Analysis of the collision induced far infrared spectrum of ethane

Analysis of observed far i.r. collision-induced absorption in ethane and ethane—rare gas mixtures indicates that only a small fraction is due to the canonical multipole-induced and overlap-induced dipole moments. Nevertheless, multipole-induced dipole spectra calculated with no adjustable parameters are shown to fit the low-frequency wing ( < 60 cm⁻¹) of the observed ethane collision-induced spectrum to within 10%. More than half of the observed intensity in the ethane and ethane—rare gas far i.r. spectra has been shown to be due to rotational and vibrational modulation of dipoles induced by electric fields which do not reflect the full symmetry of the inducing molecule but depend strongly on local symmetry of the inducing charge distribution. The ethane CC stretch, ν₃ at 993 cm⁻¹, has been observed to have a collision-induced absorption which is also assigned to effects of local symmetry. In addition to the non-resonant effects due to overlap and frame distortion, we present a new theoretical analysis of a resonant intermolecular effect, also due to frame distortion, at the frequency of the torsional mode, ν₄.