Low Energy Bending Modes of Small Carbon Clusters: Laboratory Terahertz Spectroscopy

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The aim of the research proposed here within the framework of Associated European Laboratory-HiRes, is to extend and expand our present studies to a new domain of postulated interstellar molecules that have so far eluded laboratory detection and thus spectroscopic scrutiny. Amongst these are molecular ions, linear chain molecules and large monocyclic and polycyclic rings. The key innovation will be the construction of an extremely sensitive but "portable" THz spectrometer working with and probing the molecular beam produced by our laser ablation technique. By simultaneously monitoring the IR spectra by tunable diode laser spectroscopy (IR-TDL) and recording the THz spectrum, we expect that this new, highly sophisticated piece of instrumentation to be considerably more sensitive in selecting the wanted spectra than our current spectrometer, particularly when used together with a mass-selective spectrometer. The THz spectrometer in its present form is already the most powerful and only tool in the world for this kind of research we do at these frequencies. Laboratory detection of large organic molecules will be valuable in several ways. Astronomical observations of such species will improve our understanding of the physical and chemical conditions in dark and dense molecular clouds and circumstellar envelopes of carbon-rich stars. These studies also will provide unique information about the synthetic steps in the rich transition region from linear to planar and cyclic structures, data needed to test theories of carbon molecular self-assembly, including fullerene formation. And lastly, the proposed program is being directed towards a major, long standing and unsolved problem in astrophysics: the identification of the carriers (i.e. atomic or molecular species) which give rise to diffuse interstellar bands (DIbs). The carriers, now generally thought to be fairly large molecules, most likely long carbon chain containing species, are important because they may constitute by mass a significant fraction of all the chemically bonded matter in nature. Over the past few months we have worked out essentially all details of the design of the new TZz spectrometer and its successful attachment to the existing TDL laser ablation molecular jet spectrometer, relying on the simultaneous measurement of THz and IR spectra. We expect that this next generation laboratory spectrometer will be well adapted to the measurement of the bending motion or ring puckering of even larger known molecules. By extension we hope that the laboratory spectra of postulated even larger interstellar molecules can be detected with it.