The structure of the diffuse interstellar bands
It is shown that the diffuse interstellar bands (DIB) at 5780, 5797, 6196, and 6203 A are composed of Doppler-shifted components of appreciable widths in those directions where several interstellar sodium clouds have been identified. The optical properties of the intervening clouds may differ appreciably; some of the clouds are more likely to contain the carrier(s) of one DIB family than that of another. The DIBs observed in the spectra of distant objects are therefore generally ill-defined averages over a number of clouds along the line-of-sight. As every component also introduces its particular radial velocity, a substantial widening of some of the profiles is observed. In some cases, a resolution of the bands into a number of Gaussian components has been possible. In general 'intrinsic bands', defined with the aid of nearby stars, have been successfully fitted to complex bands observed in distant stars. These 'intrinsic' bands are presented as normalized profiles in the case of the 5780- and 5797-bands.