High-resolution jet-cooled laser absorption spectra of SF6 at 10.6 μm

Sulfur hexafluoride (SF6) is a greenhouse gas of very long lifetime. Its infrared absorption spectrum is very important in modeling the atmospheric radiation balances. The SF6 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 SF6 at room temperature is very dense, which poses a great challenge to monitoring the relative abundances of different SF6 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 32SF6 and 33SF6 at 10.6 mu m. The spectrometer works in a segmented rapid-scan mode. The gas mixtures (SF6:Ar:He = 0.12:1:100) are expanded through an 80 mm x 300 mu m pulsed slit nozzle. Two QCLs running at room temperature are used and each one covers a spectral range of about 3.0 cm-1. The v3 fundamental bands of both 32SF6 and 33SF6 are observed. The rotational temperature of 32SF6 and 33SF6 in the ground state in the supersonic jet are both estimated at 10 K and the linewidth is about 0.0008 cm-1 by comparing the simulated spectrum with the observed spectrum with the PGOPHER program. A new weak vibrational band centered around 941.0 cm-1 is observed and tentatively assigned to the (v1+v2+v3)- (v1+v2) hot band of 32SF6. The effective Hamiltonian used to analyze the rovibrational spectrum of SF6 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-1. 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 32SF6 and the v3 fundamental band of 33SF6 is proposed for measuring the relative abundance of 33SF6/32SF6.