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

本文介绍了一种可用于毫米波波段的傅里叶变换微波谱仪，用于探测具有天文意义的各种自由基和分子离子的转动谱线。利用微波合成器驱动的倍频器输出端与稳频古恩振荡器输出端之间的耦合作为毫米波源，发现毫米波源的主要难点是如何将毫米波辐射与法布里-珀罗腔耦合。为了在较宽的频率范围内获得良好的性能，尝试了各种耦合环和耦合孔。耦合孔的尺寸是实现高灵敏度光谱仪的关键。利用此方法观察到C_3S和C_3H_2的强谱线。此外，还观察到了环C_3H、CCS、SO和HC_3S等自由基的谱线，并得到了较好的信噪比。沿着上述工作，我们用常规的源调制谱仪详细研究了星际分子（c-C_<>H_<>Si，线性C_3H和线性C_2H）的转动谱。此外，通过NRAO 12 m望远镜的天文观测，还发现H_2CN自由基是一种星际分子。

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.