1998A&A...337..261S

We present ISO-SWS spectroscopy of eight strong infrared sources with large extinction through the diffuse interstellar medium. These are five late-type Wolf-Rayet stars, the blue hypergiant Cyg OB2 #12 and the Galactic Center Sources 3 and 4. The spectra show a number of absorption features that can be ascribed to interstellar dust and gas. Features at 3.0, 3.4, 4.66 and 5.95µm were already known from ground and airborne observations, while features at 4.27 and 6.21µm are for the first time observed by ISO. We ascribe the 3.0, 4.27 and 4.66µm features to H₂O ice, CO₂ ice and gaseous CO, respectively, located in dense clouds along four lines-of-sight in our study. Based on an excellent correspondence with the well-known emission feature observed in many other objects, the 6.2µm absorption band is identified with aromatic materials. The profile of the feature indicates that its carriers are PAHs or PAH clusters containing up to a few thousand carbon atoms. The lack of strong infrared features of hydroxy (OH), carbonyl (C=O) and aromatic CH groups towards the objects of the present study shows that the carbonaceous dust material in the diffuse ISM is poor in oxygen as well as hydrogen.