Microwave and laser spectroscopy of carbon chains and rings

Abstract

The interstellar gas and the gas ejected into space by cool stars are now known to contain a rich collection of molecules. Most of these are organic compounds, some of which are familiar to the terrestrial chemist and can be found in a standard chemical stockroom, but many are entirely new molecules which have been detected and identified in space for the first time. Most of the nearly 100 polyatomic molecules so far detected in astronomical sources by radio telescopes are highly unsaturated carbon chains, a structure which is explosively unstable at even moderate density, and therefore difficult to study spectroscopically on Earth. For this reason laboratory detection has until quite recently lagged well behind the astronomical discovery of many carbon chains. The application of Fourier transform microwave and laser cavity ringdown absorption spectroscopy to supersonic molecular beams has largely overcome this difficulty, yielding in this laboratory during the past four years more than 75 new reactive molecules, including long carbon chains, chains attached to rings, silicon–carbon rings, and several protonated molecular ions. All or nearly all of these are candidates for astronomical discovery because they are highly polar and are close in structure and composition to molecules which have already been discovered in space. Seven have already been found with large radio telescopes, six as a result of the laboratory work described here. These discoveries include the largest interstellar molecule HC11N, the largest ring, SiC3, and the largest positive ion, HC3NH+. One of our most interesting recent laboratory discoveries is the laser detection of a strong molecular band at λ4429, the wavelength of the strongest and best known diffuse interstellar band.