Laboratory and Astronomical Detection of New Carbon Chains and Rings

Abstract

Discoveries of large new organic molecules in space help provide a foundation for understanding the role of the chemical bond and organic chemistry on a cosmic scale, and by extension, an opportunity to begin to address the interesting but far more complicated questions of chemical and biological origins and the issues that relate to them. Most of the organic molecules so far detected in the interstellar gas and circumstellar shells by radio telescopes are highly unsaturated carbon chains, a configuration of linear carbon which is unstable at high density, and therefore unfamiliar on Earth. For this reason laboratory detection until quite recently has lagged behind astronomical discovery of many new carbon molecules. The application of Fourier transform microwave spectroscopy to supersonic molecular beams has now largely overcome this obstacle, yielding in just over three years the laboratory detection of forty-four new carbon chains and eight new carbon ring-chains and rings, including two rhomboidal isomers of SiC3. The set consists of 14 polyynes, 20 carbon chain radicals, and 18 free carbenes. For four other triplet chains HC7H, HC9H, HC11H, and HC13H, which by symmetry are nonpolar, strong electronic spectra have been measured in the gas-phase by cavity ringdown absorption spectroscopy. Almost all of these molecules are good candidates for detection in space, and six in fact have now been detected in at least one astronomical source with large radio telescopes during the past two years. The laboratory astrophysics of the entire set is complete for the time being, in the sense that nearly all the rotational transitions of interest to radio astronomy can be calculated to a small fraction of 1 km sec–1 in equivalent radial velocity. With powerful new radio facilities planned or under construction it would be surprising if many more could not eventually be found. For each of the four new symmetric chains detected in the visible, the wavelength of the origin band has been measured to high accuracy, permitting deliberate searches for these molecules in diffuse and translucent molecular clouds.