the observed 6-9 µm spectrum depends on the type of object. All ISM-like sources and a few PNe and Post-AGB stars belong to the first group while isolated Herbig AeBe stars, a few Post-AGB stars and most PNe belong to the second group. We summarise existing laboratory data and theoretical quantum chemical calculations of the modes emitting in this wavelength region of PAH molecules. We discuss the variations in peak position and profile in view of the exact nature of the carrier. We attribute the observed 6.2 µm profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3 µm component and substituted/complexed PAHs representing the 6.2 µm component. The 7.6 µm component is well reproduced by both pure and substituted/complexed PAHs but the 7.8 µm component remains an enigma. In addition, the exact identification of the 8.22 µm feature remains unknown. The observed variations in the characteristics of the IR emission bands are linked to the local physical conditions. Possible formation and evolution processes that may influence the interstellar PAH class are highlighted.

Laboratory spectra through the mid-infrared (4000 to 500 cm-1 [2.5-20 micrometers]) have been used to calculate the optical constants (n and k) and integrated absorption coefficients (A) for a variety of pure and mixed molecular ices of relevance to astrophysics. The ices studied were H2O, CH3OH, CO2, OCS, CH4, CO2 + CH4, CO2 + OCS, CO + CH4, CO + OCS, O2 + CH4, O2 + OCS, N2 + CH4, N2 + OCS, H2O + CH4, H2O + OCS, and H2O + CH3OH + CO + NH3. In addition, the measurements have been extended through the far-infrared (500 to 50 cm-1 [20-200 micrometers]) for the H2O, CH3OH, and H2O + CH3OH + CO + NH3 ices.

The infrared emission band spectrum associated with many different interstellar objects can be modeled successfully by using combined laboratory spectra of neutral and positively charged polycyclic aromatic hydrocarbons (PAHs). These model spectra, shown here for the first time, alleviate the principal spectroscopic criticisms previously leveled at the PAH hypothesis and demonstrate that mixtures of free molecular PAHs can indeed account for the overall appearance of the widespread interstellar infrared emission spectrum. Furthermore, these models give us insight into the structures, stabilities, abundances, and ionization balance of the interstellar PAH population. These, in turn, reflect conditions in the emission zones and shed light on the microscopic processes involved in the carbon nucleation, growth, and evolution in circumstellar shells and the interstellar medium.

This paper presents the results of an investigation of the molecular characteristics that underlie the observed peak position and profile of the nominal 6.2 m interstellar emission band generally attributed to the CC stretching vibrations of polycyclic aromatic hydrocarbons (PAHs). It begins with a summary of recent experimental and theoretical studies of the spectroscopic properties of large (>30 carbon atoms) PAH cations as they relate to this aspect of the astrophysical problem. It then continues with an examination of the spectroscopic properties of a number of PAH variants within the context of the interstellar 6.2 m emission, beginning with a class of compounds known as polycyclic aromatic nitrogen heterocycles (PANHs; PAHs with one or more nitrogen atoms substituted into their carbon skeleton). In this regard, we summarize the results of recent relevant experimental studies involving a limited set of small PANHs and their cations and then report the results of a comprehensive computational study that extends that work to larger PANH cations including many nitrogen-substituted variants of coronene + (C 24 H þ 12 ), ovalene + (C 32 H þ 14 ), circumcoronene + (C 54 H þ 18 ), and circum-circumcoronene + (C 96 H þ 24 ). Finally, we report the results of more focused computational studies of selected representatives from a number of other classes of PAH variants that share one or more of the key attributes of the PANH species studied. These alternative classes of PAH variants include (1) oxygen-and silicon-substituted PAH cations; (2) PAH-metal ion complexes (metallocenes) involving the cosmically abundant elements magnesium and iron; and (3) large, asymmetric PAH cations.Overall, the studies reported here demonstrate that increasing PAH size alone is insufficient to account for the position of the shortest wavelength interstellar 6.2 m emission bands, as had been suggested by earlier studies. On the other hand, this work reveals that substitution of one or more nitrogen atoms within the interior of the carbon skeleton of a PAH cation induces a significant blueshift in the position of the dominant CC stretching feature of these compounds that is sufficient to account for the position of the interstellar bands. Subsequent studies of the effects of substitution by other heteroatoms (O and Si), metal ion complexation ( Fe + , Mg + , and Mg 2+ ), and molecular symmetry variation-all of which fail to reproduce the blueshift observed in the PANH cations-indicate that N appears to be unique in its ability to accommodate the position of the interstellar 6.2 m bands while simultaneously satisfying the other constraints of the astrophysical problem. This result implies that the peak position of the interstellar feature near 6.2 m traces the degree of nitrogen substitution in the population, that most of the PAHs responsible for the interstellar IR emission features incorporate nitrogen within their aromatic networks, and that a lower limit of 1%-2% of the cosmic nitrogen is sequestered within the interstellar PAH populati...

We present spectra of the 3.3 m and 11.2 m polycyclic aromatic hydrocarbon (PAH) features of a large number of stellar sources, planetary nebulae, reflection nebulae, H ii regions, and galaxies, obtained with Infrared Space Observatory Short Wavelength Spectrometer. Clear variations are present in the profiles of these features. Most of the sources show a symmetric 3.3 m feature peaking at $3.290 m, while only very few show an asymmetric 3.3 m feature peaking at a slightly longer wavelength. The profiles of the 11.2 m feature are distinctly asymmetric. The majority of the sources has a 11.2 m feature peaking between 11.20 and 11.24 m, with a very steep blue rise and a low tail-to-top ratio. A few sources show a 11.2 m feature with a peak position of $11.25 m, a less steep blue rise, and a high tail-to-top ratio. The sources are classified independently on the basis of the 3.3 and 11.2 m feature profiles and peak positions. Correlations between these classes and those based on the 6-9 m features (Peeters et al.) are found. In particular, sources with the most common profiles in the 6-9 m region also show the most common 3.3 and 11.2 m feature profiles. However, the uncommon profiles do not correlate with each other. Also, these classifications depend on the type of object. In general, H ii regions, nonisolated Herbig AeBe stars and young stellar objects show the same profiles for all 3-12 m features. Many planetary nebulae and post-asymptotic giant branch stars show uncommon feature profiles. The three galaxies in our sample show the same profiles as the H ii regions for all but the 11.2 m feature, being similar to that of evolved stars. The observed pronounced contrast in the spectral variations for the CH modes (3.3 and 11.2 m bands) versus the CC modes (6.2, 7.7, and 8.6 m bands) is striking: the peak wavelengths of the features attributed to CC modes vary by $15-80 cm À1 , while for the CH modes the variations are $4-6.5 cm À1 . We summarize existing laboratory data and theoretical calculations of the modes emitting in the 3-12 m region of PAH molecules and complexes. In contrast to the 6.2 and 7.7 m components, which are attributed to PAH cations, the 3.3 m feature appears to originate in neutral and/or negatively charged PAHs. We attribute the variations in peak position and profile of these IR emission features to the composition of the PAH family. The variations in FWHM of the 3.3 m feature remains an enigma, while those of the 11.2 m can be explained by anharmonicity and molecular structure. The possible origin of the observed contrast in profile variations between the CH modes and the CC modes is highlighted.