Laboratory Detection of a Molecular Band at λ4429

Abstract

A fairly strong molecular absorption band at Å , closely centered on the strongest diffuse 4429.27 ‫ע‬ 0.04 interstellar band at Å , has been found in a supersonic molecular beam among the products of a 4428.9 ‫ע‬ 1.4 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 band, 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 C 3 H and the dimethylene allenyl ϩ 5 radical C 5 H 5 , 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.