Collision-induced absorption in gaseous sulphur hexafluoride

Abstract

The absorption spectrum of gaseous sulphur hexafluoride has been measured in the spectral region from approximately 20 to 140 cm−1 at 339 K. The experiments were performed over a range of densities from 65 to 109.6 amagat (for SF6, 1 amagat = 45.3 mol m−3), using FTIR spectroscopy. From the density dependence of the absorption, the collision-induced spectrum was clearly separated from the strong allowed transitions of vibrational difference bands at 26 and 91 cm−1. Our results for the induced spectrum differ substantially, both in overall magnitude and in shape, from earlier data. We interpret the absorption as due to anisotropic overlap induction; however, a substantial contribution from a hexadecapolar mechanism cannot be ruled out. We can place an upper limit of approximately 7 × 10−42 esu on the hexadecapole moment. The induced line shape is calculated using information theory, based on our knowledge of the zeroth and second moment of the frequency spectrum. Reasonable agreement is found between theory and experiment.