High-resolution jet-cooled laser absorption spectra of SF<sub>6</sub> at 10.6 μm
Authors: Yuan Hong-Rui , Liu Tao , Zhu Tian-Xin , Liu Yun , Li Xiang , Chen Yang , Duan Chuan-Xi ,   Research Institute of Physical and Chemical Engineering for Nuclear Industry, Tianjin 300180, China,   Science and Technology on Particle Transport and Separation Laboratory, Tianjin 300180, China,   College of Physical Science and Technology, Central China Normal University, Wuhan 430079, China
Published: January 2023

Sulfur hexafluoride (SF<sub>6</sub>) is a greenhouse gas of very long lifetime. Its infrared absorption spectrum is very important in modeling the atmospheric radiation balances. The SF<sub>6</sub> is also a prototypical system for studying the principles and techniques of laser isotope separation using powerful infrared lasers. As a very heavy molecule, the infrared spectrum of SF<sub>6</sub> at room temperature is very dense, which poses a great challenge to monitoring the relative abundances of different SF<sub>6</sub> isotopomers by direct absorption spectroscopy. Supersonic jet expansions have been widely used to simplify the gas phase molecular spectra. In this work, astigmatic multi-pass absorption cell and distributed feed-back quantum cascade lasers (QCLs) are used to measure jet-cooled rovibrational absorption spectra of <sup>32</sup>SF<sub>6</sub> and <sup>33</sup>SF<sub>6</sub> at 10.6 μm. The spectrometer works in a segmented rapid-scan mode. The gas mixtures (SF<sub>6</sub>∶Ar∶He = 0.12∶1∶100) are expanded through an 80 mm <inline-formula><tex-math id="M1">\begin{document}$ \times $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20222285_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20222285_M1.png"/></alternatives></inline-formula> 300 μm pulsed slit nozzle. Two QCLs running at room temperature are used and each one covers a spectral range of about 3.0 cm<sup>–1</sup>. The <i>v</i><sub>3</sub> fundamental bands of both <sup>32</sup>SF<sub>6</sub> and <sup>33</sup>SF<sub>6</sub> are observed. The rotational temperature of <sup>32</sup>SF<sub>6</sub> and <sup>33</sup>SF<sub>6</sub> in the ground state in the supersonic jet are both estimated at 10 K and the linewidth is about 0.0008 cm<sup>–1</sup> by comparing the simulated spectrum with the observed spectrum with the PGOPHER program. A new weak vibrational band centered around 941.0 cm<sup>–1</sup> is observed and tentatively assigned to the (<i>v</i><sub>1</sub>+<i>v</i><sub>2</sub>+<i>v</i><sub>3</sub>)–(<i>v</i><sub>1</sub>+<i>v</i><sub>2</sub>) hot band of <sup>32</sup>SF<sub>6</sub>. The effective Hamiltonian used to analyze the rovibrational spectrum of SF<sub>6</sub> is briefly introduced. A simplified rotational analysis for this hot band is performed with the XTDS program developed by the Dijon group. The band-origin of this hot band is determined to be 941.1785(21) cm<sup>–1</sup>. The rotational temperature of this hot band is estimated at 50 K. A new scheme by measuring the jet-cooled absorption spectrum of this hot band of <sup>32</sup>SF<sub>6</sub> and the <i>v</i><sub>3</sub> fundamental band of <sup>33</sup>SF<sub>6</sub> is proposed for measuring the relative abundance of <sup>33</sup>SF<sub>6</sub>/<sup>32</sup>SF<sub>6</sub>.