Microwave spectroscopy of transient interstellar molecules

The author has recently identified five new interstellar molecules; C6H(2Πi), CCS(3Σ-), C3S, cyclic-C3H(2B2), and CH2CN(2B1) which are transient in laboratory. The C6H radical was identified only from the analysis of the astronomical spectral lines observed. Detailed knowledge about the rotational energy of the doublet linear radical was essential for the identification, because the ground electronic state of C6H turned out to be 2Πi, entirely different from those of related even-number carbon-chain radicals, C2H and C4H. The identification of the CCS radical, including the most intense interstellar line left unidentified at that time, required knowing the triplet fine structure of the linear molecule having a large spin-splitting which is comparable in energy to the cloud temperature. The CH2CN radical showed the most complicated spectral pattern among those of interstellar molecules detected so far. Twenty two precise molecular constants including the hyperfine coupling constants for the nitrogen and hydrogen nuclei made it possible to identify 26 components for a rotational transition at 40.24 GHz. The identification of these new transient interstellar molecules has been substantiated by a close collaboration of laboratory microwave spectroscopy and radio astronomy