Microwave Spectroscopic Detection and Structure Determination of Active Species in Interstellar Space and Low-Temperature Plasma

近年来，人们对短寿命分子(离子、自由基和不稳定的化学活性分子)非常感兴趣，这些分子不能用通常的化学方法来合成。为了观察这类分子的光谱，尤其重要的是使用高灵敏度的光谱仪，有效地产生不稳定的物种，并基于对密切相关分子结构的高级量子化学计算，从估计的结构中尽可能精确地预测光谱频率。在本研究中，我们构建了一台微波光谱仪，该光谱仪应该满足上述条件，目的是观察瞬变物种的转动光谱。该光谱仪的特点是：(1)使用长2米、直径10厘米的吡咯x自由空间池，在池中使用辉光放电电极原位产生不稳定的分子；(2)使用双方波源调制并以二阶导数的形式观察光谱；(3)用锁相环稳定来自速调管的10 GHz频率微波，作为高频毫米波的频率测量标准；(4)光谱测量和数据简化(积分、基线失真校正、吸收线频率的测定等。)在个人电脑的帮助下。通过观察OCS在振动激发态的弱吸收线，验证了该光谱仪的灵敏度和精密度。观察到光谱强度为1×10^&lt；-8；cm^&lt；-1&gt；的谱线。测得的频率与文献报道的分子常数计算值吻合得很好。通过低压SiCl4的辉光放电观察到了不稳定的SiCl2的光谱，在HSiCl3的放电中观察到了未知物种的光谱。

Recently much interest is directed toward short-lived molecules (ions, free radicals and unstable chemically active molecules), which cannot be synthesized with a usual chemical method. For the observation of the spectrum of such molecules, it is especially important to use a spectrometer of high sensitivity, to produce labile species efficiently and to predict the spectral frequencies as precirely as possible from the estimated structure based on the high-grade quantum chemical calculation of the structure of closely related molecules. In the present study we constructed a microwave spectrometer, which should meet the above conditions and aim to observe the rotational spectrum of the transient species. The characteristic features of the spectrometer are (1) use of a pyrex free-space cell, 2 m in length and 10 cm in diameter, with glow-discharge electrodes in the cell to produce in situ the labile molecule, (2) use of the source modulation by double square waves and observation of the spectrum in the form of the second derivative, (3) stabilization , with a phase-lock loop, of the 10 GHz frequency microwave from a klystron, which is used as the standard of the frequency measurement for the high-frequency millimeter wave and (4) spectrum measurement and data reduction (integration, correction of baseline distortion, determination of the frequencies of the absorption lines, etc.) with the aid of a personal computer. The sensitivity and precision of the spectrometer was tested by observing weak absorption lines of OCS in vibrationally excited states. The lines with the spectral intensity of 1X10^<-8> cm^<-1> could be observed. The measured frequencies agreed well with those calculated from the molecular constants reported in the literature. The spectrum of labile SiCl_2 was observed through the glow discharge of low-pressure SiCl_4. The spectrum of an unknown species was observed in the discharge of HSiCl_3.