2002A&A...395..969G

We present an analysis of spectra of six stars taken with high resolution (R=220000). The stars are reddened by molecular clouds that differ by the relative strength of the 5797 and 5780 diffuse interstellar bands (DIBs). The high signal-to-noise ratio of the spectra (S/N≃700-1000) shows that the abundance of the linear molecule C₃ with respect to E_B–V varies considerably from one star to an other. There is no correlation with E_B–V. The strong variations in the abundance of C₃ must therefore be caused by another circumstance. We point out that this may be the case: from an analysis of the interstellar potassium lines in the same spectra we conclude large differences in the state of ionization produced by interstellar photons with energies below the ionization potential of hydrogen. The ratio of the abundances of C₃ and C₂ varies considerably in different directions, even when the ratio between the strengths of various DIBs remains approximately constant.