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| Detection of a Chemical Reaction by a 1～18 GHz Chirped-Pulse Fourier Transform Microwave Spectrometer |
| JIAO Chao1, DUAN Sheng-wen1, XU Ke-ya2, WU Yi1, SUN Ming1*, LI Li1, GU Wen-hua1, XIAN Lun-lun1, ZHANG Yu-zhen1,3, CHEN Qian1,3*, LI Ya-ming4, KANG Lu5 |
1. School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
2. College of Energy and Electrical Engineering, Hohai University, Nanjing 211100, China
3. Jiangsu Key Laboratory of Spectral Imaging and Intelligent Sense, Nanjing 210094, China
4. Department of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian 116028, China
5. Department of Chemistry and Biochemistry, Kennesaw State University, Kennesaw, GA 30144, USA |
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Abstract 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.
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Received: 2019-02-12
Accepted: 2019-06-20
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Corresponding Authors:
SUN Ming, CHEN Qian
E-mail: msun@njust.edu.cn；chenq@njust.edu.cn
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