Homologous Series of Diffuse Interstellar Bands

Recent experiments suggest that some of the stronger diffuse interstellar bands (DIBs) are due to unsaturated carbon-chain molecules. Laboratory and BIB data are here discussed in terms of the free-electron model, which relates the electronic spectra of homologous molecules of different chain lengths. Five possible series are described, From a comparison with laboratory data, it is suggested that the carrier of the 4428 A DIB, which has the largest equivalent width in most sources, is the C9 molecule, and the possibility that the metastable linear isomers of higher odd C_N molecules are responsible for an observed series of DIBs is discussed.