Knowledge Bank

Acetylene-helium mixtures were expanded to provide supersonic beams that were collimated with skimmers. Spectra were obtained with a color center laser scanned under computer control; absorption was detected by changes in beam energy deposition on a helium-cooled bolometer. A number of different bands have been found in the CH stretching region that can be associated with various acetylene clusters. Those due to dimers and trimers have already been $reported.3,2$ Two bands appear centered at 3261.3 and $3262.2cm-1$; these are assigned as perpendicular and parallel fundamentals, respectively, of a symmetric top molecule and arising from the monomer $\nu 3$ mode coupled in different ways in the cluster. The structure in the perpendicular band has been assigned and fit as an oblate symmetric top. The rotational constants, combined with relative intensities of the parallel and perpendicular bands, lead to a suggested structure that has the molecules displaced from a symmetrical square planar arrangement. The molecular centers of mass are displaced above and below a reference plane, and molecular axes are also tilted with respect to the plane.