傅里叶变换微波光谱仪是测量分子转动跃迁的主要工具, 是研究分子转动光谱学的重要仪器。 以量子力学为基础的转动光谱学对物质分子的结构分析以及破解射电望远镜所捕获的深空分子信号至关重要, 这使得微波光谱仪在相关领域能发挥不可或缺的作用。 目前, 世界各国都在致力于研制和改进微波光谱仪以提高仪器的分辨率、 灵敏度、 以及应用范围, 我国也正在该类仪器研制上进行积极的探索, 期望为该领域做出应有的贡献。 介绍了一种工作频段在1～18 GHz的宽带啁啾脉冲式傅里叶变换微波光谱仪的设计和研制。 该光谱仪用于线性频率扫描的宽带啁啾脉冲信号由采样速率为1.25 GS·s-1的任意波形发生器产生。 宽带啁啾脉冲信号经混频和放大后可覆盖特定的频率范围, 随即通过喇叭天线传播到样品真空室与超音速膨胀的气相样品分子束相互作用。 样品分子被激发后发出的分子自由感应衰减信号由接收电路导出并放大, 然后直接在高速数字示波器上数字化。 该微波光谱仪的诸多电子器件均由计算机控制, 利用开发的LabVIEW程序可实现仪器的自动化控制。 应用气体喷嘴技术能有效降低待测样品气体束在检测室的转动温度, 使仪器获得更好的检测灵敏度。 应用多脉冲自由感应衰减技术能大幅度提高信号采样频率从而进一步提高仪器灵敏度。 利用实验室研制的啁啾脉冲式傅里叶变换微波光谱仪对盐酸和叔丁醇的化学反应进行了监测, 并成功检测到了该反应的产物叔丁基氯。 通过测量天然丰度下反应产物叔丁基氯及其单取代37Cl同位素异数体的分子转动光谱数据, 并利用光谱分析软件拟合这些数据后得到了叔丁基氯精准的光谱参数（转动常数, 离心畸变常数, 核四极耦合常数等）和分子结构信息。 将以上参数和结构信息与高斯计算结果对比后证实了本实验宽带光谱仪检测到的光谱数据具有高精准度。 通过对比前人所测的光谱数据进一步展示了该实验宽带光谱仪在低频范围内杰出的测试性能。

Fourier transform microwave spectrometer is the main tool for measuring molecular rotational transitions and an important instrument for researching molecular rotational spectroscopy. Based on quantum mechanics, rotational spectroscopy is essential for the structural analysis of molecules and for deciphering molecular signals from deep space captured by radio telescopes, thus making microwave spectrometers indispensable in those fields. At present, researchers from countries all over the world are working on the instrumentation of microwave spectrometers to improve the resolution, sensitivity, and application range as well, while Chinese researchers are also exploring such instrument development actively, and expect to make due contributions to those fields. In this paper, the design and development of a chirped-pulse Fourier transform microwave spectrometer are presented with a frequency coverage of 1～18 GHz. The broadband chirped pulse of a linear frequency sweep is generated by the arbitrary waveform generator with a sampling rate of 1.25 GS·s-1. After mixing and amplification, the chirped pulse with a certain frequency coverageis broadcast through a feedhorn antenna into the vacuum chamber, where it interacts with a supersonically expanded molecular beam. The free induction decay (FID) signal emitted by the excited molecules is induced and amplified by the receiving circuit and then directly digitized on a high-speed digital oscilloscope. Many electronic devices of the microwave spectrometer are controlled by a personal computer, and their automatic operation can be achieved by a LabVIEW program. The gas nozzle technology is applied to improve the sensitivity of the spectrometer by effectively reducing the rotational temperature of gas samples in the vacuum chamber. Multiple free induction decay (multiple FID’s) technology is also applied to further improve the sensitivity by dramatically increasing the signal sampling rate of the spectrometer. By using this broad-band chirped-pulsed Fourier transform microwave spectrometer developed in the laboratory, a chemical reaction of hydrochloric acid and tertiary butanol was monitored, with the reaction product tert-butyl chloride detected successfully. The rotational spectra of tert-butyl chloride and its singly-substituted 37Cl isotopologue were measured in their natural abundance, and were then fit by the spectrum analysis software to provide accurate spectral parameters (rotational constants, centrifugal distortion constants, and the nuclear quadrupole coupling constants, etc.) and molecular structure. The high accuracy of spectral data measured by the spectrometer was proved by comparison with Gaussian calculation. The spectrometer’s excellent performance in the low frequency range was also demonstrated when compared with the spectral parameters measured by predecessors.