The 6 $mu$m Feature as A Tracer of Aliphatic Components of Interstellar Carbonaceous Grains

An unidentified infrared emission (UIE) feature at 6.0 $mu$m is detected in a number of astronomical sources showing the UIE bands. In contrast to the previous suggestion that this band is due to C=O vibrational modes, we suggest that the 6.0 $mu$m feature arises from olefinic double-bond functional groups. These groups are likely to be attached to aromatic rings which are responsible for the major UIE bands. The possibility that the formation of these functional groups is related to the hydrogenation process is discussed.