Identification of the carriers of the Diffuse Interstellar Bands is often considered the biggest challenge in astronomical spectroscopy. The observational data comprise over 500 interstellar absorption features, predominantly at visible wavelengths, which are observed towards bright O and B stars with significant foreground material. Although this field of research was first established nearly one hundred years ago, efforts to determine the carriers of any of the bands have not met with success. It is anticipated that solving this problem would open up a new probe of interstellar conditions and processes, possibly with far-reaching implications for our understanding of interstellar chemistry.

We present an analysis of the profiles of diffuse bands with particular emphasis on the 6614 Å absorption band which is unusual in having fine structure. The precise band shape depends on the line-of-sight and the differences in band profile are attributable to the level of internal excitation of the carrier. The profile is shown to arise from the superposition of a main origin band with weaker hot-bands, the strengths of which depend on interstellar cloud conditions. Rotational contour modelling suggests that the carrier is a large carbon-based molecule with about 20 heavy atoms. The results are discussed with respect to other models of diffuse band spectral line shape and complementary bands observed in emission from the Red Rectangle.