Formation of C4H4 from photolysis of icy C2H2 with 175 nm at 60 K

The absorption spectrum of solid acetylene (C2H2) at 10 K was measured in wavelength range 107-250 nm with UV light from a synchrotron source. Based on the absorption spectrum, we irradiated at 175 nm the icy acetylene at 60 K; the results yielded a new set of IR absorption lines with similar temporal profiles of production during the photolysis periods. From icy (C2H2)-C-12, these new features were recorded at 3276.8, 1598.7, 963.0, 936.3, and 671.3 cm(-1), which are assigned to modes nu(1), nu(6), nu(14), nu(15), and nu(11) (mixing with nu(17)), respectively, of vinylacetylene (C4H4) based on the results of C-13- and D-isotopic experiments and quantum-chemical calculations. These calculations using density-functional theory (B3LYP/6-311++G**) predict vibrational wavenumbers and IR intensities of vinylacetylene that agree satisfactorily with our experimental results. This work enhances our understanding of the photochemistry of icy acetylene in cold astronomical environments; for instance, the present result can be applied directly to the photochemical processes on Titan's surface.