Laboratory detection of a molecular band at λ4429

A fairly strong molecular absorption band at 4429.27 +/- 0.04 Angstrom, closely 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 wavelength to a few parts in 10(4) strongly suggests a common carrier. The width of the laboratory band is significantly less than that of the diffuse interstellar hand, but this difference may be the result of the very low rotational 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 are the allyl cation C3H5+ and the dimethylene allenyl radical C5H5, both bent carbon chains that may be close enough to linear to reconcile the puzzling appearance of P-, Q-, and R-branches in the laboratory band with the five hydrogen atoms suggested by the observed deuterium isotopic shifts.