Detection of New Interstellar Molecules by Fourier Transform Millimeter-wave Spectroscopy

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A Fourier transform microwave spectrometer which can be used in the millimeter-wave region is developed in order to destect rotational spectral lines of various free radicals and molecular ions with astronomical interest. An output of a frequency doubler driven by a microwave synthesizer to an output of a frequency stabilized Gunn oscillator is used as a millimeter-wave source.A main difficulty in the millimeter-wave region is found to be how to couple the millimeter-wave radiation with a Fabry-Perot cavity. Various coupling loops and coupling holes are tried in order to obtain good performance in a wide frequency region. Dimensions of the coupling hole is found to be essential to realize a sensitive spectrometer.A discharge nozzle system is constructed to observe spectral lines of free radicals and molecular ions. By use of this, the intense spectral lines of C_3S and cyclic C_3H_2 are observed. Furthermore, the spectral lines of some free radicals such as cyclic C_3H,CCS,SO,and HC_3S are also observed with a good S/N ratio. In order to cancel the Earth's magnetic field, three Helmholtz coils are employed.Along with the above efforts, rotational spectra of interstellar molecules (c-C_<>H_<>Si, linear C_3H,and linear C_2H) are investigated in detail by using a conventional source modulation spectrometer. Furthermore the H_2CN radical is detected as an interstellar molecule through astronomical observations with the NRAO 12 m telescope.