Laboratory detection of a molecular band at lambda 4429

A fairly strong molecular absorption band at 4429.27 +/- 0.04 Angstrom, clo sely centered on the strongest diffuse interstellar band at 4428.9 +/- 1.4 Angstrom, has been found in a supersonic molecular beam among the products of a discharge through benzene and other hydrocarbons. This agreement in wa velength to a few parts in 10(4) strongly suggests a common carrier. The wi dth of the laboratory band is significantly less than that of the diffuse i nterstellar hand, but this difference may be the result of the very low rot ational temperature in the supersonic beam-possibly as low as 2 K-relative to that of a weakly polar molecule in the diffuse interstellar gas (100-200 K). Several candidate carriers are discussed. Two of the more promising ar e the allyl cation C3H5+ and the dimethylene allenyl radical C5H5, both ben t carbon chains that may be close enough to linear to reconcile the puzzlin g appearance of P-, Q-, and R-branches in the laboratory band with the five hydrogen atoms suggested by the observed deuterium isotopic shifts.