ISO-SWS observations of infrared absorption bands of the diffuse interstellar medium:The 6.2 mu m feature of aromatic compounds

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 5.95 mu m were already known from ground and airborne observations, while features at 4.27 and 6.21 mu m are for the first time observed by ISO. We ascribe the 3.0, 4.27 and 4.66 mu m features to H(2)O ice, CO(2) 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 6.2 mu m 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.