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

Abstract

Sulfur hexafluoride (SF_６) is a greenhouse gas of very long lifetime. Its infrared absorption spectrum is very important for modeling the atmospheric radiation balances. SF_６ 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_６ at room temperature is very dense, which poses a great challenge for monitoring the relative abundances of different SF_６ isotopomers by direct absorption spectroscopy. Supersonic jet expansions have been used widely to simplify 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 spectrum of ³²SF_６ and ³³SF_６ at 10.6 μm. The spectrometer works in a segmented rapid-scan mode. The gas mixtures (SF₆:Ar:He = 0.12:1:100) are expanded through a 80 mm×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^-1. The v₃ fundamental bands of both ³²SF_６ and ³³SF_６ are observed. The rotational temperature of ³²SF_６ and ³³SF_６ in the ground state in the supersonic jet is estimated to be about 10 K and the linewidth is about 0.0008 cm^-1 by comparing the simulated and 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 ³²SF_６. The effective Hamiltonian used to analyze the rovibrational spectrum of SF_６ 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 to be about 50 K. A new scheme by measuring the jet-cooled absorption spectrum of this hot band of ³²SF₆ and the v₃ fundamental band of ³³SF_６ is proposed for measuring the relative abundance of ³³SF_６／³²SF_６.