LABORATORY EVIDENCE FOR HIGHLY UNSATURATED-HYDROCARBONS AS CARRIERS OF SOME OF THE DIFFUSE INTERSTELLAR BANDS

THERE are many absorption lines in the visible and near-infrared spect ra of stars located on the far side of diffuse interstellar clouds. Th e origin of these 'diffuse interstellar bands' (DIBs) has remained an unanswered question since their discovery almost 70 years ago1,2. Ther e are now over 100 known bands1,3-6 and it is clear from the range of line widths, depths and shapes that the lines are unlikely to come fro m a single 'carrier'. Many of the proposed carriers, such as gas-phase carbon chains7, fullerenes8 and dust grains9, fail in having ultravio let absorption lines where none has yet been observed in the stellar s pectra. Polycyclic aromatic species such as C16H10+ (ref. 10) and C10H 8 (ref. 11) were recently claimed to be good candidates for carriers o f some of the DIBs. Here we present laboratory evidence that highly un saturated hydrocarbons with carbon numbers 6-12 may be the carriers of some of the DIBs in the range 480-1,000 nm. We deposit mass-selected molecules in a neon matrix at 5 K and measure their near-infrared, vis ible and ultraviolet spectra. Not only do these species have visible a nd near-infrared lines corresponding to fifteen DIBs, but they also sh ow no absorption lines in the ultraviolet, consistent with astronomica l observations.