Collision-induced absorption in a highly symmetric molecule—SF₆

The far-infrared absorption spectra of gaseous (298 K) and liquid (273 K, 233 K) SF6 obtained by Rosenberg and Birnbaum [1] are analysed in terms of recent theoretical developments. Analysis of the band shape in the gas and liquid phases indicates that hexadecapole-induced absorption accounts only for the low frequency portion of the collision-induced band. The major part of the absorption can be attributed to a much shorter range mechanism such as anisotropic overlap. The resulting induced dipole which varies very rapidly with intermolecular separation and is consistent with a (7–28) interaction energy gives a good description of the liquid spectra. Collision-induced absorption and depolarized light scattering in SF6 are compared. From a decomposition of the collision-induced gaseous spectrum, the value of the hexadecapolar energy, Φ2/σ9, is 2·7 × 10-24 J (0·20 K) which yields a value of the hexadecapole moment, Φ, of 18 × 10-60 C m4 for σ = 0·468 nm.